Femtosecond Time-Resolved Photoelectron Spectroscopy in ... · Betreuer: Prof. Dr. U. Heinzmann Priv. Doz. Dr. M. Drescher Gutachter: Prof. Dr. U. Heinzmann Priv. Doz. Dr. F. Stienkemeier

Femtosecond Time-Resolved

Photoelectron Spectroscopy

in the

Extreme Ultraviolet Region

Dissertation

zur Erlangung des Doktorgrades

der Fakultät für Physik

der Universität Bielefeld

vorgelegt von

Peter Šiffalovič

aus Bratislava, Slowakei

März 2002

Betreuer: Prof. Dr. U. Heinzmann

Priv. Doz. Dr. M. Drescher

Gutachter: Prof. Dr. U. Heinzmann

Priv. Doz. Dr. F. Stienkemeier

Prof. Dr. W. Wurth

Tag der Disputation: 10.06.2002

Teile der Arbeit sind veröffentlicht in:

• Laser-based apparatus for extended ultraviolet femtosecond time-resolved

photoemission spectroscopy

P. Siffalovic, M. Drescher, M. Spieweck, T. Wiesenthal, Y. C. Lim, R. Weidner, A.

Elizarov, and U. Heinzmann, Rev. Sci. Instrum. 72, 30 (2001)

• Application of monochromatized high harmonic EUV radiation for inner-shell

photoelectron spectroscopy

M. Drescher, P. Siffalovic, M. Spieweck, and U. Heinzmann, J. Electron. Spec. Relat.

Phenom. (to be published)

Contents

Contents

Acknowledgements __________________________________________________________1

Introduction________________________________________________________________4

Theoretical Background ______________________________________________________9

2.1 Light–Solid Interaction _________________________________________________9

2.1.1 Photoexcitation and Photoemission _____________________________________9

2.1.2 Relaxation Mechanisms in Metals _____________________________________13

2.1.3 Relaxation Mechanisms in Semiconductors______________________________19

2.2 Pump-Probe Technique________________________________________________26

2.2.1 Principle of Pump-Probe Technique____________________________________26

2.2.2 Generation, Amplification and Application of Femtosecond Light Pulses ______29

Experimental Setup_________________________________________________________34

3.1 Descriptions of Experimental Setup______________________________________34

Test Measurements _________________________________________________________47

4.1 Photoelectron Spectroscopy on Metals, Semiconductors, Adsorbates and Gases _47

Time-Resolved Photoelectron Spectroscopy on Pt(110) Surface _____________________54

5.1 Measurement of Hot Electrons on a Pt(110) Surface ________________________54

5.2 Pump-Probe Cross-Correlation in the EUV Region ________________________62

Time-Resolved Photoelectron Spectroscopy on GaAs(100) Surfaces__________________66

6.1 Measurements of Transient Ga-3d Core-Level Shifts on GaAs(100) Surfaces ___66

6.2 Dynamics of the band-bending for p-GaAs(100) ___________________________77

Contents

Conclusions and Outlook ___________________________________________________ 82

Appendix_________________________________________________________________ 84

A Knife-edge Beam Profile Measurement __________________________________ 84

B Auger Electron Spectroscopy on Pt, Cu and GaAs Surfaces _________________ 85

C Surface Photovoltage Transients and Surface Preparation __________________ 87

References _______________________________________________________________ 89

Acknowledgements 1

Acknowledgements

It was in summer 1998 when I was speaking for the first time with Prof. Ulrich

Heinzmann who got me an opportunity to join his working group and work on a project –

“Anwendung von mit Multilayern spektral selektierten hohen Harmonischen von fs-

Laserstrahlung für zeitauflösende Photoemissionexperimente” (Utilisation of high harmonics

selected with the multilayer optics for the femtosecond time-resolved photoemission

experiments). To him I owe an immense debt of gratitude for this great possibility to realise

this project. I thank him also for the possibility to be involved in a new SFB project “Physik

von Einzelmolekülprozessen und molekularer Erkennung in organischen Systemen” (Physics

of the single molecule processes and molecular recognition in organic systems). He gave me

unrestricted freedom in realising my experimental ideas. I really enjoyed having exercise

sessions for his lectures - Einführung in die Physik I (Introduction to Physics I), which were

really refreshing and enriching when compared with fade and repeating practical sessions.

At that summer I also met Dr. Markus Drescher who with excitement and full of

enthusiasm introduced to me his idea of the femtosecond visible-pump/extended ultraviolet-

probe (EUV) experiment. I wonder whether he knew at that time that this project is going to

be enormously successful. With his wide experiences I have realised pioneering experiments

on the field of femtosecond time-resolved photoelectron spectroscopy in the EUV region. We

have together achieved the first cross-correlation of femtosecond visible-EUV pulses on metal

surface (April, 2000) and the first time-resolved pump-probe core-level experiment on

semiconductor surface (July, 2001). With his great experimental skills and endurance he was

also main contributor to the fascinating cutting edge experiment in Vienna leading to the first

2 Acknowledgements

measurement of the single attosecond pulses. At this point I would like to say one great

“Thanks” for all what he has done for my work.

I am endless in dept to Prof. Štefan Luby and Dr. Eva Majková in Slovakia. They were

initiators of my PhD study in Bielefeld and they have continually supported my study and

experiments here in Bielefeld. I am looking forward to continuing this great and magnificent

cooperation.

It has been a great pleasure to work with former member of our group Dr. Michael

Spieweck. It seems almost incredible that we have built 0.2 TW femtosecond laser system in

only 18 months - a small miracle! He is also spiritual father of my German language skills. I

have never experienced better experimental cooperation than with him.

I gratefully acknowledge the work of our former diploma student Dip.-Phys. Tobias

Wiesenthal. He gave a birth to our “mechanical wonder” – monochromator for EUV

femtosecond pulses. He has done a good job!

Not less I thank Dr. YongCheol Lim who deposited multilayer mirrors for our

monochromator. These mirrors are until now world-wide unprecedented in their spectral

selection and low group velocity dispersion.

I would like to thank Dr. Norbert Müller for many helpful suggestions leading to the

better understanding of the experiments. His contributions to our experimental apparatus are

also immensely acknowledged.

I thank very much to Dipl.-Phys. Michael Sundermann and Dipl.-Phys. Armin

Brechling for their great collaboration on the self-assembled monolayers preparation.

My thanks to Dr. Frank Hammelmann and Prof. Peter Jutzi for providing me with

W(CO)6 and information on the subject of its vacuum deposition. Thanks to working group of

Prof. Jochen Mattay for synthesis of Br and I terminated self-assembled monolayers. Thanks

also to Prof. Markus Donath for supporting as with p-GaAs(100) crystal.

I wish to thank Dr. Ulf Kleineberg for his motivating and animating discussions. My

thanks go to former member Dr. Bernd Schmidtke who helped me occasionally with laser

alignment and was “real good brain” to tackle my small theoretical problems. I appreciate all

creative contributions from my close-working colleges Thorsten Uphues, Dipl.-Phys. Martin

Michelswirth.

My thanks go also to the members of our working group “Molekül- und

Oberflächenphysik”: Dipl.-Phys. Christian Meier for his technical help by “disobeying”

computers and his creative physical ideas, Dipl.-Phys. Martin Pohl for his critical remarks on

my technical drawings and introduction to the PEEM secrets, Kay Lofthouse for her

Acknowledgements 3

professional approach by the correcting of my english paper, Karla Schneider for her every

time helping hand in all kinds of bureaucratic problems, Volker Schimmang for his helping

by “mechanical” troubles, Dr. Oliver Wehmeyer for his web design, Dipl.-Phys. Thomas

Westerwalbesloh for his multilayer mirror calculations, Dipl.-Phys. Tarek Khalil for his

readiness to help, Dr. Wiebke Hachmann for her SEM images, Mag. Frank Persicke for his

introduction to the laser tweezers, Dipl.-Phys. Toralf Lischke for his Berlin’s “how to get

there?” knowledge, and Dipl.-Phys. Andreas Aschentrup for introduction to the LIP system.

Most importantly, my great thanks go to my entire family for support and

encouragement throughout my years at Bielefeld University.

4 Chapter 1 – Introduction

Chapter 1

Introduction

The first article1 in the column “Research Highlights” of the Nature Physics Portal

from 29th November, 2001 was published under the synonym - “The Age of Attophysics”. The

way to this scientific breakthrough2,3 is directly linked with the generation of light in the

extreme ultraviolet (EUV) region proposed some ten years ago4. Since then a rash progress5-7

has ended in the experimental evidence of a single attosecond pulse with a duration of

650±150 as. Considering that the attosecond pulse of such length is a coherent superposition

of electromagnetic radiation with a bandwidth§ of 2.8 eV or 680 THz, which is likely the

whole visible part of the electromagnetic spectrum, leads us to the conclusion that the carrier

of such ultra-short pulses would be the EUV region. Consequently, in the following years we

will be witnesses of the fast developing femto-and-attosecond time-resolved spectroscopy in

the EUV region.

The development8 of short laser pulses and their applications for time-resolved

measurements began in the early 70’s. The progress on laser pulse shortening is shown in

Fig. 1.1. The first lasers working in the femtosecond region have been dye lasers. Practically

all fascinating time-resolved measurements9-15 has been demonstrated in the 80’s. The fast

changes of the transmittance and reflectivity of semiconductors, metals and liquids linked

with inter- and intra-band transitions in semiconductors and metals, inter- and intra-molecular

transitions in complex organic molecules, the rearrangement and breaking of chemical bonds

§ For chirp-free gaussian pulse the relation between the bandwidth and time duration is given as

ν∆ τ∆

441.0=∆⋅∆ τν

Chapter 1 – Introduction 5

as well as the observation of phonons and excitons have been shown more than 10 years ago.

Although the dye laser stability has been lower when compared to contemporary femtosecond

lasers a pulse duration of less than 10 fs has been achieved with additional pulse spectral

broadening via a self-phase modulation and subsequent pulse compression16,17. In the early

90’s rapid development of femtosecond solid-state lasers resulted in the renaissance of the

“old” time-resolved techniques and stimulated growth of new and more sophisticated

measurements. Also the pulse duration18,19 of the femtosecond pulses in the visible has

reached its limits of some 4.5 fs.

dye technologyTi:sapphire technology

SPM & compression

high harmonics

Puls

e du

ratio

n

P

t

f

(

t

i

e

a

E

e

Fig. 1.1 Road map of the laser pulse duration (taken from Ref. 20 and updated)

arallel to these experiments in the visible, the nonlinear conversion of the visible light into

he EUV, soft X-rays and hard X-rays regions stimulated by the development of TW

emtosecond lasers21 has achieved great success and pushed the development of the theories

Fig. 1.2). The classical nonlinear optics22 based on the perturbation theory being a powerful

ool of explaining processes has been not appliable to the new observed phenomena at

ntensities higher than 10

)(nχ14 W/cm2. At these and higher intensities23 one is dealing with laser

lectric fields comparable to those in the atoms themselves and the electrons become ionized

lthough the photon energies are much lower than the ionization potential of the atoms.

ffects such as above-threshold ionization, high harmonic generation but also high energetic

lectrons24 and positrons25 generation has been observed and theoretically explained. Already


at currently available intensities of some 1020 W/cm2 one can observe relativistic effects in the

light-plasma interaction26-30.

Regimes of Nonlinear Optics

Perturbative regime Strong-field regime

Bound electrons Free electronsχ(2) Processes

χ(3) Processes

Second harmonic generationOptic parametric generationOptic rectification

Third harmonic generationStimulated Raman scatteringSelf-phase modulationSelf-focusing

Multiphotonabove-thresholdionization

Laser ablation

Long distanceself-channeling

High harmonicgeneration

Sub-fs x-ray andelectron pulses

Self-defocusing

Relativisticregime

hard x-rays

Multi-MeVelectrons

Self-focusingand channeling

1011 1012 1013 1014 1015 1016 1017 1018 1019

Intensity (W/cm )2

Fig. 1.2 Overview of the new regimes of nonlinear optics (taken from Ref. 23)

Another challenge that is closely connected with the generation of the light pulses with

energies of few tens to few thousands of electronvolts is the measurement of their temporal

duration and possible application for ultra-fast time-resolved techniques. The chart showing

the progress on this field is compiled in Fig. 1.3. The pulse duration of the low-orders of high

harmonics with energies up to 13.5 eV has been successfully measured with autocorrelation

techniques based on two- and three-photon ionization31-33. However, this method is not

extendable to higher photon energies because of the extremely low transition probabilities of

the non-resonant two and more photon ionization by the current high harmonics intensities.

Recently, high harmonics up to 40 eV photon energies have been characterized with

cross-correlation techniques and utilised in time-resolved photoelectron spectroscopy

measurements on adsorbates34, isolators35 and in the gas phase36,37. Femtosecond pulse

duration of high harmonics with a photon energy of 70 eV has been measured via hot

electrons on a Pt surface in our Bielefelder-group described in chapter 5. Also stunning

measurement of attosecond pulse duration of the high harmonics with an energy of 90 eV has

been measured in the cooperation Vienna-Bielefeld1. The development of X-ray sources based

on plasma recombination generated with high power femtosecond laser pulses on solid state

Chapter 1 – Introduction 7

surfaces initiated the first hard X-ray time-resolved diffraction measurements. Recently,

femtosecond pulse duration at photon energies of 4.5 keV38,39 and 8 keV40,41 were reported.

IRUV

VUV

Extreme Ultraviolet - EUV

Soft X-rays

Hard X-rays

1 eV 10 eV 100 eV 1 keV 10 keVPhoton energy

Fig. 1.3 Chart of demonstrated femtosecond or sub-femtosecond pulse duration with

respect to their location in the electromagnetic spectrum.

The aim of this thesis is to present two measurements that are contributing to the rapid

growing field of femtosecond time-resolved spectroscopy in the EUV region. The first one

demonstrates the selection of a single high harmonic without temporal distortion in a

dedicated multilayer monochromator as proved by the measurement of its temporal duration

with a cross-correlation technique based on hot electrons at the Pt surface. I have already

shown a spectral selection35-47 of one high harmonic order. In chapter 5 I will report on

the measurement48-50 of the 45th high harmonic temporal duration. The peak photon flux of

built high harmonic source compared to that of 3rd generation storage ring51 (BESSY II) is

shown in Fig. 1.4. The main advantage of high harmonic source as compared to such large

scale facilities is the femtosecond time duration of high harmonic pulses. There are already

first advances52,53 of a formation of synchrotron radiation with femtosecond pulse duration but

until now no experiment has been demonstrated utilizing these pulses. The current drawback

of the high harmonic sources is the low repetition rate of a few kHz compared to that of few

hundred MHz of the storage rings51. The second significant measurement presented in

chapter 6 is the first application of a spectrally selected single high harmonic order for

time-resolved core-level photoelectron spectroscopy. Recently published papers34-36 on

time-resolved photoelectron spectroscopy using high harmonics are dealing exclusively with

the valence bands or highest occupied molecular orbitals. Here I report on the first

application54-57 for a femtosecond time-resolved study involving core-level electrons on the

GaAs surface.


The extreme surface sensitivity of the photoemitted Ga-3d core-level electrons served for an

observation of the charge transport and recombination after photoexcitation with the visible

pump pulse.

High Harmonic

Peak

flux

e

Fig. 1.4 Performance of built high harmonic source compared to BESSY II (based on data

from Ref. 51)

Chapter 2 – Theoretical Background 9

Chapter 2

Theoretical Background

2.1 Light–Solid Interaction

2.1.1 Photoexcitation and Photoemission

Under photoexcitation one understands the process when an electron after absorption

of a photon will be excited into a binding state localized in the bulk or on the solid state

surface. Photoemission stands for the process where the electron is excited into the continuum

and escapes to vacuum. A simplified model of the metal and semiconductor electronic

structure§ is shown in Fig. 2.1. The lowest photon energy needed for the photoemission is

defined as the work function for the metals and for the semiconductors, where is

the electron affinity and is the semiconductor band gap.

φ gE+χ χ

gE

The probability of the photon absorption and electron transition from the initial state i to the

final state f is given by60

)(2 2ωδπh

h−−= ififi EEiHfP (2.1)

§ The negative binding energies throughout this thesis are given relative to the Fermi level for the metals; relative

to the top of the valence bands for semiconductors; and relative to the vacuum level for the rare gases.

10 Chapter 2 – Theoretical Background

core-levels

Metal

core-levels

E =0F

Evac

E (eV)bin

EF

Evac

Semiconductor

CBM

VBM

φχ

Eg

E =0VBM

E (eV)bin

T

p

i

H

g

e

d

f

t

Fig. 2.1 Scheme of the electronic structure of metal and semiconductor

iE and are the energies of the initial and the final electron states respectively and fE

Ae= km ⋅Hi , where is the electron momentum operator and is the vector potential.

he energy conservation law is enforced with the delta function. In the case of the

hotoemission when the final state is not a bound state, the kinetic energy of the free electron

s given by (2.2) and (2.3) for metals and semiconductors respectively

k A

φω −+= ikin EE h (2.2)

gikin EEE −−+= χωh (2.3)

owever, the probability of the photoexcitation and photoemission in the solid state is not

iven with the simple equation (2.1), but has to include the specific electronic structure of the

xamined specimen. After correction due to the variety of initial and final states and their

ispersion with the electron momentum one can define the imaginary part of the dielectric

unction60, which is proportional to the absorption coefficient62 by the photoexcitation and to

he photoemission cross section in the case of photoemission63,64 respectively, as follows

∑∫ −−

=

fiifi kdkEkEkiHkf

me

,

322

0

))()(()()(1))(Im( ωδωπε

ωε h (2.4)


photoexcitation photoemission
Fig. 2.2 Imaginary part of the dielectric function for Pt and GaAs (based on data from Ref. 62)
Fig. 2.3 Electron inelastic mean free path for Al and GaAs (taken from Ref. 67, 68) and their

lattice constants (taken from Ref. 61)


For illustration the imaginary parts of the dielectric function62 for two relevant solid states, Pt

and GaAs, are shown in Fig. 2.2. Only the photons with an energy exceeding approx. 5.5 eV

cause photoemission from the Pt and GaAs surfaces. Described as a three-step process65, the

photoemission consists of excitation, transport and escape of electrons from the solid state.

During transport to the surface the final kinetic energy is reduced due to electron-electron

(e-e) scattering and electron-phonon (e-p) scattering. As consequence of the inelastic

scattering processes the collision mean free path of the electrons is drastically reduced as seen

in Fig. 2.3. Electrons with a kinetic energy of about 50 eV typically possess an inelastic mean

free path of less than 1 nm66-68. Correspondingly, techniques probing electrons with kinetic

energies around 50 eV are particularly surface sensitive, because 86% of all electrons are

originating from the first and second atomic layer of the solid state. A characteristic feature of

the low kinetic energy electron spectra is a huge secondary electron background as result of

the inelastic scattering processes.

EF

Evac

Ebin MPE

φ−= ωhmEkin

φωh

o

p

s

o

m

s

e

Fig. 2.4 Multiphoton photoemission processes

As already noted, the photons with energies less than the work function for the metals

r sum of the electron affinity and the band gap energy for the semiconductors can not

romote electrons into vacuum. The situation changes when a huge amount of photons in a

hort time participate in the photoexcitation processes and nonlinear processes can be

bserved. Typical many-photon processes are summarized in Fig. 2.4. Let us speak about

ultiphoton emission69 (MPE) when an absorption of an appropriate number of photons

timulates the electron transition over the vacuum barrier. At even higher light intensities an

lectron that is already free due to MPE undertakes additional photon stimulated transitions in


continuum resulting in its final kinetic energy is , where is the number of the

absorbed photons

φω −hm m70 (see Fig. 2.4).

2.1.2 Relaxation Mechanisms in Metals

In general the relaxation mechanism describes the transition of the excited system back to

the equilibrium. Such mechanisms are usually characterized with the energy and the phase

relaxation times71-73.

ω10h

1

0

S

r

b

w

r

Fig. 2.5 Scheme of a two level system coupled to the heat sink

uppose we have brought the simple two-niveau system (Fig. 2.5) in the excited state with the

esonant light pulse. The relaxation induced via a dissipative system (heat sink) is described

y the following equations in the density matrix formalism73

)(1)()( 102

101010 tT

titdtd ρρωρ −−= (2.5)

))((1)( )(1111

111

etT

tdtd ρρρ −−= (2.6)

)()( *1001 tt ρρ = (2.7)

)(1)( 1100 tt ρρ −= (2.8)

here denotes the thermal equilibrium and T , are the energy and the phase

elaxation times, respectively. The elements of the density matrix can be calculated as

)(11

eρ 1 2T

ijρ


jiij ρρ ˆ= i (2.9) 1,0, =j

where is the density operator. The energy relaxation time Tρ̂

11ρ

1 characterizes the occupation

decay of the upper level of the two-niveau system )(t

)exp(~)(1

11 Ttt −ρ (2.10)

The phase relaxation time T describes the decay of the induced polarization in the system

with the excitation light pulse

2 P

)exp(~~2

10010110 TtppP −+ ρρ (2.11)

where and are the matrix elements of the atomic dipole operator. The energy

relaxation time as well as the phase relaxation time are experimentally observable.

10p 01p

0,1 1 10 100 1000

0,1 1 10 100 1000

Time scale (fs)

Dynamicalscreening

Electronic decoherence

One optical cycle of400 nm light (1.33 fs)

e-p Scattering

e-e Scattering

Surface state quenching

Fig. 2.6 Chart of the time response of metals to light excitation (taken from Ref. 72)

Most fundamental processes governing the excitation and relaxation processes in metals are

summarized in Fig. 2.6. Dynamical screening describes the primary response of the valence

electrons to the excitation light pulse. A macroscopic effect manifesting the dynamical


screening of electrons is the polarization of metals. Therefore all fundamental effects such as

light reflection but also non-linear effects like second harmonic generation depending on the

polarization of metals evolve on the attosecond time scale72,74. Loss of the collective

excitation coherence in the absence of the driving excitation light pulse is referred to as

electronic decoherence72. The phase relaxation time T in the equations (2.5)-(2.8) is

describing the loss of coherence in the observed system. Macroscopically, it can be observed

as an exponential decay of the light-induced polarization of metals. In the last decade the

interferometric time-resolved two-photon photoemission technique made direct observation of

the electronic decoherence possible

2

76.

The reason for the electronic decoherence are mutual interactions of electrons such as

the impurity and e-e scattering72,77. In the case of electron scattering by charge impurity the

electrons interact with charge impurities through the Coulomb potential77. This process is

elastic, that is the electron energy is conserved and only the direction of the electron

momentum is changed. The electrons with lower kinetic energy are deflected more than

electrons with higher kinetic energy by charged impurities. One of the most effective

processes leading to decoherence is the e-e scattering72. To clarify the e-e scattering let us use

the concept of hot electrons, which are electrons excited above the Fermi level. A hot electron

with momentum interacts with one electron below the Fermi level with the momentum

through the Coulomb interaction to produce two hot electrons with energies less than the

energy of the primary hot electron and momentum and . The electron energies and

momenta are given by

1k 2k

1k ′ 2k ′

2121 kkkk ′+′=+ (2.12)

)()()()( 2121 kEkEkEkE ′+′=+ (2.13)

The characteristic e-e scattering time for hot electrons with the initial energy difference

to the Fermi level derived by Fermi liquid theory (FLT) is

ee−τ

FEE − 72

2

0

−

=−F

Fee EE

Eττ (2.14)

where is given exclusively by the density of the electron gas0τ 72. Direct measurement of the

e-e scattering times / hot electron life times has been performed with the time-resolved two-


photon photoemission technique for series of noble and transition metals78-89 but also with

traditional optical linear and non-linear time-resolved techniques94-97. Typical measured hot

electrons relaxation times89 for noble metals such as Ag and transition metals as Ni are shown

in Fig. 2.7.

r

r

t

c

d

e

t

Fig. 2.7 Measured hot electron relaxation times for Ag and Ni metals (taken from Ref. 89)

A very important electron energy loss mechanism in the femtosecond and picosecond

egion is the e-p scattering71,72,77. Populating the phonon modes in the e-p scattering leads to a

ise of the lattice temperature. Typical energies of the phonons are a few tens of meV and

herefore a lot of electron-phonon interactions are needed to lower the hot electron energy

onsiderably. Phonons are acting as a heat sink for the thermalized hot electrons. The

ynamics of the electrons and phonons is described by the following coupled differential

quations in terms of the electron gas temperature T and phonon gas (lattice)

emperatures T

e

l98-100

)()()( tGTTgTTt

C leerree +−−∇⋅∇=∂∂ κ (2.15)

)( lell TTgTt

C −=∂∂ (2.16)


where and C are the heat capacities of the electrons and lattice, is the optical

generation term and is the electron-phonon coupling constant. The hot electron diffusion

term with the electronic heat conductivity is dominant in the first few picoseconds and

therefore the term describing phonon diffusion has been neglected. To illustrate the “cooling”

of hot electrons after initial pulsed photoexcitation of the platinum surface with an intensity of

32 GW/cm

eC l )(tG

g

κ

2 the electron and lattice (phonon) temperatures are shown in Fig. 2.8. Rapid

cooling of the thermalized hot electrons can be observed in the first few picoseconds.

I = 32 GW/cm2

t

a

h

t

p

w

A

e

Fig. 2.8 Cooling of the electron gas via e-p scattering after photoexcitation with laser pulse on

the Pt surface (taken from Ref. 100).

All time-resolved techniques probing surface dynamics are measuring electron life

imes on surfaces, therefore surface-states85,86 which are real states localized on the surface

re playing a significant role in the hot electron relaxation. There is possibility of transfer of

ot electrons to the surface-states and so to prolong electron life times. Also an electron

ransfer to empty states of adsorbates has been observed and measured72,85,91-92. Auger

rocesses leading to energy relaxation of hot-electrons in metals have been recognized as

ell72.

A very important aspect of the electron relaxation in metals are transport effects101,103.

lmost every technique used to measure hot electron life times is surface sensitive, therefore

lectron transport into the bulk is affecting measured relaxation times104. Summarized


electron relaxation processes together with the electron diffusion into the bulk are shown in

Fig. 2.9. Pulsed photoexcitation promotes electrons into the unoccupied states above the

Fermi level and produces a hot-electron population. At the same time the electrons are

ejecting into the bulk and leaving the surface region with the velocity ~ 1 nm/fs (Fig. 2.9a).

hν

EF E

DOS

laser

ballistic electronmotion

v ~ 1 nm/fs

surface

znonequilibrium electrons

kTe

EF E

DOS

v ~ 10 nm/ps

z hot electrondiffusionelectrons in thermal

equilibrium

kTl

EF E

DOS

v ~ 100 m/ sµ µz thermal diffusion

electrons and latticein thermal equilibrium

a) t = 0

b) t = T > T

τe-e

e l

c) t = T = T

τe-p

e l

electrondensity

electrondensity

electrondensity

I

v

l

r

t

o

t

Fig. 2.9 Relaxation and transport of hot-electrons in metals (taken from Ref. 103)

n a short phase of few tens of femtoseconds after photoexcitation the hot electrons thermalize

ia e-e scattering and are characterized with the electron temperature T . During this time the

attice temperate T is much smaller than the electron temperature T , while the surface

egion is depleting of electrons with the velocity of 10 nm/ps (Fig. 2.9b). The third phase of

he electron relaxation is characterized through the e-p scattering when “lattice heating”

ccurs due to thermalization of the electrons and phonons (T ). Typical electron

ransport velocities are about 100 µm/µs which is conventional thermal diffusion.

e

T→

l e

le


2.1.3 Relaxation Mechanisms in Semiconductors

There are many similarities of relaxation processes in semiconductors with the

processes in metals. The main differences have their origin in the band structure of the

semiconductors and the correspondingly smaller density of free electrons. An overview of

typical relaxation processes and their characteristic times is given in Fig. 2.10.

1 fs 1 ps 1 ns 1 sµ10 fs 100 fs 10 ps 100 ps 10 ns 100 ns

1 fs 1 ps 1 ns 1 sµ10 fs 100 fs 10 ps 100 ps 10 ns 100 ns

carrier-carrier scattering (1)

intervalley scattering (3)

intravalley scattering (2)

Auger recombination (4)

carrier diffusion

radiativerecombination (5)

defect recombination (6)

VBM

CBM

hν 54

4

2

1

3

6defect state

initial electronpopulation distribution

Fig. 2.10 Electron relaxation processes in semiconductors (taken from Ref. 105,106)

The coherent regime initiated with the photoexciting pulse is similar to the one in metals and

is described with the coherence dephasing time (see eq. (2.11)).


(arb

. un.

)

R

f

d

t

c

e

s

I

v

d

w

r

r

t

e

a

Fig. 2.11 Time-resolved change of the GaAs transmittance for 1017 cm-3 photoexcited

carrier densities (adopted from Ref. 107)

apid loss of coherence has the same reason as in metals and is primarily connected with the

ast e-e scattering71. The e-e scattering is broadening the initial electron population

istribution to the hot thermalized distribution (Fig. 2.10-1). At this stage the electron gas

emperature is different from the phonon gas (lattice) temperature. The following phase is

haracterized through the e-p scattering108. Phonon scattering processes are decreasing the

lectron gas temperature and increasing the lattice temperature. In semiconductors the e-p

cattering is divided into two main groups – intervalley scattering and intravalley scattering.

n intravalley scattering the electron - after interacting with phonon - stays in the same

alence band valley (Fig. 2.10-2). The intervalley scattering transfers the electron to a

ifferent valley (Fig. 2.10-3). So electrons are scattering to the bottom of the conduction band

here much slower processes occur, such as radiative recombination (Fig. 2.10-5), Auger

ecombination (Fig. 2.10-4), defect recombination (Fig. 2.10-6) in the bulk or the

ecombination via surface states. To illustrate the discussed processes Fig. 2.11 shows the

ime-resolved differential transmission measurement of a 0.5 µm GaAs film for 1017 cm-3

xcited carrier density107. A fast decrease of the transmittance in the first few femtoseconds is

ttributed to e-e scattering whereas a slower decay is pointing to the e-p scattering.


Time-resolved studies based on two-photon photoemission has been applied to measure the

electron dynamics on semiconductors surfaces as well109-111.

The process of carrier diffusion is much more complex for semiconductors as

compared to metals in the surface vicinity. The charge localized in the surface states (SS) is

compensated via bulk electrons. For metals with a high density of electrons in the valence

band this compensation is resulting in a dipole layer of only a few angstroms near the

surface131. Semiconductors with lower electron density in the valence band are building a

space-charge region (SCR) which can extend few tens of nanometers into the bulk. Within the

SCR the surface charge is compensated, leading to a band-bending in the surface

vicinity132-138. To shed more light on the phenomenon of band-bending let us suppose a n-type

semiconductor with acceptor-type surface states. Electrons will be transferred to the empty

surface states which results in a negative surface charge Q . To achieve equilibrium we need

the same amount of positive space charge under the surface to satisfy the net charge

neutrality

SS

SCQ136

0=+ SSSC QQ (2.17)

As a result, the depleted zone of electrons – SCR will be formed. All potentials in the SCR

have to fulfill Poisson’s equation136 for the new charge redistribution which results in an

upward band-bending towards smaller effective binding energy. For p-type semiconductors

with the donor type of surface states the situation will reverse whereby the surface charge

is positive and the space charge is negative. In this case all bands bend downwards. In a

first approximation the SCR width , the doping density and the band-bending Y at the

surface are coupled as follows

SSQ

SCQ

w N 0

136,139

eNYw 02ε

= (2.18)

where is the dielectric constant and e is the elementary charge. For example the p-GaAs

with =10

ε

N 19 cm-3 and Y =0.7 V has the SCR length of 10 nm. The potential change of 0.7 V

over the range of 10 nm is responsible for very high electric fields of few hundreds of kV/cm

in the SCR

0

139. Additional free electrons and holes injected by light absorption can change the

band-bending. This phenomenon is called surface photovoltage (SPV) effect132-138.


surfacerecomb.

defect(SRH)

Y0

-

SS

Vacuum p-GaAs

+

CBM

VBM

SCR

-

Auger

-

+ +

-

+

-

+

--

+

++

- -

radiative

Y

Ebin

EF

z

E

Set

m

c

T

e

a

t

d

R

r

Fig. 2.12 Scheme of the band-bending for p-GaAs

The dynamics of the photogenerated carriers - electrons and holes - neglecting the

entioned relaxation processes such as e-e and e-p scattering are described by the following

oupled differential equations139-143

[ nnn RtzntzEzt

tznDtzGt

tzn−

∂∂

+∂

∂+=

∂∂ ),(),(),(),(),(

2

2

µ ] (2.19)

[ ppp RtzptzEzt

tzpDtzGt

tzp−

∂∂

−∂

∂+=

∂∂ ),(),(),(),(),(

2

2

µ ] (2.20)

[ )()(),(),(),(),(2

2

zNzNtzntzpez

tzEz

tzVad −+−=

∂∂

=∂

∂ε

] (2.21)

he time-resolved band-bending Y is given by the time-depended densities of

lectrons and holes as well as by the densities of ionized donors and

cceptors . The dynamics of the electron density and hole density determine

he continuity equation where G is the optical generation term, and are the

iffusion coefficients for electrons and holes, and are the electron and hole mobility.

ecombination terms and are describing recombination processes

),0( tzV ==

)t

),( tz

nµ

),( tzn

aN

,(zp

pR

dN

),( tzn

pµ

),( tzp

DnD p

nR 143 such as radiative

ecombination139, defect – Shockley-Read-Hall recombination144 and Auger recombination143.


Surface recombination dynamics is given by two boundary conditions for electrons and

holes143

),0())0((

0

tznD

zESSzn

e

ntnn

z

==−+

=∂∂

=

µ (1.22)

),0())0((

0

tzpD

zESSzp

p

ptpp

z

==−+

=∂∂

=

µ (1.23)

where and are the surface recombination velocities and and are the carrier

transfer velocities characterizing processes like tunneling or thermionic emission. Equations

(2.19)-(2.23) have no analytic solution

nS pS

0

tnS t

pS

143. To illustrate the competing processes evolving after

photoexcitation of a semiconductor surface let us suppose a p-type GaAs semiconductor

surface (Fig. 2.12). Pulsed photoexcitation with a photon energy larger than the band gap

produces electron-hole pairs. The presence of the high surface electric field separates the

photogenerated electrons and holes, whereby the electrons move to the surface and the holes

move further into the bulk. The change of the carrier density in the SCR flattens the original

band-bending Y to a new value Y . The electron mobility saturates139,140 for electric fields of

some 100 kV/cm, for even higher fields the drift velocity therefore does not exceed 50 nm/ps.

In this simple estimation the electron needs about 200 fs for transit through the SCR of 10 nm

width145.

Fig. 2.13 Spatio-temporal evolution of the space charge field (taken from Ref. 169)


p

l

f

e

i

f

g

a

t

p

s

h

m

r

s

r

m

a

p

t

Fig. 2.14 Normalized time-resolved THz pulse wave and its spectrum (taken from Ref. 167)

The result of a detailed numerical study169 of the space charge field after delta-like

ulsed photoexcitation of n-GaAs for the low carrier excitation density of 4x1016 cm-3 and the

ow doping concentration of 1015 cm-3 is shown in Fig. 2.13. The decrease in the space charge

ield accompanied with an oscillatory behavior is pointing to a complex band-bending time

volution already at relatively small excitation intensities. Fast electron transfer in the SCR

mmediately after photoexcitation leads to radiative emission of the broad-band femtosecond

ar-infrared (FIR) THz pulses170-179. The FIR field strength due to transient charge transfer is

iven directly as the second derivative of the space charge field167. Measured FIR-THz pulse

t different excitation densities of 1x1015, 1x1016 and 1x1017 cm-3 in p-GaAs (4x1014 cm-3)

ogether with their spectra are shown in Fig. 2.14. Femtosecond optically generated THz

ulses in the SCR of semiconductors have found widespread application in THz time-resolved

tudies180-183.

Once the electrons and holes have been separated and a new value of the band-bending

as been achieved the relaxation processes take place. Besides the bulk recombination

echanisms like radiative, defect or Auger recombination, the surface offers new

ecombination possibilities like the surface recombination via surface states. The surface

tates act as recombination centers in the same manner as defect states144 in the bulk. Surface

ecombination146 can be relatively fast on the picosecond time scale147-153. Direct

easurements of the SPV dynamics on nanosecond and coarse picosecond time scales have

lready been published154-164. These were based on an excitation laser synchronized with the

robing soft x-ray storage ring pulses. In spite of these advances the fast SPV transients on

he p-GaAs(100) surface have not been resolved as shown in Fig. 2.15. The SPV effect of the


anisotropic materials such as GaAs has been also measured with the pure optical

time-resolved methods142,166-168 making use of the electrooptic effect165 on the surface.

Fig. 2.15 SPV shift of the Ga-3d photoelectrons in p-GaAs at 125 K as a function of the delay

time between soft x-ray synchrotron pulse and excitation laser pulse (taken from Ref. 161)


2.2 Pump-Probe Technique

2.2.1 Principle of Pump-Probe Technique

One can observe ultrafast dynamics applying the so-called pump-probe

techniques73,184. The general principle common to all types of pump-probe techniques is

schematically shown in Fig. 2.16.

timeτ - delay

pumppulse

probepulse

system inequilibrium

relaxation detection ofphotons or

electron

excitationof the system

T

p

c

r

g

s

p

f

t

t

Fig. 2.16 General scheme of pump-probe techniques

he system initially in the equilibrium is optically excited into unoccupied states with the

ump pulse. Once excited, the system relaxes back to the equilibrium states with the

haracteristic relaxation times (see section 2.1.2). A probe pulse with variable time-delay with

espect to the pump pulse “maps” the changes in the relaxing system. Photons or electrons

ated (=photoemitted, photodiffracted, up-converted etc.) by the probe pulse reflect the actual

tate of the system at the time of the probe pulse arrival. Scanning the time delay between

ump and probe pulses one can learn about the relaxation dynamics of the system.

τ

Let us assume that relaxation of the system back to the equilibrium is described by the

unction after the excitation with the delta-like pump pulse . The actual response of

he system is described by the following convolution

)(tf

g

)(tδ

)(t 73,165 with the pump pulse’s

emporal intensity profile )(thpu


∫+∞

∞−

−⋅=×= ξξξ dthfthtftg pupu )()()()()( (2.24)

or Fourier transformed

)()()( ωωω puHFG = (2.25)

The measured signal gated by the time delayed probe pulse with the temporal intensity

profile can be written as convolution in the form

)(tm

)(thpr

∫+∞

∞−

−⋅=×= ξξξ dthgthtgtm prpr )()()()()( (2.26)

or Fourier-transformed

)()()()( ωωωω prpu HHFM = (2.27)

Now we can distinguish two limiting cases which simplify the derived equation (2.27). Let us

assume that pump and probe intensity pulse profiles are delta-like functions. With this

simplification the measured signal equals the function . It means that we measure

exclusively the relaxation of the investigated system without any influence of pumping and

probing pulses. Another limiting case is when the relaxation function is a delta-like

function. The measured signal m now equals a cross-correlation of pump and probe pulses.

This case gives an opportunity to measure the linear cross-correlation signal when the pump

duration is considerably larger than the relaxation time of the system.

)(tm )(tf

)(tf

)(t

In pump-probe techniques one measures the photons or the electrons directly gated

with the time delayed probe pulse under the assumption that the intensity of the probe pulse is

smaller than that of the pump pulse. This means, that the influence of the probe pulse on the

relaxation dynamics will be negligible. In practice, the following observables can be detected:

- absorption, transmission, reflection, diffraction, polarization of the “probe” photons

- nonlinear effects like second harmonic generation (SHG) of the “probe” photons

- fluorescence photons gated with the “probe” photons

- photocurrent due to photoexcited electrons by the “probe” photons


- photoelectrons emitted by the “probe” photons

Another difference between various pump-probe techniques is the propagation of pump and

probe pulses. There are two principal possibilities of the mutual pump and probe propagation

- collinear and non-collinear. Finally, time-resolved techniques can be phase sensitive or

intensity sensitive. The phase sensitive time-resolved techniques are pushing the

time-resolution under the duration of the pulse optical period cycle (400 nm = 1.33 fs)76.

A special class of the time-resolved techniques is the time-resolved photoelectron

spectroscopy (TR-PES). This technique combines the classical photoelectron spectroscopy

with the possibility to directly observe the transient changes of the electronic structure after

photoexcitation. Photoexcitation of the valence electrons results in a transient population of

normally unoccupied orbitals of molecules or bands of the solid state. The TR-PES techniques

specializing on these transient electron populations utilize probe photons in the visible or

ultraviolet region. A typical representative of this TR-PES class is the time-resolved two-

photon photoemission (2PPE) technique72. Its principle is based on the following scheme for

metals or semiconductors. The pump pulse excites the valence electrons into the unoccupied

states above the Fermi level. After photoexcitation the probe pulse causes photoemission from

the transiently occupied states. This technique provides one immediately with the relaxation

times of the transiently occupied states. It has been developed for metals78-89,

semiconductors109, clusters112 and molecules113. Exciting electrons into the unoccupied states

can result in the photodissociation or photoisomerization of molecules, charge transport on

the surfaces in semiconductors, desorption of adsorbates on metal surfaces and so forth. All

mentioned processes can be detected as chemical shifts304 in the core-levels or changes in the

valence band structures. Consequently, the probe photon energy must be in the vacuum

ultraviolet (VUV) or EUV region for the valence band and core-level studies, respectively.

The latter condition requires femtosecond pulses at least in the EUV region. Pioneering work

in the field of EUV TR-PES was performed by Haight et al. who studied photoexcited

semiconductor surfaces with few hundreds of femtoseconds resolution114-130. Recently, this

techniques has also been applied to the adsorbate dynamics on a Pt surface34, the molecular

dissociation of Br236,37, and hot-electron dynamics in quartz35.


2.2.2 Generation, Amplification and Application of Femtosecond Light

Pulses

A light pulse of the femtosecond duration is a superposition of large number of the

planar waves with different wavelengths but well defined phase conditions. Such pulse can be

conveniently described in the following form73

ωωπ

ωωφ deeEtE tii ⋅= ∫+∞

∞−

)()(21)( (2.28)

where is the amplitude and is the phase of the complex spectral density

E(ω) . Generally the phase can be written as

)(ωE

)(ω ieE

)(ωφ

)(ωφ)(ωφ= 185

...)()(61)()(

21)()()()( 3

002

00000 +−⋅′′′+−⋅′′+−⋅′+= ωωωφωωωφωωωφωφωφ (2.29)

where )( 0ωφ ′′ is called group velocity dispersion (GVD) term, )( 0ωφ ′′′

φ

is called third order

dispersion (TOD) term and so on. A pulse with a linear phase is a so-called

Fourier-limited pulse and is the shortest achievable pulse for a given bandwidth . The

action of a linear optical system on femtosecond light pulses is given by the convolution of

the impulse response function of the linear optical system

)(ω

)(ωE

)(ts 73,75,165 and the femtosecond

pulse and after Fourier transformation is simply given in the form )(tE

Eout(ω) E)()( ωϕω ieS= in(ω) (2.30)

where Ein(ω) is the complex spectral density of the in-going pulses and Eout(ω) is the complex

spectral density of the transformed pulses leaving the system. The linear system changes the

amplitude of the complex spectral density by the S(ω) and introduces the phase-shift ϕ(ω).

Let us suppose that the linear system acts solely in the phase domain. In this case .

As result of such transformation the Fourier-limited pulse after the propagation through the

linear system receives , and higher orders of the linear system

phase components. In other words, the pulse will stretch in time via accumulated phase-shifts

1)( =ωS

)(GVD 0ωϕ ′′≡ )(TOD 0ωϕ ′′′≡


while propagating through the linear system. This is illustrated in Fig. 2.17 for the linear

system with . 1)( =ωS

E Eout in= ( )eS ω i ( )ϕ ω

linear systemω

∆ωin

t

∆τin

ω

∆ω =∆ωout in

t

∆τ ≠∆τout in

A

c

b

C

p

g

s

g

c

o

m

a

T

e

p

o

c

b

r

d

o

Fig. 2.17 Femtosecond pulse transformation through the linear system

Fourier-limited pulse stretched by the linear system with a positive (negative) GVD term is

alled up-chirped (down-chirped)73. The GVD term is responsible for a symmetrical

roadening of the pulses whereas the TOD term causes an asymmetrical broadening.

ontributions of higher terms than TOD are important only for few-cycles optical

ulses21,23,186.

Generation of femtosecond pulses requires a laser medium with a broadband spectral

ain like the Ti-doped sapphire crystal. This medium exhibits a sufficiently large emission

pectrum to support pulses even less then 10 fs duration20.The principle of femtosecond pulse

eneration is based on the locking of the phase relationship between different laser modes in a

avity which is random under normal circumstances. To achieve a fixed phase relationship

ne can use active or passive mode locking techniques73. In most cases the passive Kerr-lens

ode locking (KLM) technique23,187-190 is used to generate the femtosecond light pulses. An

rtificially induced perturbation of the laser cavity leads to sudden high intensity fluctuations.

heir propagation in the specially designed laser cavity is more favourable due to the Kerr

ffect and the laser starts pulsed operation. Further amplification of the femtosecond laser

ulses without any additive arrangements would lead to a destruction of the laser media and

ptics due to the very high intensity leading to self-focusing and other nonlinear effects. The

hirped pulse amplification14,191-207 (CPA) technique tackles this problem of material damage

y stretching – “up-chirping – of the femtosecond pulses before amplification and

ecompressing – “down-chirping” – of the amplified pulses back to the initial short pulse

uration. In detail, the femtosecond pulses from the master oscillator are stretched in an

ptical device with very high positive GVD – the stretcher. Such long pulses are routinely


amplified in two types of optical amplifiers – regenerative and multipass amplifiers.

Regenerative amplifiers consist of an optical cavity with an optical relay being able to confine

and release optical pulses within the cavity. In this way one can regulate the number of passes

through the amplifying medium. A disadvantage of the regenerative amplifiers is the

relatively large GVD and higher dispersion terms leading to an additional hard controllable

stretching of the pulses208. On the other hand, the multipass amplifiers have very low

dispersion but are not flexible in the choice of the number of passes through the amplifying

medium which are realized as “optical-bench-fixed” paths multiplexing at small mutual

angles in the amplifying medium. After amplification the pulses are recompressed back to

almost the original duration by a device with large negative GVD – the compressor.

The high intensities of femtosecond light pulses are predestining them for the

stimulation of nonlinear optical effects22,58,59. Classical nonlinear optics is based on the

perturbation theory22. At low intensities (< 1013 W/cm2) nonlinear phenonena23 have been

successfully described with the Maxwell equations and a medium polarisation ansatz73

nn EEEEEEP )(

03)3(

02)2(

0)1(

00 ...)( χεχεχεχεχε ++++== (2.31)

where are known as the nonlinear optical susceptibilities of n)(nχ th order. A representative

class of the second order nonlinear effects like second harmonic generation, sum frequency

mixing, optical rectification, optical parametrical generation are governed by the

susceptibility. Third harmonic generation, self-focusing, two-photon absorption, self-phase

modulation are modelled on the susceptibility. At higher light intensities (> 10

)2(χ

)3(χ 13 W/cm2)

the light electric field is not merely a small perturbation of the atomic Coulomb potential. To

describe the nonlinear effects accompanying such high light intensities one has to perform an

ab initio quantum mechanical calculation of the system atom and high intensity light field.

The key-process powering all experiments throughout this thesis is the high harmonic

generation, theoretically209-224 and experimentally225-262 explored over one decade now. In this

process the high harmonic photons in the VUV, EUV and soft X-ray region are produced by

the interaction of the intense laser field (>1014 W/cm2) with an atomic gas. The classical

interpretation of the high harmonic generation is based on a three-step model209: In the first

step, an electron tunnels through the Coulomb barrier suppressed by the intense laser field.

Once free, the electron is oscillating and gaining energy from the laser field and under certain


circumstances recombines with the mother ion. The electron recombination process209 attends

the photon emission with the energies

ωh⋅= nEhh (2.32)

where is the laser field frequency and is odd integer number. To estimate the

maximum extractable photon energy we can use the classical trajectory

T/2πω = n

E

209-211 ) of the

free electron born at the time t in the laser field E

,( 0ttx

0 )sin( tω=

)cos()())sin()(sin(),( 00000

00 tvttttvxttx ωωωω

−+−+= (2.33)

The kinetic energy of the oscillating electron in the laser field is then given simply as

[ ])(cos)cos()cos(2)(cos2),( 02

02

0 ttttUttE Pkin ωωωω +−= (2.34)

Electron RecombinationPath

Electron kinetic energyin U unitsP

no recombinationpossible

no recombinationpossible

Fig. 2.18 Kinetic energy (contour plot) and recombination times (shown as white paths) of the

free electron born at the time t0 in the laser field with period T.


where ωmeEv =0 and 222 4 ωmEep =

T64.0

U is the ponderomotive energy of the electron in the

laser field. To estimate the maximum energy of the emitted high harmonic photons we have to

know the actual kinetic energy of the electron colliding with the mother ion. The contour plot

in Fig. 2.18 shows the electron’s kinetic energy given by equation (2.34) in the

oscillating laser field at arbitrary normalized time t born in the laser field E at the

normalized time . Solutions of the equation (2.33) for shown in Fig. 2.18 as

white “recombination paths” constitute the only possible recombination times for electron

with mother ion in the oscillating laser field. The graphical solution (Fig. 2.18) of the electron

recombination process makes clear that only electrons born in the second and fourth quarter

of the laser field period will return to the mother ion and can radiative recombine. The

electrons born in the first and third quarter of the laser field period will never return to the

mother ion again and therefore do not contribute to the high harmonic generation (shown by

hatched area in Fig. 2.18). The electrons born in the laser field at the times

will return to the mother ion period later with the maximum possible kinetic energy

obtained in the laser field (shown with the arrow in Fig. 2.18). Radiative

recombination for this case constitutes the cutoff energy for the high harmonic photons

defined as

),( 0ttEkin

T/

Tt /0 0),( 0 =ttx

}8.0,3.0{0 TTt =

PU17.3

209,212

PPhh UIE 17.3max += (2.35)

where is the ionization potential of the atomic gas. In the case of a few-cycles light pulses

the possibility of a single attosecond pulse generation near the cutoff photon energy has been

proposed

PI

23 and recently experimentally confirmed1.

34 Chapter 3 – Experimental Setup

Chapter 3

Experimental Setup

3.1 Descriptions of Experimental Setup

The constructed experimental apparatus for photoelectron spectroscopy in the EUV

region consists of four main parts (Fig. 3.1) :

• femtosecond high-power laser system (0.2 TW)45

• conversion chamber for the high harmonic generation45

• EUV multilayer monochromator with low GVD46

• UHV analysis chamber

Chapter 3 – Experimental Setup 35

high harmonicgeneration

E=15mJ@50Hz=800nm

t=70fsλ

Ne

delaystage

MCP+phosphor

TOF

Xe, HePt(110)

UHV analysischamber

LEEDQMAAES

CCDcamera

330 l/s

500 l/s

210 l/s

1000 l/s

ML (r=1m)

f=50 cm

XUVphotodiode+ Al-filter

ML

MSP+quad-photocathode

Fig. 3.1 Layout of the built experimental apparatus


In the framework of this theses a femtosecond laser system based on the CPA technique

has been built45. A scheme of the system is shown in Fig. 3.2., a detailed true-to-scale

technical drawing is depicted in Fig. 3.4.

masteroscillator stretcher regenerative

amplifier

multipassamplifier compressor

35 fs5 nJ

220 ps3 nJ

220 ps1 mJ

220 ps20 mJ

70 fs15 mJ

T

f

a

2

l

s

a

s

w

s

t

w

m

s

r

F

D

e

w

m

p

m

Fig. 3.2 Scheme of the 0.2 TW femtosecond laser system

he master-oscillator – a KLM Ti:Sapphire femtosecond oscillator190 pumped with 3.6 W

rom a diode-pumped frequency-doubled solid-state laser (Spectra-Physics Millenia) - serves

s a seed laser with p-polarized pulses of 5 nJ energy at a rate of 77 MHz , a bandwidth of

0 nm (FWHM) at a center wavelength of 800 nm, and a pulse duration of 35 fs (Fourier-

imited pulse). The pulse spectrum is monitored with a compact Czerny-Turner type

pectrometer45. The pulse duration can be measured with a scanning second harmonic

utocorrelator45. Complete characterization of the oscillator pulses can be performed with the

ingle-shot second harmonic frequency-resolved optical gating (SHG-FROG) technique263,

hich is presently in the test stage. Operation in the mode-locking regime is started with a

light reversible disalignment of the laser cavity. This can be observed as a sudden change in

he spectrum of the laser pulses. The single line of the continuous mode spectrum is replaced

ith a broadband spectrum of nearly gaussian distribution being a signature of the pulsed

ode. Once in the pulsed regime, the oscillator runs stable over more than 12 hours with a

light shift of the spectral maximum of the order of 1-2 nm. Maintenance of the oscillator

ests upon a small adjustment of the oscillator end-mirror approximately every two months.

emtosecond pulses from the master oscillator enter the pulse stretcher before amplification.

ue to high positive GVD the femtosecond pulse duration after passing the stretcher is

xtended to approximately 220 ps at an energy of 3 nJ. The all-reflective pulse stretcher

orking in the Littrow configuration265 has been designed with particular attention to

inimize space requirements. Changing the angle of the stretcher grating also

re-compensates TOD of the following amplification stages. The pulse stretcher is

aintenance-free and only precise alignment of the laser beam into the stretcher is required.


The chirped pulses leaving the pulse stretcher are propagating through the polarizer, quartz

rotator and Faraday rotator so that the polarization is changed from p to s-polarization.

12.9 ns

pre-pulse : pulse = 1 : 85

T

w

l

r

i

l

a

r

e

t

t

c

e

c

P

s

Fig. 3.3 Main pulse and pre-pulses of the femtosecond CPA laser system

he regenerative amplifier cavity provides high quality only for p-polarized light pulses

hereas s-polarized light pulses are rejected. The Pockels-cell - synchronized with the pump

aser and master oscillator - changes the polarization from s to p for a single laser pulse in the

egenerative cavity. The confined laser pulse propagates in the cavity and becomes amplified

n the Ti:Sapphire crystal pumped with 27 mJ from a frequency-doubled pulsed Nd:YAG

aser (λ=532 nm, Spectra Physics LAB-150-50) at 50 Hz repetition rate. After the pulse

mplification saturates, the pulse propagates additional two or three round-trips in the

egenerative cavity before the Pockels-cell changes its polarization back to s and the pulse is

jected from the regenerative cavity. The additional round-trips after the saturation are aimed

o stabilize pulse-to-pulse energy fluctuations. The amplified pulse propagates back through

he Faraday and quartz rotator but in this propagation direction the pulse polarization is not

hanged and the polarizer redirects the amplified pulse to the multipass amplifier with a pulse

nergy of about 1.7 mJ. The maintenance of the regenerative amplifier consists mainly of the

ompensation of a small day-to-day “beam-walk” of the pump laser and tilting of the

ockels-cell. The pre-amplified pulse from the regenerative amplifier propagates in five

uccessive passes through the Ti:Sapphire crystal pumped with 100 mJ from the Nd:YAG


laser. The multipass amplifier boosts the pulse energy up to 20 mJ. The cavity-free design of

the multipass amplifier causes its high sensitivity to an optical misalignment. As a result, a

complete new alignment of all five passes is necessary once every three months. The last

component of the CPA system is an all-reflective pulse compressor working in Littrow

configuration with an easily adjustable net negative GVD. After propagation through the

pulse compressor the laser pulse has an energy of 15 mJ and a pulse duration about of 70 fs.

The less then 100% efficiency of the polarizer in the regenerative cavity gives rise to a

leakage of pre-pulses during the amplification. The main pulse and its pre-pulses are shown in

Fig. 3.3. The pre-pulse/pulse contrast ratio of 1:85 is typical for normal day-to-day operation.

The pulse duration is minimized by means of maximalization of the pulse spectral broadening

by the supercontinuum generation266,267 after focusing the pulse in the air. The whole laser

system is situated in a separate air-conditioned room with laminar flow-boxes above the laser

bench guaranteeing a particle-free atmosphere.

A detailed down-scaled drawing of the setup for pump-probe experiments is shown in

Fig. 3.5. The laser beam is divided with beamsplitter into a pump and a probe path with an

energy ratio of 3:7. The probe beam is magnified two times with an off-axis reflective Galileo

telescope and focused with a lens (f = 500 mm) into the conversion chamber to an intensity of

1015 W/cm2 in front of the 0.8 mm diameter nozzle of a pulse valve (Lasertechnics LPV 300)

backed with 6 bar of Ne gas. Due to synchronized pulsed operation with an open duration of

145 µs a 330 l/s turbomolecular pump is sufficient to maintain a pressure of 4x10-4 mbar in

the conversion chamber. A 100 nm thin Al filter (Lebow company) as an entrance window to

the monochromator chamber reflects the fundamental and low harmonic pulses and acts as a

bandpass filter (T ~ 60%) for high harmonics with the energies between 15 eV and 73 eV.

The Al filter is placed on a motorized arm and can be moved in and out of the high harmonics

propagation path according to requirements. Under operating conditions the monochromator

vacuum chamber, pumped by a 500 l/s turbomolecular pump maintains a pressure of

1x10-6 mbar. A microchannel plate (MCP) intensified phosphor screen can be inserted into the

beam path at distance of 1 m from the pulsed valve for visualizing the high harmonics spatial

profile. The divergence (full angle) of all high harmonics within the Al-bandpass was

measured to be less than 10 mrad. For small fundamental laser energies the high harmonics

emission is very directional with a small divergence as can be seen in Fig. 3.6. The

conversion efficiency saturates at about 5 mJ of the fundamental laser energy.


Fig.

3.4

Tru

e-to

-sca

le te

chni

cal d

raw

ing

of th

e 0.

2 TW

CPA

fem

tose

cond

lase

r sys

tem

100

mm

mas

ter o

scill

ator

compressor

mul

tipas

s am

p.

rege

nera

tive

amp.

to e

xper

imen

t

stretcher


TOF

targ

et

300

mm

mul

tilay

erm

onoc

hrom

ator

conv

ersi

onch

ambe

rpu

mp-

prob

eop

tical

set

up

diffe

rent

ial

pum

p-st

age

UH

V-ch

ambe

r

from

CPA

lase

r sys

tem

Fig.

3.5

Tru

e-to

-sca

le te

chni

cal s

ketc

h of

the

visi

ble

/ EU

V p

ump-

prob

e ex

perim

ent


50 mm

HHG intensitymax

min

fundamental laser energy5 mJ

H

h

o

v

T

m

a

d

c

a

t

s

m

Fig. 3.6 Spatial profile of all high harmonics passing Al filter as a function of fundamental laser

energy. For the energies higher than 5 mJ the high harmonics spatial profile deteriorates.

igher fundamental laser energies above 5 mJ result in a spatial distortion of the high

armonic profile as shown in Fig. 3.6 and decreased conversion efficiency as a consequence

f an exceedingly high plasma density in the interaction volume which can be easily observed

isually as an incoherent plasma radiation.

he multilayer monochromator46 has to fulfill three basic requirements :

- select a single high harmonic order from the harmonic spectrum while preserving its

femtosecond pulse duration – the selecting element must have a small GVD

- refocus the diverging high harmonic beam into a well defined target volume and thus

providing a high light intensity

- conserve the propagation direction of the high harmonics

The first requirement has been realized with specially designed molybdenum/silicon

ultilayer mirrors43,47,268 with 30 periods of 9.9 nm thickness and a narrow bandwidth of

bout 2.9 eV (FWHM). Given the thickness of a complete multilayer structure of some

300 nm one can expect the temporal broadening of the pulses not to exceed the time

elay between reflection from the multilayer surface and the substrate, respectively

=d τ∆

2)cos(2 ≈⋅ θcd<∆τ fs where is the light velocity and is the light incident angle. The

alculations

c θ269 of the broadening based on the Fresnel-equations actually yields fs for

70 fs EUV pulse.

1<∆τ

The second and the third requirement is fulfilled with the Z-fold mirror design where

he first multilayer is deposited on a planar substrate and the second one on a concave

pherical substrate with a radius of 1 m. The wavelength selective reflection of the multilayer

irrors is ruled by the Bragg condition270,271. By changing the angle of incidence I can select


a narrow energy band and focus it to an area of ~ 1 mm2 in the UHV experiment chamber. A

variable position of both mirrors along the beam direction accomplishes a stable focus

location independent on the chosen angle of the multilayers. The currently used multilayer

mirrors are tailored for effectively selecting the high harmonics of orders 43, 45, and 47. By

changing the set of the multilayer mirrors the tuning range of the monochromator can be

extended. A EUV photodiode (XUV-100C, UDT Sensors Inc.) provides routinely control of

the photon flux available in the target which is typically 104 photons per laser shot at an

energy of 70 eV. The overall transmittance of the multilayer monochromator including Al

filter is estimated to be 5% for a single harmonic order.

The pump beam propagates through a delay stage with a translatory precision of

1.2 µm (~ 4 fs) and is then imaged onto a BBO crystal (thickness 0.3 mm) with an off-axis

reflective Galileo telescope demagnified by a factor of 2.5. The hygroscopic BBO crystal is

located in a vacuum steel chamber with Brewster-cut windows to avoid surface deterioration.

The second harmonic at 3.1 eV (λ = 400 nm) generated in the BBO crystal is focused with a

lens (f = 4 m) into the monochromator chamber where two additional mirrors are redirecting

the pump beam into the UHV analysis chamber. All mirrors after frequency doubling are

wavelength selective for 3.1 eV light energy. I have also performed experiments with a pump

photon energy of 1.55 eV (λ = 800 nm). It this case the demagnifying optics and the BBO

crystal have been removed and all the mirrors after the BBO crystal have been exchanged

according to the fundamental light energy.

The radiation of a selected high harmonic probe pulse together with a temporally

delayed pump pulse are spatially overlapped at an angle of 12 mrad onto a target in the UHV

chamber. The differential pumping stage bridging the pressure difference between the

monochromator and the experiment chamber consists of two apertures and a 210 l/s

turbomolecular pump. On demand, removing the Al filter in the monochromator and moving

a bending mirror into the pump and probe path allows an imaging of the interference pattern

of both pump and probe pulses on a CCD camera for in situ calibration of the time delay.

With this technique I can set equal optical paths for pump and probe pulses. In this case to

avoid multilayer degradation I use only unamplified femtosecond pulses directly from the

master oscillator which provides me also with a better signal statistics due to its high

repetition rate. When the second harmonic is used as pump pulse the additional BBO crystal

(thickness 0.1 mm) is introduced into the optical path of probe pulses. Correspondingly, a

blocking filter for the fundamental light is placed in front of the CCD camera and only

interference on the second harmonic is observed. After finding equal optical lengths for pump


and probe paths I have to correct this value by the optical thickness of the BBO crystal with

respect to vacuum (~ 166 fs). Finally the Al filter is moved back to its original position and

despite small readjustments required before running the pump-probe experiment the

pump-probe temporal coincidence is stable within about 100 fs for a few days.

pump-probeinterference

t = 0 fs∆

T

p

o

p

m

e

m

m

t

p

r

§

a

Fig. 3.7 Pump-probe interference pattern in case of temporal overlap and measured E-field

autocorrelation function at 1.55 eV photon energy

he observation of the interference pattern of pump and probe pulse is also informing about

ossible wave-front deformations and the influence of dispersive optical elements in the

ptical paths deteriorating the interference contrast. In Fig. 3.7 the interference of pump and

robe pulses with photon energy 1.55 eV is shown. A linear variation of the interference

aximum-to-maximum spacing is giving information on the wave-front tilt. This is to be

xpected because of the astigmatic optical setup of the multilayer mirrors§ in the

onochromator. Graph in Fig. 3.7 shows the mean value of the interference pattern

odulation for a horizontal line-scan in the middle of the interference spot as a function of

ime-delay between pump and probe pulses. Assuming two identical gaussian Fourier-limited

ump and probe pulses with an intensity profile defined as

, the acquired pump-probe coherence function

epresenting the E-field autocorrelation is given as

)/2ln4exp()()()( 220

* τtItEtEtI ⋅−=⋅= )(tΓ

272,273

The calculated normal reflectivity62 of the topmost Si layer of the multilayer mirror is 35% at 800 nm and 47%

t 400 nm light wavelength.


∫+∞

∞−

⋅−≈−⋅=Γ ))2/(2ln4exp()()()( 22* τςςς tdtEEt (3.1)

yielding the relation where (FWHM) is the measured width of the autocorrelation

function . The measured autocorrelation width of = 94 fs then yields a pulse duration

of = 47 fs, which matches the pulse duration of 45 fs determined with the SHG

autocorrelator quite well. This result ascertains that both pump and probe optical paths do not

contain optical elements with considerable dispersion. Also the astigmatism resulting from the

monochromator mirrors setup is not critical for pump-probe experiments.

ττ 2=a aτ

)(tΓ aτ

τ

HHG intensity100

0

ML

ML

43. HH

45. HH

47. HH

Fig. 3.8 Selectivity of the multilayer monochromator vs. photon energy of the high harmonics and

the angle of incidence of the multilayer mirrors The inset shows monochromator layout

The UHV experimental chamber has a base pressure of 3x10-10 mbar rising to

9x10-10 mbar when operating the Ne gas jet in the conversion chamber and is equipped with

the standard surface preparation equipment and analysis methods such as an ion bombardment

gun, low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and

quadrupole mass analyzer (QMA). As targets solid samples as well as gases can be

investigated. The position of the selected high harmonic within the chamber is controlled with

a quadrant metal photocathode intensified with a microsphere plate (MSP). In gas phase

experiments the low 50 Hz repetition rate of the laser system enables usage of the


synchronized pulsed valve providing a high local target gas density in front of the electron

spectrometer at low background pressure (10-6 mbar). Due to an unfavorable duty cycle of the

piezo-driven pulsed valve, such technique becomes less efficient with high repetition rate

(kHz) laser sources. For energy analysis of photoelectrons or Auger electrons excited with the

high harmonic pulses a time-of-flight (TOF) electron spectrometer is used45. A benefit of the

TOF technique is its capability to simultaneously analyze electrons of different kinetic

energies. An electrostatic lens system at the entrance of the µ-metal shielded electron drift-

tube (length 40 cm) lets me dynamically modify the acceptance solid angle of the TOF

spectrometer from 0.06% to 5% of 4π at the expense of energy resolution. To achieve better

resolution at electron kinetic energies >20 eV a retardation voltage has been applied. The

time-resolved electron signal detected on a 40 mm diameter MSP detector is transmitted to a

discriminator (Philips Scientific, Model 6904) followed by a fast multiscaler (FAST, Model

7886) with 0.5 ns time resolution. A digitizing oscilloscope (Tektronix DSA 602) proved to

be useful for alignment purposes.

As mentioned, the main challenge in the selection of a single harmonic order from the

high harmonics spectrum driven by 1.55 eV femtosecond pulses is to achieve sufficient

contrast between the desired high harmonic and the neighboring odd orders. I have used

photoionization of He gas as a precise and convenient method to measure the selectivity of

the multilayer monochromator45,46. If only one particular high harmonic is selected, only a

single photoelectron peak with the energy h is observed in photoelectron spectrum,

where is the photon energy and = 24.6 eV is the first ionization potential

PI−ν

νh PI 283 of He.

Hence, the photoelectron spectrum of He gas directly reflects the spectral distribution of the

exciting radiation. The measured selectivity of the monochromator for systematically varied

angles of the multilayer mirrors is shown in Fig. 3.8. By changing the multilayer angle of

incidence between 11 and 26 degrees selection of one high harmonic with a photon energy

between 66 eV and 73 eV has been performed. A strong absorption274 at the Al L2,3 -edge

leads to the small observed intensity of the high harmonic at 73 eV photon energy. The

highest selectivity has been achieved for 70 eV photon energy corresponding to an angle of

20 degrees. The contrast ratio to the neighboring high harmonics at 70 eV is measured to be

better than 1:10. Recently, it has been shown that the yield of one specific high harmonic can

be optimized through the variation of the driving pulse phase255,276. Applying this technique275

would enhance the contrast ratio between the selected and adjacent high harmonics.

The temporal stability of the 45th high harmonic measured as total photoelectron yield

from GaAs target is shown in Fig. 3.9. It depends on the laser system stability which is a


function of the pump laser pointing stability. Summary of the femtosecond CPA laser system

and the high harmonic femtosecond EUV source is given in Tab. 3.1. Detailed description of

the photon flux measurement at 70 eV photon energy and the estimation of the EUV pulse

spectral bandwidth is given in Ref. 45. The EUV pulse duration of 70 fs has been measured

by means of time-resolved photoelectron spectroscopy on Pt surface (see section 5.2). The

EUV focus size before the entry aperture of the TOF electron spectrometer has been measured

with the knife-edge technique (Appendix A). The polarization of high harmonics243 is the

same as the polarization of driving laser.

Fig. 3.9 Measured temporal stability of the 45th high harmonic over four hours

Table 3.1 Summary of the properties of the CPA fs-laser system and fs-EUV source.

Chapter 4 – Test Measurements 47

Chapter 4

Test Measurements

4.1 Photoelectron Spectroscopy on Metals, Semiconductors,

Adsorbates and Gases

High harmonics proved to be efficient tools for the static photoelectron spectroscopy

although the repetition rate of the high harmonic sources is 105 times smaller than that of the

storage rings. The first application of high harmonics as a compact table-top equivalent of

storage rings in the EUV region was demonstrated by Haight et al.120,121 Availability and easy

operation of the high harmonic sources279 made systematic and extremely time consuming

studies possible. This has lead to detailed studies of the electronic structure of metals and their

adsorbates277,278.

To illustrate the capability of my system to obtain well resolved photoelectron spectra,

I have recorded the photoelectron spectrum from the Pt(110) surface shown in Fig. 4.1. The Pt

crystal has been cleaned with acetone before transfer to the UHV vacuum chamber. In

vacuum it has been cleaned with the Ar+ ion bombardment (1.6 keV), heating (630 K) in an

oxygen atmosphere (8x10-8 mbar) and flashing (1000 K). The cleanness has been confirmed

with an Auger electron spectrum which was free of all contamination (Appendix B –

Fig. B.1). The proper surface reconstruction has been verified with LEED which displayed

well ordered (1x2) surface reconstruction280,281.

48 Chapter 4 – Test Measurements

Fig. 4.1 Photoelectron spectrum of the Pt(110) crystal excited with the 45th high harmonic.

Fig. 4.2 Photoelectron spectrum of the p-GaAs(100) crystal excited with the 45th high harmonic


The photoelectron spectrum excited with the high harmonic of 70 eV photon energy shows a

dominant and well resolved Pt valence band282. In order to increase the energy resolution of

the TOF spectrometer the retarding voltage† of 35 V has been used. The light of 70 eV photon

energy generates core-holes283,284 in the 5p3/2 (Ebin = -51.7 eV) and 5p1/2. (Ebin = -65.3 eV)

states. The kinetic energy§ of the 5p3/2 photoline excited with the 70 eV photon energy is only

12.9 eV and has not been detected with the used retarding voltage. The photoemission from

the 5p3/2 state with 70 eV photon energy has enhanced cross section (see equation (2.4)) due

to the high density of final states278,285 between 18 eV and 20 eV above the Fermi niveau. The

O3VV Auger electrons (Fig. 4.1) resulting from the 5p3/2 core-hole decay are therefore well

pronounced278,286-288.

The photoelectron spectrum of the p-GaAs(100) crystal is shown in Fig. 4.2. The

retarding voltage of 5 V has been used. The GaAs crystal has been cleaned290 in toluene,

acetone, tricholoethylene, acetone and ethanol before transfer into vacuum. In vacuum a clean

p-GaAs(100) surface has been achieved with repeated cycles of sputtering with 1.5 keV Ar+

ions and annealing at 500 °C. After such preparation the Auger electron spectra showed no

carbon and oxides contamination (Appendix B – Fig. B.3). The GaAs(100) surface exhibits a

variety of reconstructions which are conveniently recognized observing the photoline

intensity ratio Ga-3d/As-3d136,295-297. The photoelectron spectrum clearly resolve Ga-3d and

As-3d photolines. The Ga-3d photoemission cross section291,292 has a maximum for 70 eV

photon energy and the photoelectrons emitted from the surface exhibit maximum surface

sensitivity68. The inset of the Fig. 4.2 shows the As-3d resolved spin-orbit splitting293,294 of

0.7 eV. The photoemission cross section for the GaAs valence band at 70 eV photon energy is

more than one order of magnitude smaller as compared to the Ga-3d cross section. The

measured selectivity ratio of the 45th to 47th high harmonic is about 6%. This can be further

reduced to a virtually pure single high harmonic under the special circumstances shown later

in Fig. 6.4.

To demonstrate the capability of the built apparatus also the photoelectron spectra of

adsorbates have been measured. Fig. 4.3(b) shows the photoelectron spectrum of

wolframhexacarbonyl W(CO)6 adsorbed on a Si(100) surface together with the W(110)

photoelectron spectrum (Fig. 4.3(a)) for comparison. The TOF retarding voltage of 0 V has

been used. The W(110) crystal has been cleaned by several flashing and oxidation cycles.

† The kinetic energy of electron throughout this thesis is refereed to the electron’s kinetic energy direct after

emission from the target and not after the retardation in the TOF spectrometer. § The work function289 of Pt(110) (1x2) surface is 5.4 eV.


He - UPSI

O O

O

Ga

N

N

N

Fig. 4.4 Photoelectron spectrum of Gaq3. The UPS spectrum is taken from Ref. 307.

Fig. 4.3 Photoelectron spectra of the W(110) crystal (a) and W(CO)6 adsorbate (b)


Surface cleanness has been confirmed by the observation of well pronounced 4f5/2 and 4f7/2

photolines in the photoelectron spectrum (Fig. 4.3(a)) which are surface sensitive. On the

basis of known values of the binding energies283,301-303 for 4f5/2 (Ebin = -33.6 eV) and 4f7/2

(Ebin = -31.4 eV) states the photoelectron spectrum has been converted to the electrons’

binding energies to make the direct comparison with W(CO)6 possible. The W 4f binding

energies in Ref. 283 are given with the accuracy of ±0.1 eV which sets the upper limit on the

chemical shift error evaluation although the peak position for the W 4f5/2 is determined by the

fitting with Gauss function with an error of 10 meV. The W(CO)6 has been evaporated onto

Si(100) surface terminated with an OTS (Octadecyltrichlorosilane) self-assembled

monolayer305 with the Si(100) substrate held on -100°C with a liquid nitrogen cooler. After

evaporation the W(CO)6 adsorbed layer could be observed visually as the reflectivity of

substrate had changed. The adsorbed W(CO)6 photoelectron spectrum (Fig. 4.3(b)) shows

well resolved molecular orbitals belonging to the CO groups298-300 as well as both core-level

4f lines. The photoelectron spectrum has been converted to the electrons’ binding energies on

the knowledge of the CO groups position298. The distinctive chemical shift304 ∆EEC = 1.2 eV

of 4f core-levels points on the different electronic environment of the W atoms in the W(110)

surface and the W(CO)6 adsorbate, respectively. This demonstrates the usefulness of this

apparatus for electron spectroscopy for chemical analysis (ESCA) in the EUV region.

Recently the technical relevance of the organic materials306 for light-emitting diodes

was recognized. One of the promising and extensively studied materials is tris(8-

hydroxyquinolinato) gallium (Gaq3)307-313. Fig. 4.4 shows the photoelectron spectrum of Gaq3.

The TOF retarding voltage of 30 V has been applied. The Si(100) crystal has been stripped of

its natural oxide layer in a buffered HF solution and transferred into the vacuum chamber

where it was cleaned with heating cycles up to 830 °C. For the evaporation314 of Gaq3 a Al2O3

single-ended tube (Friatec AG) resistively heated with molybdenum wire has been used at

temperatures up to 280 °C. The presence of a Gaq3 film has been checked as a visual change

in the substrate reflectivity. The photoelectron spectrum in Fig. 4.4 shows the well resolved

Gaq3 molecular orbitals excited with 70 eV photon energy together with a spectrum measured

with He I-UPS for comparison307. The Ga-3d core-level photoline in Fig. 4.4 points to the

excellent sensitivity of this measurement method for a single atomic species embedded in a

large molecular complexes.


Fig. 4.5 Photoelectron spectrum of I2 adsorbate excited with the 45th high harmonic after

subtraction of the secondary electron background. The inset shows the originally measured

spectrum.

30 35 40 45 50 55 600

2

4

6

8

10

5p5s5s-Satellites

binding energy (eV)

inte

nsity

(a.u

.)

kinetic energy (eV)

hν = 73 eV

N4,5O2,3O2,3-Auger

5p3/25p1/25s1/2

Xe

25 23 21 19 17 15 13 11 90

2

4

6

8

10
Fig. 4.6 Photoelectron and Auger spectrum of Xe after photoionization with 73 eV.


I have also investigated the photoelectron spectra of iodine films with the objective of

Auger 4d core-hole decay observation. Iodine films have been prepared by condensing I2

molecules from an electrochemical AgI cell315,316 onto Si(100) surface terminated with OTS

self-assembled monolayer cooled with liquid nitrogen to –100 °C. The photoelectron

spectrum of a I2 adsorbate excited with 70 eV photon energy is shown in Fig. 4.5. The TOF

retarding voltage of 0 V has been used. The iodine 4d3/2 and 4d5/2 photolines are obscured

with a massive secondary electron background as shown in inset of Fig. 4.5.

Double-exponential background subtraction reveals both 4d core-level photolines. The

measured spin-orbit splitting317 is ∆EFS = 1.7 eV. The N4,5O2,3O2,3 Auger decay of the 4d

core-hole is observed as a broad less prominent feature318 between 20 eV and 30 eV kinetic

energy.

The first measurements45, however, were performed in the gas phase photoionizating

He for spectral characterization of the multilayer monochromator (see section 3.1). For

completeness, Fig. 4.6 shows the photoelectron and Auger spectra of Xe gas after the

photoionization with 73 eV photon energy and TOF retarding voltage of 15 V. The

photoionization of the 5s and 5p shells gives rise to the photoelectron peaks at energies

49.5 eV and 61 eV, respectively. Increasing the retardation voltage of the TOF spectrometer

to 35 V and using the photon energy of 70 eV, I can observe the spin-orbit splitting of the 5p

energy level (inset in Fig. 4.6). The photon energy of 73 eV is sufficient to create a hole state

in the 4d shell which relaxes through the characteristic NOO Auger decay. Electrons

corresponding to the N4,5O2,3O2,3 Auger decay as well as 5s-satellites are well distinguished in

the electron spectrum. The measured linewidth of 0.9 eV (FWHM) of the Auger electron peak

N5O2,3O2,3 (1S0) is considerably larger than the natural linewidth319 of 120 meV. Since Auger

linewidths are generally independent on the bandwidth of the exciting radiation, this value

represents the energy resolution of the TOF spectrometer at 30 eV electrons’ kinetic energy

and 15 V retardation potential. The variation of the TOF energy resolution has been analyzed

in Ref. 45 in detail. Taking into account the limited resolving power, the photoelectron and

Auger electron spectrum are in reasonable agreement with spectra measured with synchrotron

radiation320,321.

54 Chapter 5 – Time-Resolved Photoelectron Spectroscopy on Pt(110) Surface

Chapter 5

Time-Resolved Photoelectron Spectroscopy on

Pt(110) Surface

5.1 Measurement of Hot Electrons on a Pt(110) Surface

The idea of the experiment is based on the photoexcitation of electrons from occupied

states of the valence band into unoccupied states above the Fermi level with a visible pump

pulse. These excited short-lived electrons (see section 2.1.2) (also called “hot electrons”) are

then emitted with an EUV probe pulse into vacuum where they are energy-analyzed. The time

delay between pump and probe pulses can be continuously changed. In this way, I obtain

information on the transient population above the Fermi level at the surface. The

implementation of the principle for the Pt(110) surface is shown in Fig. 5.1. The pump pulse

with 1.5 eV photon energy and 70 fs duration excites electrons from occupied d-band states322

in the valence band into unoccupied states above the Fermi level as shown in the energy-time

representation in Fig. 5.1(a).

Chapter 5 – Time-Resolved Photoelectron Spectroscopy on Pt(110) Surface 55

Fig. 5.1 Principle of the visible-pump/EUV-probe hot electron spectroscopy on a Pt surface

shown in (a) energy-time representation and (b) space-time representation


The total absorbed light intensity of 1.6x1011 W/cm2 excites 2.2% of all valence band

electrons§. These hot electrons relax very rapidly via e-e scattering. It has been shown89 that

the hot electron’s life times of transition metals are remarkably shorter than the life times of

noble metals. This phenomenon is due to the large phase space for e-e scattering caused by

the high density of states322 (d-band) at the Fermi edge. On the contrary, totally filled d-bands

of the noble metals and solely contribution of the s-bands to the e-e scattering at the Fermi

edge are responsible for the relatively long-lived hot electrons. The probe pulse with a photon

energy of 70 eV and duration of less a 70 fs projects the current electron distribution into the

vacuum as shown in Fig. 5.1(a). Changing the time delay between pump and probe pulses I

can track the temporal evolution of the electronic population at the Fermi edge. Here I define

the time delay as positive when the pump pulse precedes the probe pulse. The kinetic energy

of the hot electrons emitted with the probe pulse is about 65 eV. Such electrons are mainly

(86% of all registered electrons) originating326 from only 0.88 nm depth. At this point I have

to assume also a ballistic transport (1 nm/fs) and a diffusion (10 nm/ps) of hot electrons103

from the probe spot at the surface (Fig. 5.1(b)). Consequently, the net observed relaxation

times of hot electrons at Pt surface is expected to be smaller than 3 fs for the hot electrons of

1.5 eV kinetic energy with respect to the Fermi niveau.

I have used a variant of the experimental setup shown in Fig. 3.1 without the

frequency doubling unit for pump pulses. In this experiment the repetition rate of the laser

system was 10 Hz and the time-resolution of used TOF electron detection system was 1.5 ns.

The clean Pt(110) surface has been prepared as described in section 4.1. The p-polarised

pump pulses (800 nm, 1.55 eV) with energy 0.5 mJ and duration of 70 fs were focused

together with the 70 eV p-polarised probe pulses on the Pt(110) surface. The pump and probe

incident angle measured from the surface normal was 53°. The axis of the TOF spectrometer

and surface normal made an angle of 8° and the retardation voltage was 35 V. The

photoelectron spectra for –400 fs, 0 fs, and +400 fs delay are shown in Fig. 5.2. The

photoelectron spectra for ±400 fs are identical in the region above the Fermi level indicating

that hot electrons for +400 fs are complete thermalized. The photoelectron spectrum for time

delay 0 fs shows a clear deviation when compared with the spectra for ±400 fs. When pump

§ Total number of absorbed photons NA = 8.4x1014 is given by NA = (1-R)NI where NI=2x1015 is the number of

incident photons and R = 0.58 is the reflectivity62 of Pt for the incident angle 53° and photon energy 1.55 eV.

The 86% of all photons are absorbed62 in the depth of 26 nm. The pump spot at the surface was 19x10-3 cm2,

resulting in total excited electron density of 1.5x1022 cm-3. The electron density61 of Pt valence band (5d, 6s

electrons) is 6.6x1023 cm-3, yielding the net valence band electronic excitation of 2.2%.


and probe pulses hit the Pt surface in temporal coincidence I have to observe the highly

non-equilibrium electron distribution above the Fermi edge. This has been observed as shown

in the inset of Fig. 5.2. The probe pulse with 70 eV photon energy creates a core-hole in the

5p3/2 niveau. This core-hole relaxes as shown in section 4.1 via Auger decay seen as a O3VV

Auger peak on the top of a secondary electron background. The relation between the O3VV

peak shape and the valence band density of states (DOS) is not elementary and in the first

approximation is given by the self-convolution of the valence DOS327-330.

Fig. 5.2 Photoelectron and Auger electron spectra of Pt(110) surface for 0 fs and ±400 fs delays

between pump and probe pulses.


Fig. 5.3 Fermi edge photoelectron spectra of the Pt(110) surface for time delays of 0 fs and

±100 fs between pump and probe pulses.

Naively, one would expect a simple broadening of the O3VV peak due to new possibilities in

the Auger recombination process connected with the transient hot electron distribution in the

normally unoccupied states above the Fermi level. As shown in the inset of Fig. 5.2, the

O3VV Auger peak shifts to lower kinetic energies. A detailed theoretical study of this

phenomenon is necessary to account for the observed O3VV Auger peak shift. In the

following I will concentrate exclusively on the hot electron dynamics at the Fermi edge which

gives more transparent and easily interpretable results. More detailed photoelectron spectra

for the three delay times 0 fs and ±100 fs at the Pt Fermi edge measured with the retardation

voltage of 45 V are shown in Fig. 5.3. A considerable non-equilibrium electron population

above the Fermi edge is present at zero time delay. The spectrum for +100 fs shows no signs

of hot electrons above the Fermi edge revealing a very fast hot electron thermalization as

expected. The inset of the Fig. 5.3 shows the photoelectron spectrum generated exclusively

with the pump pulses which can be attributed to multiphoton emission processes331 at such

high pump intensity. Only by using probe photons with energies exceeding 40 eV the

photoelectron signal can be prevented from being obscured by these background electrons. I

attribute the observed time-dependent spectral variation of the photoelectron signal at the Pt

Fermi edge to a transient hot electron population rather than to a space-charge broadening


induced by the high local electron density because i) the distinct spectral feature observed at

65.5 eV in Fig. 5.3 does not indicate a simple broadening of the Fermi-edge and ii) due to a

velocity of <3.5 µm/ps for electrons with energies lower than 35 eV I would not expect an

ultrafast space charge relaxation. A more detailed discussion on the space-charge phenomena

is given in section 6.1.

65.5 eV

pump h = 70 eVprobe h = 1.5 eV

ν ν

E+

70 e

VF

probe pump probepump

A

p

w

s

n

p

(

s

d

Fig. 5.4 Contour plot of the photoelectron yield vs. kinetic energy and time delay between

pump and probe pulses measured at the Pt(110) surface. The time-resolved signal at

photon energy 65.5 eV (dashed line) is shown in Fig. 5.8.

contour plot of the photoelectron yield vs. kinetic energy and time delay between pump and

robe pulses (Fig. 5.4) exhibits a fast relaxation of the hot electrons within 100 fs. The data

ere acquired in four subsequent runs with 96 min duration, each. During this time, no

ignificant changes in the temporal structure of the spectra has been observed. For large

egative time delays (<-100 fs) when the probe pulse precedes the pump pulse the measured

hotoelectron spectra reflect the non-disturbed Fermi edge. For large positive delays

>+100 fs) the hot electron population has already decayed and the measured photoelectron

pectra are same above the Fermi edge as the spectra for large negative time delays. For time

elays between ±100 fs I observe a symmetrical hot electron distribution above the Fermi


edge with respect to zero time delay. When I assume the temporal profile to be a delta-like

function for pump and probe pulses one has to observe the exponentially decreasing hot

electron distribution for positive time delays with a maximum for zero time delay. For

negative time delays there should be no hot electron population. Consequently, this

pump-probe method is not symmetrical for time delay inversion. The actually measured

symmetrical distribution for time delays between ±100 fs is caused by the finite time duration

of pump and probe pulses which is considerably larger than the characteristic hot electrons

life times (see equation (2.27)). This means that the measured hot electron distribution is

proportional to the cross-correlation between pump and probe.

Fig. 5.5 Fermi edge time-resolved photoelectron spectra for the Pt(110)

The decrease of the time-resolved photoelectron yield for the electron energies lower

than 64 eV (representing the Fermi edge), correlated with the hot electron population, can be

understood as an electron depletion of the valence band. At the absorbed intensity of

1.6x1011 W/cm2 I deal with a photoexcitation of 2.2% of all valence band electrons. Even

larger photoexcitation can lead to the short time scale destabilization of the crystal

structure324,325. This in turn can be observed as an alteration of the metal dielectric

function333-335. There are also theoretical studies336,337 for semiconductors where the

photoexcitation of 10% of all valence band electrons leads to collapse of the electronic

structure338-348 observed as semiconductor band-gap collapse, semiconductor-to-metal


transition, and structural changes leading to an ultra-fast femtosecond melting of the

semiconductor. This is the first direct study of the hot electron population and valence band

changes at such high pump intensities. The traditional studies79 have been performed at

intensities more than 10 times lower than in this case. The used pump intensity in my study is

still considerably lower than the threshold for plasma formation at metal surfaces332 of

1013 W/cm2.

beforephotoexcitation

afterphotoexcitation EF

Fig. 5.6 Photoelectron spectra near the Pt Fermi edge before and after photoexcitation

Upgrading of the laser system to a repetition rate of 50 Hz has lead to a faster data

acquisition. Additionally, a better time-resolution of the TOF electron detection system of

0.5 ns enabled to do the same experiment on the Pt(110) surface with shorter accumulation

times and better photoelectron energy resolution. The measured time-resolved photoelectron

spectra are shown in Fig. 5.5. The shown spectra have been measured in two sequential runs,

each of 52 min duration. The Fermi edge at zero time delay shows the highly non-equilibrium

hot electron distribution. The e-e scattering is leading to the hot electron thermalization within

few femtoseconds. However e-p interactions occur on a time scale up to a few picoseconds

leading to lattice heating (see Fig. 2.8) resulting in a less pronounced – less sharp Fermi

edge61. The photoelectron spectra showing the Fermi edge region of the Pt valence band

before and after photoexcitation are shown in Fig. 5.6. The observed difference in the

photoelectron spectra is not due to a high harmonic intensity drop off, because the shown

spectra were acquired in two sequential runs and have been normalized so that the integral of


all registered photoelectrons for all spectra is constant. I therefore associate the observed

difference at the Fermi edge with the growing lattice temperature after photoexcitation.

5.2 Pump-Probe Cross-Correlation in the EUV Region

The measurement of femtosecond (or even attosecond) pulse duration in the EUV

region represents a great experimental challenge. In the infrared, visible and ultraviolet

traditional techniques based on nonlinear effects are used for the pulse duration

measurement73 yielding not only amplitude, but also phase information263,264. The current

EUV pulse intensities are not high enough to make nonlinear effects observable. Accordingly,

the autocorrelation techniques based on nonlinear effects are not available. The tendency is to

use instead cross-correlation techniques with a high-intensity visible or infrared “dressing”

pulse and an EUV pulse in an experimental setup with a generalized scheme like in Fig. 5.7.

time delay

EUV pulse“dressing”

pulse e-to spectrometer

Fig. 5.7 Scheme of the “dressing-pulse” cross-correlation techniques in the EUV region

One measures photoelectron350 or Auger electron349 spectra of the noble gases as a function of

the time delay between ionizing EUV pulse and “dressing” pulse with an intensity higher than

1011 W/cm2. The photoelectron spectrum in the presence of the strong “dressing” pulse field

exhibits side-bands6,350 of photoelectron peaks as well as peak’s broadening and shifting1,7.

This technique has been successfully applied even to the measurement of the trains of

attosecond pulses6 and single attosecond pulses1 as well. Assuming chirp-free EUV pulses

one can estimate their duration also with linear effects like interference227,357-359. However,

daily usage of the femtosecond pulses for the time-resolved spectroscopy in the EUV region

is calling for a UHV compatible measurement method utilizing lower intensities than

1011 W/cm2. The cross-correlation method utilizing the ultrafast relaxing hot electrons on


transition metal surfaces seems to be very efficient and reliable tool for the measurement of

femtosecond EUV pulses.

In the section 5.1 I have shown the measurement of a very fast decaying hot electron

distribution at the Fermi edge of the Pt(110) surface. The measured transient signal from the

hot electron distribution does not allow extraction of the hot electron’s life-times of some few

femtoseconds but is proportional to the pump-probe cross-correlation. Let us consider hot

electrons with a kinetic energy of 1.5 eV referring to the Fermi level photoexcited with the

pump pulse modeled by the gaussian intensity profile . The

photoemission of these hot electrons into vacuum with the EUV probe pulse with the intensity

profile defined as results in a photoelectron signal depending on

the time delay between pump and probe pulses. The photoelectron signal at the kinetic energy

of 65.5 eV corresponding to these hot electrons is shown in Fig. 5.8. This signal corresponds

to a line-scan shown through the contour plot in Fig. 5.4. Assuming that the registered

photoelectrons are governed by the Poisson probability distribution

)/2ln4exp( 220pupupu tII τ⋅−=

)/2ln4exp( 220prprpr tII τ⋅−=

361 the error bars in

Fig. 5.8 show the standard deviation§ of the measured photoelectron counts supposing these to

be the mean value. The evaluated FWHM error of the cross-correlation signal is given as the

standard deviation of the Gauss function width fitting parameter.

Under the approximation of a delta-like fast hot electron relaxation the measured

cross-correlation signal as a function of time delay between pump and probe pulses is

given as

)(τS τ

))/(2ln4exp()()()( 222∫+∞

∞−

+⋅−≈−= prpuprpu tdIIS ττςτςςτ (5.1)

On the basis of this result the measured square of FWHM of the cross-correlation signal is

proportional to the sum of the pump and probe pulse duration (FWHM) squares. The

measured pulse width of the pump pulse45 of 70 fs and the cross-correlation width of 100 fs

yield the EUV probe pulse duration of approximately 70 fs neglecting the hot electron

lifetime. A spectral and spatial drift of the driving femtosecond laser and mechanical system

instabilities over the long photoelectron spectra acquisition times have been slightly reduced

§ Presumed is the mean of the Poisson probability distribution the standard deviation is given by µ µ


utilizing a new pump laser of 50 Hz resulting in the shorter width of cross-correlation signal

show in Fig. 5.8(b).

An application of this method for pulse duration measurements in the EUV region has the

following advantages:

- reduced pump intensity of one or two orders compared to the “dressing” pulse

intensity higher than 1011 W/cm2 necessary in gas-phase cross-correlation

- UHV compatible environment during the measurement compared to the some

10-4 mbar in the “dressing” pulse cross-correlation methods

FWHM = 100 ± 5 fs

FWHM = 92 ± 7 fs

inte

nsity

(arb

. uni

ts)

inte

nsity

(arb

. uni

ts)

Fig. 5.8 Cross-correlation of pump and probe pulses corresponding to hot electrons with a

kinetic energy of 1.5 eV above the Fermi level as measured with (a) 10 Hz and (b) 50 Hz

laser system.


Until now, I have supposed an instant hot electron relaxation but remembering the finite

relaxation times of hot electrons restricts this method to measurements in the femtosecond

region. But the perspective of higher excitation energies of some 3–4 eV of the pump pulse is

leading to even shorter hot electron relaxation times which can push the applicability of this

method even to the femto-atto-second edge.

66 Chapter 6 – Time-Resolved Photoelectron Spectroscopy on GaAs(100) Surfaces

Chapter 6

Time-Resolved Photoelectron Spectroscopy on

GaAs(100) Surfaces

6.1 Measurements of Transient Ga-3d Core-Level Shifts on

GaAs(100) Surfaces

The principle of the pump-probe measurement used throughout this section is shown

in Fig. 6.1. The pump pulse (3.1 eV) with an intensity of 1x1010 W/cm2 generates

approximately 1.6x1020 cm-3 electron-hole pairs§ in the SCR at the p-GaAs(100) surface. The

SCR electric field of few hundreds of kV/cm separates generated electron-hole pairs in less

than 1 ps. The electrons drifting to the surface partially compensate the SCR electric field

which is leading to the bands flattening. The core-levels362 are bending in the same manner

and with the same amplitude as conduction and valence bands. The time-delayed probe pulse

with energy of 70 eV emits the electrons from the Ga-3d core-level into vacuum.

§ Total number of absorbed photons NA = 1.25x1013 is given by NA = (1-R)NI where NI=3.14x1013 is the number

of incident photons and R = 0.6 is the reflectivity62 of GaAs for the incident angle 53° and photon energy 3.1 eV.

The 86% of all photons are absorbed62 in the depth of 30 nm. The pump spot at the surface was 23x10-3 cm2,

resulting in total excited electron-hole density of 1.6x1020 cm-3. The electron density105 in GaAs valence band is

2x1023 cm-3, yielding the net valence band electronic excitation of 0.1%.

Chapter 6 – Time-Resolved Photoelectron Spectroscopy on GaAs(100) Surfaces 67

Fig. 6.1 Principle of the band-bending after photoexcitation detected by measuring the

kinetic energy of the photoelectrons from the Ga-3d shell.


Accordingly, the kinetic energy of Ga-3d photoelectrons is shifting by the same amount as

that of valence bands. The Ga-3d photoelectrons’ shifts are therefore giving information on

the band-bending as a function of time delay between pump and probe pulses. The surface

carrier recombination leading to band-bending receding to the original value is observed as

the back-shift of the Ga-3d photoelectrons’ kinetic energy. The high excitation intensity of the

pump pulses is giving rise also to multiphoton processes seen as a low-energetic electron tail.

The kinetic energy of the Ga-3d photoelectrons emitted with photons of 70 eV energy retains

maximal surface sensitivity of circa 1 nm depth.

Fig. 6.2 Photoelectrons excited through multiphoton processes with a pump pulse (3.1 eV)

of intensity 10 GW/cm2 from the p-GaAs(100) surface

Before discussing the experimental results on transient changes in the SPV at the

GaAs surface I elucidate some particular challenges connected with the time-resolved

photoelectron spectroscopy at high pump intensities. While in my visible-pump/EUV-probe

scheme I aim for a single-photon electronic excitation of the semiconductor, the large number

of photons reaching the surface in a short time interval (about 1013 in 70 fs corresponding to

~10 GW/cm2) also leads to multiphoton emission (MPE)363,364 of energetic electrons, as

depicted in Fig. 6.2. In the case of low energetic probe photons in the visible or UV range the

strong MPE electron background will obscure the pump-probe photoelectron signal and has

therefore restricted such studies in the past to considerably lower intensities. I overcome this


problem by use of EUV probe photons with 70 eV energy and so I achieve sufficient energy

separation between the measured signal and the MPE low energetic electron tail becoming

confined to energies below 11 eV for 10 GW/cm2 pump intensity. However, the signal

electrons will still be affected owing to vacuum space charge156.

I

b

p

r

a

m

w

t

C

a

b

s

p

Fig. 6.3 Valence band photoelectron spectra of the Cu(110) with and without pump pulse

indicating the influence of vacuum space charge decoupled from the SPV effect.

n the following I will use the maximum kinetic energy - MPE cut-off - of the MPE

ackground electrons rather than pump intensity which depends on the spatial pump pulse

rofile. The study of the copper valence band photoelectron spectra in the highly excited

egime has been selected as an example of a system which does not exhibit the SPV effects

nd therefore permit an independent study of the vacuum space charge influence on the

easured photoelectron spectra without interfering SPV effect. The Cu(100) surface excited

ith pump pulse of 1.55 eV photon energy producing the MPE background electrons with up

o 35 eV kinetic energy has been probed with probe pulse of 70 eV photon energy. The

u(100) crystal has been cleaned with repeated cycles of Ar+ sputtering (1.5 keV) and

nnealing at 580 °C, in the last cycle only at 200 °C to avoid surface contamination from the

ulk. The surface cleanness has been verified by a contamination-free Auger electron

pectrum (Appendix B – Fig. B.2). The Cu valence band photoelectron spectra excited with a

robe pulse in the presence and absence of pump pulses are shown in Fig. 6.3. I have decided


to use the pump pulse with photon energy of only 1.5 eV to avoid direct photoexcitation from

the d-bands322 of Cu valence band and so be able to observe a well defined d-bands edge also

at high pump intensities. The Cu valence band photoelectron spectra for time delays ±100 fs

between pump and probe are identically shifted to higher kinetic energies without significant

broadening of spectral features. This can be understood as an acceleration of the leading fast

probe photoemitted electrons by Coulomb repulsion from the trailing cloud of slow MPE

pump photoemitted electrons. In the following I therefore express the measured SPV

transients in terms of a time-dependent relative shift with respect to the situation when the

probe pulse precedes the pump pulse by 800 fs (negative delay).

CBM

VBM

Evac

As 3d ~ 41.5 eV

E ~ 5.5 eVth

valenceband

Fig. 6.4 Photoelectron spectra of the p-GaAs(100) excited with a spectrally pure 45th high

harmonic. The inset shows the simplified GaAs electronic structure.

Time-dependent photoinduced changes of the band-bending were studied for p-doped

(Zn: 3x1019 cm-3) GaAs(100) as well as n-doped (Te: 6x1017 cm-3) GaAs(100) surfaces. The

clean surfaces were prepared as described in detail in section 4.1. The photoelectron spectrum

of the clean p-GaAs(100) surface is shown in logarithmic scale in Fig. 6.4. The photoemission

cross section for Ga-3d core-level electrons is nearly two orders of magnitude higher than that

for the valence band electrons. This is the reason why the currently feasible short

photoelectron accumulation times are prohibiting the direct observation of the electron

population dynamics in the conduction band or surface states as a result of insufficient

photoelectron signal statistics. Accordingly, in the further text I deal exclusively with the


Ga-3d core-level photoelectrons. The inset of Fig. 6.4 shows also a simplified (without

band-bending) electronic structure of the GaAs crystal. The most stunning feature of the

photoelectron spectrum in Fig. 6.4 is the absence of the adjacent high harmonics

photoelectron signal vanishing in the secondary electron background. This can be achieved by

the interplay of the high harmonics driving fundamental wavelength and the incident angles

of multilayers. This method is yielding a nearly spectrally clean single high harmonic

selection.

p-GaAs(100)

CBM

VBM

n-GaAs(100)

CBM

VBM47.47

47.42

47.80

47.66

Fig. 6.5 Shift of the Ga-3d core-level of (a) p-GaAs and (b) n-GaAs probed 400 fs before and

after photoexcitation. The calculated peaks’ center-of-gravity are shown as vertical continuos

lines for +400 fs and vertical dashed lines for -400 fs. The insets show schematically the

photoinduced band-bendings.


For an energy of 270 µJ/cm2 deposited by each 3.1 eV pump pulse neither changes of the

Ga-3d core-level spectra by photochemical decomposition155 nor any macroscopic alterations§

of the GaAs surface have been observed after the pump-probe experiment with irradiation

over several hours. The deposited energy by the pump pulses is lower than the known

photochemical decomposition intensity threshold of some 20 mJ/cm2 specified for p-

GaAs(100) in Ref. 161 or 1 mJ/cm2 for n-GaAs(110) in Ref. 155. In Fig. 6.5(a) are presented

photoelectron spectra of the Ga-3d inner shell of the p-GaAs(100) at positive and negative

pump-probe delay. The observed transient shift of the Ga-3d photoline center-of-gravity

towards higher energies after photoexcitation (+400 fs) corresponds to a partial screening of

the electric field in the SCR by the mobile carriers created by the optical pump pulse.

According to Fig. 6.1 this in turn leads to a reduction of the effective binding energy of the

Ga-3d state at the semiconductor surface. Due to the different doping concentrations of n and

p-GaAs crystals I am not expecting the same amplitude of the shifts for these two crystals but

finding the corresponding shift for n-doped GaAs(100) at lower kinetic energies as indicated

in Fig. 6.5(b) I can exclude this effect as being induced by repulsion from MPE electrons as

discussed in the case of vacuum space charge. Rather, the opposite sign is compatible with a

reduction of band-bending towards higher effective binding energy as expected for n-doping.

Since the p-doped crystal exhibited a more pronounced transient energy shift, I focused my

investigation on the p-GaAs(100) as indicated in Fig. 6.6 as a sequence of photolines at

different visible/EUV time delays. To rule out possible adsorbates induced core-level

chemical shifts from residual gases, photochemical decomposition, and temporal drifts of the

laser or the electron detection system during the long photoelectron acquisition times the

spectra with subsequent time delays have been measured in a random fashion rather than in an

ascending order. The pump-probe spatial overlap was verified every time when the time delay

has been changed by more than 1 ps with the retractable pinhole of 1.2 mm diameter and a

CCD camera. The Fig. 6.6(a) shows the photoelectron yield as a function of time delay and

electrons’ kinetic energy. For negative time delays - which corresponds to the situation when

probe pulse precedes pump pulse - I observe no significant variations in the Ga-3d photoline.

For positive time delay smaller than 1 ps I notice a kinetic energy shift of Ga-3d

photoelectrons to higher kinetic energies.

§ The visual inspection of the used part of GaAs surface in the pump-probe experiment revealed no changes

when compared to the unexposed part of surface examined with a light microscope and magnification up to

200x.


(b)

probe pump probepump

(a)
Fig. 6.6 Transient in p-GaAs(100) revealed by the Ga-3d core-level shift as a function of the
pump-probe delay - (a) contour plot and (b) for selected time delays. The thin vertical lines in

graph (b) indicate the peaks’ center-of-gravity positions.


Photoexcitation of the electron-hole pairs in the SCR is followed by the charge separation

phase, where the electrons are drifting to surface and the holes into the bulk. This charge

transport is connected with gradual decrease of the SCR electric field and corresponding

flattening of the electronic bands as well as core-levels. This typical behavior can be seen also

in Fig. 6.6(b) for time delays +100 fs and +1 ps. For time delays longer than 1 ps I observe

slow gradual decrease of the Ga-3d photoelectrons’ kinetic energy. This phase is linked with

the surface recombination processes (see Fig. 6.6(b) for delay times +8 ps, +32 ps). The

measured recombination has nearly exponentially decaying character recovering the old value

of the band-bending approximately after +32 ps. Fig. 6.6(b) shows also Ga-3d photoline in the

absence of pump pulse. Accordingly, I observe a permanent shift with respect to the

non-pumped surface as a collective result of the band-bending induced by a possible pre-pulse

preceding the femtosecond pump pulse by 12.9 ns, of residual non-decayed carriers from the

previous pump pulse 20 ms before and of the vacuum space charge effect as discussed above.

SPV shift

SPV shift

space-chargeshift

no MPE

MPEcut-off 11 eV

n-GaAs(100) pump h = 3.1 eV

probe h = 70 eVνν

(a)

(b)

(c)

Fig. 6.7 Effect of the vacuum space charge created by pump pulses on the Ga-3d

photoelectrons’ kinetic energy at n-GaAs(100) surface


In order to understand the effects of the vacuum space charge created by pump pulses

on the Ga-3d photoelectrons I have measured the Ga-3d photoelectrons’ kinetic energy for

different pump fluences at n-GaAs(100). The SPV effect on the n-GaAs(100) surface shifts

the kinetic energy of the Ga-3d photoelectrons to lower kinetic energies. In Fig. 6.7 Ga-3d

photoelectron spectra are shown for three different cases - without pump pulses (Fig. 6.7(a)),

at the pump intensities when no vacuum space charge has been generating (Fig. 6.7(b)) and at

the pump intensities generating vacuum space charge consisting of the MPE electrons with

11 eV kinetic energy cut-off (Fig. 6.7(c)). In the regime when no vacuum space charge has

been generated the SPV effect is dominant and the Ga-3d photopeaks have lower kinetic

energy than the Ga-3d photopeak measured at the unpumped surface (Fig. 6.7(b)). In this case

the Ga-3d photopeaks 400 fs before and 400 fs after photoexcitation do not differ remarkably

in their position. It has been shown161 that the ratio of the amplitudes of fast and slow SPV

shifts are about 14%. In this case the fast transient SPV shift should be some 60 meV which

needs a better photoelectron statistics to prove it. In Fig. 6.7(c) the presence of vacuum space

charge built of the electrons with kinetic energy cut-off of 11 eV is shifting the Ga-3d

photopeaks to higher kinetic energies. The position of the Ga-3d photopeaks in this case is

nearly the same as the position for the unpumped surface which is associated with the

generated vacuum space charge and will be different for the vacuum space charge with other

electrons’ kinetic energy cut-off. This case clearly demonstrates how vacuum space charge

affects the kinetic energy of Ga-3d photoelectrons. In the spectra shown in Fig. 6.7 the shift

due to the SPV effect is nearly the same but opposite to that of vacuum space charge. This

case also explains the difficulty to obtain the absolute SPV shifts in the presence of vacuum

space charge. The Ga-3d photopeaks show also the mentioned fast SPV transient shift

observed as a slight shift of the Ga-3d photopeak to lower kinetic energies after

photoexcitation (Fig. 6.7(c)).

For completeness Fig. 6.8 shows the measured SPV transient for the n-GaAs(100).

The phenomena yielding Ga-3d photopeak shifts in the opposite direction is the same as for

p-GaAs(100) only the roles of the electrons and holes are exchanged. After photoexcitation

the electrons in the SCR are drifting into the bulk and holes to the surface. This is leading to

the flattening of the electronic bands as well as core-levels originally bent upwards. The

Ga-3d photoelectron spectra for the positive time delay shorter than 1 ps show shifts to lower

kinetic energies. The surface recombination processes tend to restore the original Ga-3d peak

position for time delays longer than 1 ps.


(b)

(a)

Fig. 6.8 Ga-3d core-level peak as a function of the pump-probe delay - (a) contour plot and

(b) for selected time delays. The thin vertical lines in graph (b) indicate the peaks’

center-of-gravity positions.


6.2 Dynamics of the band-bending for p-GaAs(100)

An extraction of quantitative values for the SPV changes demands some attention

because of overlapping pump and probe beams with the same diameter of approximately

1.2 mm but an inhomogeneous spatial profile as shown simplified in Fig. 6.9. Photoelectrons

originating from the less pumped outer region therefore experience a correspondingly smaller

SPV shift as compared to the central region of the pump spot and the observed net shift

actually represents an integration over the whole area.

p-GaAs

Ekin

Ga 3d

Fig. 6.9 Principle of the asymmetrical Ga-3d photopeak broadening

Correspondingly, the measured Ga-3d peak profile is given as an integral of the

unperturbed Ga-3d peak profile for the unpumped surface and a weighting function

describing the amplitude of the SPV effect as a function of the energy shift which in

turn depends on the spatial pump intensity profile

)(Es

)y

)(Ep

)( Eg ∆

,(xI

∫∆

−=max

0

)()()(E

dEpgEs ςςς (6.1)

where is the maximum observed SPV shift. Deconvolution of the photoelectron peak

profile requires exact knowledge of the spatial profile of the pump beam which has

been not recorded. Generally, there are three simple possibilities how to extract data on the

maxE∆

)(Es


transient SPV shifts observed with my measurements. I restrict the analysis to the extraction

of the maximal SPV shift induced with the most intense part of the spatial pump pulse profile.

i) In a first approximation I can fit my data with two peaks, where one is fixed and the second

one is moving. Each of the peaks is a replica of the Ga-3d peak in the unpumped condition.

This method is quite good but it can not always fit properly the profile of the measured Ga-3d

peak. ii) Second possibility is to concentrate the attention only to the highest energetic edge of

the Ga-3d peak profile. In this case I can use a second derivative to find the inflexion point

which is tracking very precisely the maximum Ga-3d kinetic energy shift. However this

method is very sensitive to the peak profile and very good photoelectrons’ peak statistics is

required. iii) The last method is the center-of-gravity determination. This very robust method

has limited sensitivity for the subtle changes at the highest energy edge of the Ga-3d peak

because of the not weighting nature of the center-of-gravity calculation. Although reduced

sensitivity for the fine changes at the highest energy edge of the Ga-3d photopeak I use this

method as primer source of the information on the observed band-bending dynamical

processes. The center-of-gravity and its standard deviation (shown as error bars) has

been calculated as follows

CE CE∆

269

=C

∑∑⋅=

EE

EE

C NEN

E 1 (6.2)

∑∑⋅−∆

EEC

EE

NEEN

E 2)(1 (6.3)

where denotes the number of photoelectrons with kinetic energy and summation in

runs over the region of interest. In the derivation of equation (6.3) the Poisson probability

distribution of the registered photoelectrons has been utilized.

EN E E

In Fig. 6.10 the transient shift of the Ga-3d photopeak is shown as a function of the

time delay between pump and probe determined with the center-of-gravity calculation. The

reliability of the center-of-gravity shifts has been verified by repetitive measurement of the

shift for +1 ps time delay one hour later under the same experimental conditions. These

measured shifts are shown inside of the gray bar in Fig. 6.10(b). The calculated error bars in

Fig. 6.10 with equation (6.3) are showing the Gaussian standard error interval361

corresponding to 68.3% of all measured shifts.

σ1


(a)

(b)

Fig. 6.10 Temporal evolution of the Ga-3d peak’s center-of-gravity in linear (a) and

semi-logarithmic (b) time scale after photoexcitation of the p-GaAs(100) surface.


However the shift for +1 ps time delay measured one hour later does not fall in the standard

confidence interval and only the confidence interval based on 2 corresponding to 95%

of all measured shifts can justify the shift measured one hour later. A different value of the

observed shift points to possible deterioration of the surface conditions. To solve this problem

I have to achieve better vacuum of order 1x10

σ1 σ

-11 mbar. The pointing laser stability resulting in

the falling conversion efficiency for high harmonics (Fig. 3.9) and in the changes of the

spatial pump profile are also an error source contributing to the measured different values for

the SPV shifts. In order to reproduce the measured SPV shifts one needs the precisely same

prepared and reconstructed surface. Already small differences in the surface preparation are

yielding different SPV transients as shown in Appendix C. The temporal coincidence of pump

and probe pulses found with the interferometric method is precise within 100 fs due to small

required re-adjustments needed afterwards. Daily operation and measurements on highly

photoexcited n-GaAs(100) has proved a sudden shift of the Ga-3d peak at +100 fs. On this

basis I believe to find the pump-probe temporal coincidence in Fig. 6.10 at about +100 fs. In

Fig. 6.10 there are noticeable Ga-3d peak shifts for negative time delays. The reason for the

observed phenomenon could be found in the temporal pump pulse profile. The highly

dynamic autocorrelation measurements characterizing femtosecond pulses from CPA laser

systems365 show that the light intensity at 1 ps before the pulse arrival is some 10-6 of the

maximum intensity and at a few hundreds femtosecond some 10-3 of the maximum intensity.

Since the SPV effect is not linear136 with the pump intensity, at high pump fluences as used in

this study it tends to saturate. It is quite possible that already few hundreds femtoseconds

before the pump pulse arrival the intensities of some 10-3 of the pump pulse intensity are

contributing to the observed slight Ga-3d photopeak shifts. The Ga-3d shifts shown in

Fig. 6.10 are relative to the negative time delay at -800 fs. Fast carrier transport in the

photoexcited semiconductor bulk to the probed surface through an acceleration in the strong

SCR electric field manifests itself in a reduction of band-bending and a corresponding peak

shift within less then 500 fs after the pump pulse. The subsequent carrier relaxation by

recombination and trapping in surface states is observed to occur on a slower time scale148,151

of a few picoseconds. The dynamical behavior in Fig. 6.10 can be well fitted with a

convolution of an exponential decay and a gaussian function of the form

−+

−)

2(1

Tw

wterfe T

t

(6.4)


where is the error functionerf

15=

§. The fitting procedure based on the method of least squares is

yielding the mean values and their standard deviations of fs for the rise time

and ps for the decay time. Electrons transit times of about 500 fs in p-GaAs were

predicted by simple theoretical considerations

180600 ±=w

3±T139,145, assuming solely the electron drift with

the saturation velocity through the SCR, albeit for lower doping of 1018 cm-3 with a

correspondingly larger SCR. An extraction of more detailed information on the carrier

dynamics for the studied system will be obtained by solving the complex set of coupled

differential equations (2.19)-(2.21) connecting electron-hole creation, charge transport,

relaxation and modification of the electric field in the SCR by the transient charge

redistribution. For completeness the transient Ga-3d peak shifts for the n-GaAs(100) with

significantly lower dopant level is shown in Fig. 6.11. The observed Ga-3d peak shifts to

lower kinetic energies as has been expected. Surprisingly, the Ga-3d shift occurs within 400 fs

which is faster than for the p-GaAs surface. At this stage I need support by a theoretical

simulation to interpret the measured results.

§

Fig. 6.11 Temporal evolution of the Ga-3d peak’s center-of-gravity after photoexcitation of the

n-GaAs(100) surface.

Error function defined as ∫ −=x

t dtexerf0

22)(π

82 Chapter 7 – Conclusions and Outlook

Chapter 7

Conclusions and Outlook

In the framework of this thesis novel apparatus for the femtosecond time-resolved

photoelectron spectroscopy in the EUV region has been built. The CPA femtosecond laser

system based on regenerative and multipass amplification has been constructed and

characterised. By focusing the laser beam in the pulsed Ne gas jet the high harmonic has been

generated. The key position in the experimental apparatus is the low group velocity dispersion

EUV monochromator46 selecting single high harmonic without its temporal distortion. The

specially tailored multilayer low gamma Mo/Si mirrors47 has been applied for the selection

and refection of single high harmonic. Using the TOF electron spectroscopy and selected high

harmonic the first static photoelectron spectroscopy on the rare gases has been performed45.

The excellent spectral selectivity of the EUV monochromator has been investigated via

photoelectron spectroscopy of He gas. The UHV compatibility of this apparatus made the

photoelectron spectroscopy of the solid states surfaces also possible. The first photoelectron

and Auger electron spectra of the Pt(110) surface as well as Ga and As core-levels by the

photoemission from GaAs(100) surface has been observed. The chemical shift of the W 4f

core-levels has been studied on W(110) crystal surface and W(CO)6 adsorbate. The iodine

films 4d core-levels and valence band photoemission together with Auger electron spectra has

been observed. Also the Ga-3d core-levels and valence band photoelectron spectra of a

complex organic molecule Gaq3 has been investigated. The visible-pump/EUV-probe

technique has been implemented for the observation of the ultrafast hot electron relaxation on

Chapter 7 – Conclusions and Outlook 83

a Pt(110) surface48. The valence band electron dynamics of highly excited Pt surface has been

observed. The valence band depopulation effect due to the total excitation of 2.2% of all

valence band electrons has been examined. The following cooling of hot electrons

accompanied by the rise of crystal lattice temperate has been analysed. Due to short lifetime

of hot electrons of a few femtoseconds a visible-pump/EUV-probe cross-correlation has been

measured and constitutes the first UHV compatible technique for the measurement of

femtosecond high harmonic duration. The principle is based on the short living hot electrons

excited into the unoccupied states above the Fermi level and emitted with the time-delayed

high harmonic probe pulse. The visible-pump/EUV-probe time-resolved core-level

photoelectron spectroscopy has been utilized for tracking the charge carrier dynamics on

semiconductor surfaces. Transient changes of the surface photovoltage at the p-GaAs(100)

surface has been observed after photoexcitation with femtosecond pump pulses. The time

evolution of the band-bending has been probed by measuring the kinetic energy shifts of the

Ga-3d core-level photoelectrons after excitation with femtosecond EUV probe pulses. Carrier

transport from the bulk to the surface has been observed to occur within 500 fs after

photoexcitation while a subsequent relaxation has been determined to evolve on a time scale

of a few tens of picoseconds.

I hope that in this thesis I have convinced about usefulness, capability and great

potentials to address core-level electrons by the femtosecond time-resolved photoelectron

spectroscopy in the EUV region. An application of this experimental technique for the

observation of electron dynamics at high pump intensities on semiconductors surfaces366 and

the dynamics of chemical bonding of ordered organic layers on surfaces367 will give new

physical insights on these two promising branches of the ultrafast processes. It makes me

believe that my core-level based time-resolved experiment is predetermined to offer a deeper

view on highly photoexcited semiconductors surfaces not accessible with other time-resolved

techniques. The traditional time-resolved studies of the chemical bonds are based on the

changes induced in the bonding molecular orbitals. However, the complex organic molecules

consisting of large number of elements have the valence band structure which changes are not

easy to interpret and locate one particular site in the molecule where the chemical bond had

changed. The idea of “marker atom” embedded in the site of interest and application of

time-resolved photoelectron spectroscopy on the core-level electrons of this particular

“marker atom” will provide the information on the dynamics of complex molecular systems

in the very proximity of the “marker atom“ rather than on the “site-unspecific” changes of the

valence band.

84 Appendix

Appendix

A Knife-edge Beam Profile Measurement

The most simple technique of the EUV beam profile analysis is so-called knife-edge

technique. The EUV beam before the TOF electron spectrometer has been gradually blocked

by the knife-edge plate and the passing residual light has been measured with the MSP

intensified photocathode (see Fig. 3.1). The measured signal proportional to the EUV

intensity as a function of the knife-edge position is shown in Fig. A.1. On the assumption that

EUV beam has spatially homogenous Gaussian profile368 the measured data can be fitted with

a function

∫∞−

−=

xw

t dtewIxI2

0 2

22)( π (A.1)

where is the maximum of light intensity and is the full width at e0I w2 -2 of . Under these

conditions the fitted beamwidth is = 1.2 mm.

0I

w2

Fig. A.1 The measured EUV light intensity as a function of the knife-edge position. The grey

curve shows the corresponding fit on the assumption of the Gaussian beam profile.

Appendix 85

Appendix

B Auger Electron Spectroscopy on Pt, Cu and GaAs Surfaces

The Auger electron spectroscopy287 (AES) has been used to check on the quality of

surface cleanliness. The excitation electron beam with the kinetic energy of 1.25 keV has

been impinging under 75° to the surface normal. The AES spectra has been collected with

self-made retarding field analyser369.

Fig. B.1 The AES spectra of Pt(110) surface before (a) and after (b) cleaning procedure.

Thin vertical lines indicate the characteristic electron peaks of platinum and carbon.

The AES spectra of Pt(110) surface before and after cleaning are shown in Fig. B.1. The

initial carbon contamination has been removed during the cleaning procedure. In Fig. B.2 are

shown the AES spectra of Cu(100) surface before and after cleaning. In the region between

100 eV and 600 eV there are no characteristic lines of copper and only the main contaminants

like sulphur and carbon are visible. These were eliminated during the cleaning procedure. The

Fig. B.3 shows the AES spectra of GaAs(100) surface. Again in the measured region between

100 eV and 600 eV there are neither the characteristic lines of gallium nor arsenic. The carbon

86 Appendix

contaminants as well as native gallium and arsenic oxides were removed during the cleaning

procedure.

Fig. B.2 The AES spectra of Cu(100) before (a) and after (b) cleaning procedure. The

vertical lines show the position of electron peaks of carbon and sulphur contaminants.

Fig. B.3 The AES spectra of GaAs(100) before (a) and after (b) cleaning procedure. The

contamination due to carbon and native oxides show the vertical lines in the electron spectra.

Appendix 87

Appendix C Surface Photovoltage Transients and Surface Preparation

The SPV transients are highly sensitive probe of the semiconductor surface conditions. This is

demonstrated in Fig. C.1-C.3. The p-GaAs(100) and n-GaAs(100) surfaces has been prepared

as described in section 4.1. Although the preparation conditions varied only in a few minutes

in the duration of the sputtering and annealing times the behaviour of the SPV transients were

radically influenced. This is shown for p-GaAs(100) in Fig. C.1-C.2 (compare with Fig. 6.10).

This has been also observed for n-GaAs(100) in Fig. C.3 (compare with Fig. 6.11).

Fig. C.1 Temporal evolution of the Ga-3d peak’s center-of-gravity after photoexcitation of

the p-GaAs(100) surface.

88 Appendix


the p-GaAs(100) surface.


the n-GaAs(100) surface.

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Eidesstattliche Erklärung

Hiermit versichere ich, die vorliegende Arbeit eigenständig und ohne fremde Hilfe

durchgeführt zu haben. Alle benutzten Hilfsmittel sind in der Literaturliste kenntlich gemacht.

Bielefeld, den 19.03.2002 Peter Šiffalovič

Curriculum Vitae Personal Name : Peter Šiffalovič Date of birth : 23.05.1975 Place of birth : Bratislava Nationality: slovak Martial status: single Education 1982 – 1989 primary school in Bratislava 1989 – 1993 grammar school of J. Hronca in Bratislava 1993 – 1998 undergraduate study of physics – optics and optoelectronics at

Comenius University in Bratislava July 1996 practical training on femtosecond lasers at Technical University,

Vienna, Austria (prof. H. F. Kauffmann) May – Nov. 1997 scientific sojourn at Shizuoka University, Research Institute of

Electronics, Hamamatsu, Japan - research on nonlinear properties of organic crystals (prof. T. Nagamura)

July 1998 master in physics with diploma work - Nonlinear phenomena in χ(3) media

joined department of multilayers at Institute of Physics SAS, Bratislava (prof. Š. Luby and Dr. E. Majková)

since Sep. 1998 Ph.D. study at department of molecules and surfaces, University Bielefeld, Germany (prof. U. Heinzmann)

Awards 1995, 1996, 1997 “Laureate of student scientific conference” for works on acousto-optics,

CCD-imaging and measuring of ultrashort laser pulses via TPA

Documents

Femtosecond Time-Resolved Photoelectron Spectroscopy in ... · Betreuer: Prof. Dr. U. Heinzmann Priv. Doz. Dr. M. Drescher Gutachter: Prof. Dr. U. Heinzmann Priv. Doz. Dr. F. Stienkemeier