Lifetime enhancement of ZEKE states in molecular clusters and cluster fragmentation products with...

Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175–181www.elsevier.nl / locate /elspec

Lifetime enhancement of ZEKE states in molecular clusters andcluster fragmentation products with programmed electric pulses

*Udo Aigner, Leonid Ya. Baranov, Heinrich L. Selzle, Edward W. Schlag¨ ¨ ¨Institut f ur Physikalische und Theoretische Chemie, Technische Universitat Munchen, Lichtenbergstrasse 4, D-85748 Garching,

Germany

Received 8 March 2000; accepted 2 May 2000

Abstract

A new control scheme of fast switching of electric pulses is applied to Rydberg states of the benzene?Ar complexproducing highly stable long lived ZEKE states. Since this scheme permits complete external control of the production ofZEKE states it no longer depends on internal experimental parameters but can be applied to a much broader range ofexperiments where such parameters are often difficult to control such as synchrotron radiation experiments. This method caneven be applied for excitation of the ionic core far above the ionization limit, where these states even survive corefragmentation and can be further stabilized by fast electric pulses. 2000 Elsevier Science B.V. All rights reserved.

Keywords: ZEKE; Ionization; Fragmentation; Benzene; Cluster

1. Introduction signal can be lost due to stray fields. This can beseen most directly in the argon signal where depen-

2ZEKE spectroscopy has emerged now as a new dencies of the ZEKE signal below P ionization1 / 2

high resolution spectroscopy of molecular ions. The limit of argon depended on the electric field [1,2].spectroscopy in this high resolution form depends on This long time stability of the very high Rydbergthe formation of long lived zero kinetic energy states of molecules is thus of current interest for the(ZEKE) states which are dark states from optical spectroscopy of the eigenstates of molecular ions andhigh n Rydberg states. It has now been shown, that clusters. The unanticipated extreme lifetime of thesethese ZEKE states need not depend on vagaries of states which was first observed by Reiser et al. [3]the experiment but can now be controlled and indeed allows them to be detected in ZEKE spectroscopystabilized by external pulse programming. This is [4]. A large effort has been put to understand theimportant for new experiments particularly using nature of these long lived states and their interactionssynchrotron radiation, where this new stabilization with environmental factors connected to the ZEKEcan be of prime importance since the usual stabiliza- experiment like electric fields, ion densities, colli-tion from neighboring ions is negligible and thus the sional effects and intramolecular processes [5–26].

The current accepted explanation for the long life-time of these states is the Stark mixing of the l states*Corresponding author. Tel.: 149-89-2891-3390; fax: 149-89-induced by external stray electric fields and the m2891-3389. l

E-mail address: schlag@ch.tum.de (E.W. Schlag). mixing due to surrounding ions as it was given in the

0368-2048/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 00 )00211-5

176 U. Aigner et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175 –181

early work of Chupka [17] and others [27,28]. These In this work we now want to demonstrate thefields and ion contributions to the ZEKE experiment unusual stability of the ZEKE states obtained by thisvary between different experiments and there is a new technique for molecular clusters at the ioniza-persistent interest in finding direct methods for tion onset and even for the severe case of highlyprogrammed stabilization of Rydberg states using excited ionic core states far above the ionizationspecially tailored applied fields. limit leading to a fast dissociation of the ionic core,

There were several approaches and suggestions to which can be accompanied also by a fast autoioniza-this problem [18,26,29] and we have studied the tion. Rydberg states with a total energy above theelectric field effect on the lifetime of the ZEKE lowest ionization potential of the molecule or clusterstates experimentally [30] which could be confirmed can in principle autoionize. In an earlier experimentby theoretical computations [31]. The experiments we have shown that for the ZEKE states one has anwere performed with pulsed and static electric fields extreme decoupling of the electronic from the nu-of different magnitudes and various ion concen- clear motions and we could find very stable neutraltrations. From this it was possible to understand the ZEKE states in polyatomic molecules with lifetimesdifferences in previous experiments concerning of many tens of microseconds in three regimes —atomic and molecular ZEKE states and to give a direct below the ionization onset, above the ioniza-unified picture for the production of ZEKE states in tion onset, but just below an vibration in the ion andstatic electric fields and ionic environments [32]. for high intensity up-pumping to produce fragmentsThere exists a further need for a controlled method [38]. In all three cases ionization occurs uponfor the generation of these long lived states and application of a delayed DC field and demonstrates

¨Muhlpfordt et al. [21] found a moderate lifetime the extreme stability of the ZEKE states which areenhancement into the microsecond range for the produced from optical accessible low l RydbergRydberg states of DABCO in crossed magnetic and states. The decay channels for the ZEKE states areelectric fields. It is also possible to apply circularly given by the interaction with the ionic core. Thispolarized microwave fields which should mimic the coupling is weak for high n Rydberg states here theeffect of collisions with charged particles, as it was electron is the slowest moving particle in the systemshown by Jones et al. [33]. Here the autoionizing and the inverse Born–Oppenheimer description ap-Rydberg states with a circularly polarized field were plies [27,39]. From additional l and m mixing thisl

significantly longer than for linearly (l mixing only) coupling is further reduced and explains the anomal-polarized fields but only an approximately constant ous long lifetime of the up-pumped states far abovelifetime of 70 ps for n.18 was found. A more the ionization potential [7]. Therefore this system iscomplicated coherent control scheme for high n very much suitable to test the method of increasingRydberg lifetimes was proposed by Yu et al. [34] the longevity of such states by external controlledusing wavepacket technology. This scheme is based pulseson time-dependent, crossed electric and magnetic This work uses a simplified version of the controlfields and should be applicable to small signal ZEKE scheme whose detailed explanation can be found inspectroscopy and to Rydberg photofragment transla- Refs. [35] and [36]. In brief, the high n Starktional spectroscopy but is not yet realized in practice. manifolds are populated through photoexcitation in

21Recently we have introduced a new fast field presence of a 300 mV cm electric field which isswitching technique to produce these extremely long then switched off 10 ns after the laser excitation. Thelived ZEKE states with an external field [35,36] to non-selectively populated l-mixed Stark manifoldsovercome ambiguities of internal interactions and collapse into low-field l states (Fig. 1). The dominantinherent experimental perturbations. This method fraction of the Rydberg population is now caught inwhich was first shown for a small molecule like NO the non-penetrating high l subspace which is well[36] could also be tested for the severe case of the isolated from the unstable non-hydrogenic low l

2autoionizing Rydberg states converging to the P states at low field. This technique of inducing the1 / 2

ionization limit of xenon where it was possible to high yield of indefinitely long lived states free of lowproduce highly stable long lived ZEKE states in the l character was termed ‘l-locking’ in Refs. [35] andabsence of channel interactions [37]. [36]. The reason here for the in-field excitation is

U. Aigner et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175 –181 177

behavior would be expected for molecular clustersalthough due to the low frequency intermolecularmodes and a high density of vibrational states alarger coupling with the core could occur. Thisshould be the more true for high excited states abovethe ionization limit where fast dissociation can occurand fast autoionization rates are expected as it is seenin the photoinduced Rydberg ionization of fluoro-benzene [40] and chlorobenzene [41].

2. Experimental

The experimental arrangement of the ZEKE spec-trometer, described in detail in Ref. [7], consists of adoubly skimmed supersonic jet of benzene (10% inAr at 2 bar) interacting with two counterpropagatingpulsed laser beams between plates P1 and P2 (Fig.2). For this the second and third harmonic of aNd:YAG laser (Quanta Ray DCR1A) were used topump two separate dye lasers (Quanta Ray PDL1).

21The first frequency doubled laser at 38 588 cmFig. 1. Schematic drawing of a Stark map for a non-hydrogenicsystem. Photoexcitation occurs at a field above the onset of Stark

52]effect (right side). This limit is given by F ¯ m /n (a.u.),onset l3

where m is the quantum defect of the respective l sublevel [35].l

Here all high l Stark states are mixed and optical accessible. Thishigh l character is maintained by fast switching to zero field andno remixing with low l states can occur any more (left side). Thusthe optical excited high field states are locked in the high lsubspace which now have a long lifetime. In this scheme longlived ZEKE states are produced whereas at excitation at zero fieldonly l states with short lifetimes can be populated.

that the field-free non-penetrating states with high lcharacter are optically dark, while the bright stateswith low l decay on a fast timescale. The Starkcoupling during laser excitation to the low l statesenables to populate the high l states through theintensity borrowing, but the same coupling woulddrain all signal through the autoionization after some

Fig. 2. Schematic of the experimental setup. The laser excitationtens to a hundred of ns. That is why the Stark occurs between plates P1 and P2. Here plate P2 is at groundcoupling has to be turned off prior to the extensive potential and at plate P1 fast negative pulsed switched voltages aredecay of the Stark states. applied. For locking, this voltage is applied during the laser pulse

and immediately switched off after the laser. Again at 100 ns later,Initially, the l-locking was applied to the pre-a negative voltage is applied for removing the directly formeddissociating Rydberg states of nitric oxide [36],ions. This spoiling field is maintained until the point of extraction

where a high-intensity ZEKE signal at high n is into the RETOF between plates P2 and P3 with a positive voltageobserved even under ‘normal’ conditions, while upon applied on P3. Extraction is performed with a fast rising pulse onlocking this signal extends down to n545. A similar plate P2 29 ms after laser excitation.

178 U. Aigner et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175 –181

1excites the 6 vibrational S state of benzene in the lived ZEKE states just below the IP. The spectrum121benzene?Ar cluster. Transitions from this state shows a sharp onset on the blue at 35 801 cm and

21through the Rydberg manifold are accessed by a peak at about 35 798 cm . This has to bescanning the second frequency doubled laser in the corrected for the lowering of the IP by the spoiling

]21 21 21 21range from 35 760 cm to 35 830 cm from below field U of 300 mV cm (4 ? U cm , U in Vsp sp spœ21the ionization potential (IP) to above the threshold. cm ) which then together with the energy of the

21The laser intensities were adjusted so that the direct first laser of 38 588 cm corresponds to the ioniza-21ions produced through one-color absorptions of each tion potential of 74 389 cm of the complex. This is

21laser are negligible compared to that produced lowered by 122 cm corresponding to the barethrough two-color absorptions. The excited clusters benzene molecule as it was found before in REMPIdrift at the speed of the jet through the in-line ion experiments [43] and in accordance with PFI mea-optic plates where they are exposed to fields which surements of Krause et al. [44]. The ZEKE signal

21separate the promptly produced ‘background’ ions vanishes nearly completely 20 cm below the IPfrom the Rydberg neutrals. The ‘locking’ field is which demonstrates that effectively no ZEKE statesapplied with a voltage pulse at plate P1 and plate P2 below n575 survive for 29 ms under these con-hold at ground level. This voltage then is switched ditions. This result is in accordance with the lifetimeoff immediately after the laser pulse. The separating of Rydberg states of the bare benzene molecule.field in these experiments is generated from also Here Neuhauser et al. [45] have found a lifetime ofapplying a voltage at P1 which is delayed by 100 ns about 100 ns to 1.5 m for n varying between 65 ands

21with respect to laser excitation, thus during excita- 85 in a constant field of 200 mV cm .This picturetion, under nominally ‘field-free’ conditions, the only changes completely when a l locking pulse is

21field present is the stray field of the instrument applied. For this an electric field of 300 mV cm is21(20–40 mV cm ) [42]. A pulsed electric field of applied during laser excitation which is then

21200 V cm , applied between plates P2 and P3, 29 switched off immediately within 10 ns after the laserTs after laser excitation, ionizes the ZEKE neutrals with a fall time of about 5 ns. The cluster ex-which are extracted into the reflectron time-of-flight periences then a field free drift time of about 90 ns(RETOF) mass spectrometer. The signal from the after which the spoiling field is switched on as in theparent clusters and possible fragments is then ob- previous case. The spectra with and without thetained by putting separate detection gates at the locking pulse were obtained in a single scan, whererespective masses on the TOF signal. It was carefully at every wavelength of the scan in a ‘toggle’ modechecked by varying the spoiling field that no directly the locking pulse was applied for five lasershots andformed ions penetrate to the extraction region and was not present for another five lasershots. Thetherefore the observed ion signal arises solely from signal was averaged for both cases and allows for anfield ionized long-lived Rydberg states (ZEKE absolute signal ratio for both measuring conditions.states). It is immediately seen, that the signal for the very

high ZEKE states does not show an increase inintensity, but that the ZEKE band now extends to

3. Results and discussion lower n. In this case a strong ZEKE signal is seen21which does not stop at 20 cm below the IP but

21The delayed pulsed field ionization spectrum for extends much further than 35 cm which was thethe benzene?Ar cluster is shown in Fig. 3. The lower range of the scan in this experiment. This means thatspectrum is obtained under field free excitation with also in the case of molecular complexes the pro-only the residual stray field present. The spoiling grammed electric field pulses allow for stabilizationfield is applied 100 ns after and maintained until the far below n555 with lifetimes of many tens ofextraction pulse is applied after 29 ms. The obtained microseconds.

21ZEKE band shows a half width of about 10 cm In this experiment the excitation of the cluster iswhich is typical to such a spectrum and arises from performed in a two color scheme. The intensity ofthe unresolved rotational band contour and the long the first laser had to be sufficiently kept low to

U. Aigner et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175 –181 179

1Fig. 3. l locking of ZEKE states with fast electric pulses for the benzene?Ar cluster. The benzene?Ar cluster are excited via the 6 S1

intermediate state of the benzene molecule with a first laser and the region below the ionization potential is scanned with a second laser, as21indicated on the right side. Without the locking pulse applied no states survive below 20 cm below the ionization limit. If the locking

pulse is applied the spectrum extends much further to the red and shows that ZEKE state with n,55 are readily stabilized.

circumvent direct ionization with this laser which from the fragmentation was obtained in the samecan easily occur, as the intermediate state is above scan as for the benzene?Ar cluster and gives anhalf of the ionization energy and a one color two intensity ratio of about 25% for core fragmentation.photon absorption immediately leads to ions. There- This high fragmentation ratio indicates more than thefore, the intensity of the second laser had to be absorption of only one further photon and the widthincreased to obtain a reasonable ZEKE signal. This of the non-locked spectrum in Fig. 4, therefore, isintensity now is large enough not only to produce the partially due to simultaneously produced ions fromclusters in ZEKE states but there can also a second autoionization of the highly excited cluster which areor a third photon be absorbed which leads to an rejected by the spoiling field. The signal-to-noiseexcitation of the ionic core of at least 4.4 eV if only ratio is also much less than for the parent complex,one further photon is absorbed. This is far above the but here it is also seen, that the ZEKE signal

21dissociation limit of the van der Waals binding vanishes 20 cm below the IP from where up-energy of 71 meV for this benzene?Ar cluster [14] pumping has occurred. It is interesting to see, thatand the up-pumping from the originally excited the spectrum with the l locking pulse present nowZEKE states will lead to van der Waal’s fragmenta- also extends to lower energies, but is not as pro-tion. Therefore, putting the mass gate to the bare nounced as in the parent cluster case. The stabiliza-

21benzene molecule a similar ZEKE spectrum is tion even with the small pulse of 300 mV cm isobtained (Fig. 4) which shows the fingerprint of the achieved for low n states below n555. The lockingbenzene?Ar complex. The spectra for the benzene of ZEKE states even in the case of the core frag-

180 U. Aigner et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175 –181

Fig. 4. l locking of ZEKE states with fast electric pulses in the case of cluster fragmentation of the benzene?Ar complex. The benzene?Ar1cluster are excited via the 6 S intermediate state of the benzene molecule with a first laser and the region below the ionization potential is1

scanned with a second laser, as indicated on the right side. Due to the high intensity of this second laser, up-pumping from the Rydbergstates below the IP occurs which leads to cluster fragmentation. Even under these conditions the ZEKE states survive. Without the locking

21pulse applied this is limited to states for up-pumping not below 20 cm to the ionization limit. If the locking pulse is applied the spectrumextends much further to the red and shows that ZEKE state from up-pumping from Rydberg states with n,55 are readily stabilized.

mentation is interesting, as it shows, that the high l of the neutral ZEKE states. It is interesting to notecharacter present during excitation is maintained that even small programmed pulsed fields of only

21during the dissociation process and remains for the 300 mV cm can stabilize ZEKE states down tofragment in a high n Rydberg state. The core states with a principle quantum number of less thenfragmentation must also have occurred before the 55, even if the parent molecular cluster undergoes alocking field was switched off to allow for the dramatic change in composition. The reason of thislocking in high l states to zero field states. is given by the fact that the l locking is achieved for

In summary we have demonstrated that the new l fields above the onset of the linear Stark effect forlocking technique with fast programmed electric the corresponding n Rydberg state. This field gets thepulses is quite universal for stabilizing ZEKE states lower, the higher the possible angular momentum ofand can be applied to a wide variety of systems, the Rydberg state is which is accessible in the zerofrom small molecules to fast autoionizing atoms and field case due to the decrease of the quantum defecteven clusters with high core excitation and dissocia- of higher l states [35]. This value for p and d statestion. The initially prepared Stark states in an electric is often small and allows for small fields for stabili-field are locked in their high l character at low fields zation far below the Inglis–Teller limit for mixingmaintaining the high l character which makes them different n states. This can be employed to stabilizeimmune against remixing to low l states which optically accessible lower n Rydberg states to longinteract with the core and would lead to a fast decay lived ZEKE states which is here more important than

U. Aigner et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 175 –181 181

[14] H. Krause, H.J. Neusser, J. Chem. Phys. 99 (1993) 6278.for high n Rydberg states, as the core penetration is[15] J. Chevaleyre, C. Bordas, M. Broyer, P. Labastie, Phys. Rev.more effective due to the smaller Rydberg radius

Lett. 57 (1986) 3027.2which scales with n . [16] C. Bordas, P.F. Brevet, M. Broyer, L. Chevaleyre, P.The method described here can easily be included Labastie, J.P. Perrot, Phys. Rev. Lett. 60 (1988) 917.

into a practical ZEKE (MATI) spectrometer without [17] W.A. Chupka, J. Chem. Phys. 98 (1993) 4520.[18] F. Merkt, R.N. Zare, J. Chem. Phys. 101 (1994) 3495.interfering with the goals of high accuracy and[19] E. Lee, D. Farelly, T. Uzer, Chem. Phys. Lett. 231 (1994)resolution. It overcomes the problems from ill-de-

241.fined and randomly oriented stray fields. It also [20] E. Rabani, L.Y. Baranov, R.D. Levine, U. Even, Chem. Phys.allows to lock a dominant fraction of the total Lett. 221 (1994) 473.

¨population of primarily generated Rydberg states in [21] A. Muhlpfordt, U. Even, E. Rabani, R.D. Levine, Phys. Rev.A 51 (1995) 3922.sufficiently high l states and by further modification

[22] F. Merkt, S.R. Mackenzie, T.P. Softley, J. Chem. Phys. 103in high m states to ensure a high yield of ultimatelyl (1995) 4509.long lived ZEKE states. [23] M.J.J. Vrakking, Y.T. Lee, Phys. Rev. A 51 (1995) R894.

[24] M.J.J. Vrakking, Y.T. Lee, J. Chem. Phys. 102 (1995) 8818.[25] M.J.J. Vrakking, Y.T. Lee, J. Chem. Phys. 102 (1995) 8833.[26] H. Palm, R. Signorell, F. Merkt, Philos. Trans. R. Soc. Ser.Acknowledgements

A 355 (1997) 1551.[27] S.D. Chao, M. Hayashi, S.H. Lin, E.W. Schlag, J. Chin.

This work was supported by the SFB 377 of the Chem. Soc. 45 (1998) 491.Deutsche Forschungsgemeinschaft. LB acknowl- [28] S.D. Chao, M. Hayashi, S.H. Lin, E.W. Schlag, J. Phys. B:

¨edges the Claussen-Stiftung im Stifterverband fur die At. Mol. Opt. Phys. 31 (1998) 2007.[29] F. Merkt, H.H. Fielding, T.P. Softley, Chem. Phys. Lett. 202Deutsche Wissenschaft for support through a re-

(1993) 153.search fellowship.[30] A. Held, H.L. Selzle, E.W. Schlag, J. Phys. Chem. 100

(1996) 15314.[31] F. Remacle, R.D. Levine, E.W. Schlag, H.L. Selzle, A. Held,

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