HANDBUCH DER PHYSIK
M I T 754 F I G U RE N
SPRINGER-VERLAG BERLIN HEIDELBERG GMBH 1958
ISBN 978-3-662-35394-3 ISBN 978-3-662-35393-6 (eBook) DOI 10.1007/978-3-662-35393-6
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Originally published by Springer-Verlag OHG. Berlin • Göttingen · Heidelberg in 1958
Softcover reprint of the bardeover ISt edition 1958
Die Wiedergabe von Gebrauchsnamen, Handelsnamen, Warenbezeichnungen usw. in diesem Werk berechtigt auch ohne besondere Kennzeichnung nicht zu der Annahme, daß solche Namen ini Sinn der Warenzeichen· und Markenschutz· Gesetzgebung als frei zu betrachten wären und daher von jedermann benutzt
werden dürften.
Inhaltsverzeichnis.
Luminescence. By G. F. ] . GARLICK, Professor of Physics, Department of Physics, University of Hull, Yorkshire (Great Britain). (With 135 Figures)
I. lntroduction . . . . . . . . . . . . . . . .
Seite
a) Luminescence and its associated phenomena . . . . . 1 b) Early theories of luminescence mechanisms . . . . . 2 c) Growth of the modern theory of luminescence . . . . 5 d) The nature and charai::teristics of luminescence centres 9
II. Classes and characteristics of phosphors . . 11 a) Luminescence in monatomic solids . . . 11 b) Alkalihalide and other halide phosphors 16 c) Luminescence in some oxide phosphors 27 d) Zinc and cadmium sulphide phosphors 31 e) AlkaHne earth sulphide phosphors . 38 f) Silicate and germanate phosphors 40 g) Phosphate phosphors . . . . . . . 4 5 h) Tungstate and molybdate phosphors 47 i) Divalent manganese as a luminescence activator 49 j) Ultravialet and infra-red emitting phosphors. SO
k) Miscel!aneous phosphors . . . . . . . . . 51 1) Luminescence in organic solids and liquids. 53
111. Theories of luminescence processes in solids. . 60 a) Processes within the luminescence centres . 60 b) Phosphorescence and thermoluminescence in photoconducting phosphors 65 c) Luminescence and non-radiative processes in photoconducting phosphors 73 d) Stimulation and quenching of luminescence in photoconducting phosphors 78 e) Sensitisation of luminescence: energy transfer in non-photoconducting
phosphors . . . . . . . . . . . 81 f) The polarisation of luminescence . 85
IV. The electrical properlies of phosphors 89 a) Photoconductivity in phosphors . 89 b) The photodielectric effect in phosphors 94 c) Semiconduction in phosphors . . . . 97 d) Electroluminescence . . . . . . . . 98 e) Electrophotoluminescence . . . . . . 106 f) Luminescence due to minority carrier injection. 107 g) Electron emission from phosphors 110
V. Cathodo- and radio-luminescence . . . . . . . . 111 a) Studies of cathodoluminescence . . . . . . . 111 b) Radioluminescence: excitation of single crystals by high energy particles 115
VI. Some experimental techniques in luminescence measurements 121 a) General methods . . . . . . . . . . . . . . . . . . . 121 b) Measurement of decay times greater than microseconds . . . 126 c) Measurements of long decay times and of thermoluminescence 127
General bibliography . . . . . . . . . . . . . • . . . 128
Temperature Radiation of Solids. By Dr. G. A. W.RuTGERS, Physicist in KEMA La- boratories, Arnhem (Netherlands). (With 45 Figures) 129
A. Introduction . . . . 129
II. Black-body conditions . 137
VI Inhaltsverzeichnis.
Seite III. The radiation constants . . . . . . . . . 139 IV. International temperature scale above 1336° K 141 V. Radiation data . . . 141
C. Radiationfrommetals . . 143 D. Radiation from non-metals 161
General references. . . . 170
RAMAN Effect. By SAN-ICHIRO MIZUSHIMA, Professor of Physical Chemistry, Tokyo University, Tokyo (Japan). (With 19 Figures) . . 171
I. Introduction and experimental techniques 171 Il. Vibration of molecules . . . . . . . . . 174
111. Vibration spectra . . . . . . . . . . . 196 IV. Rotation spectra and vibration rotation spectra. 210 V. RAMAN effect in the liquid and solid states . . . 219
VI. Calculation of thermodynamic functions from spectroscopic data 222 VII. Structure of simple molecules . 229
Concluding remarks 242 General references. . . . . . . . 242
Spectroscopie dans l'infrarouge. Par JEAN LECOMTE, Professeur a la Sorbonne, La- boratoire des Recherehes Physiques, Paris (France). (Avec 555 Figures) 244 A. Introduction . . . . . . . . . . . . . . . 244 B. Methodes utilisees en spectrometrie infrarouge 247
I. Recepteur . . . . . . . . 248 a) Recepteurs selectifs . . . 249 b) Recepteurs non selectifs . 255
II. Sources d'infrarouge . . . . 262 III. Separation des radiations 266
a) Spectrographes a prisme 266 b) Spectrographes a reseau . 282 c) Interferometrie dans l'infrarouge 293 d) Dispositifs spectrographiques speciaux 294 e) Methodes speciales pour la separation des radiations infrarouges 298 f) Obtention des spectres de reflexion . . . . . . . . . . . . . 308 g) Accessoires des spectrographes . . . . . . . . . . . . . . . 310 h) Preparation des echantillons de substances pour les spectres d'absorption 317
c. Remarques generales sur les spectres d'absorption Oll de reflexion infrarouges 326
D. Spectres d'absorption infrarouges de composes organiques 343
I. Methane et deuteromethanes . 34 3 Il. Derives halogenes du methane 354
III. Ethane et deutero-ethanes . . 366 IV. Derives halogenes de l'ethane. 372 V. Carbures satures aliphatiques . 385
VI. Derives ethyleniques. . . . . 405 VII. Derives acetyleniques • . . . 428
VIII. Benz€me et derives benzeniques . 438 a) Benz€me. . . . . . 438 b) Deutero-benz€mes. . 452 c) Carbures benzeniques 453
IX. Cyclanes et cycUmes . 459 X. Alcools et phenols • 469
a) Alcools . . . . . 469 b) Phenols . . . . . 486
XI. cetones et aldehydes . 491 a) Cetones . . . . . 491 b) Aldehydes . . . . 518
XII. Acides carboxyliques . 520
XIV. Ethers-oxydes, ozonides, peroxydes XV. Amines et imines .
XVI. Amides ...... . a) Amides simples . . b) Amides complexes. c) Polypeptideset proteines d) Lactames . . . . . . . e) Urethannes ...... .
VII
Seite
570 570 578 579 587 587
XVII. Acides amines et leurs chlorhydrates . 588 XVIII. Fonctions azotees . . . . . . . . . 593
XIX. Heterocycles . . . . . . . . . . . 606 XX. Composes organiques du soufre et du selenium. 614
XXI. Composes organiques du phosphore . . . . . 619 XXII. Composes organometalliques . . . . . . . . 621
E. Spectres d'absorption infrarouges de composes mineraux . 633 I. Corps simples . . . . . . . . . . . . . 633
II. Combinaisons du carbone et de l'oxygene . . . . 653 III. Ozone 0 3 • • • • • • • • . . . • • • • • . . 669 IV. Combinaisons de l'oxygene avec !'hydrogene (ou ses isotopes) 671 V. Ammoniac-phosphine-arsine-stibine 705
VI. Fonctions azotees . . . . . 723 VII. Hydraeides halogenes 752
VIII. Combinaisons des halogenes . 765 IX. Composes du bore . . . . . 770 X. Composes du soufre, du selenium et du tellure. 775
XI. Composes du phosphore 788 XII. Oxydes metalliques . . 789
XIII. Seleniures et sulfures. . 794 XIV. Halogenures metalliques 797 XV. Sels metalliques d'acides mineraux. 808
XVI. Silicates . . . . . . . . . . . . 830
F. Application des spectres d'absorption infrarouges a l'analyse des melanges 849 I. Analyse de melanges avec des radiations dispersees. . . . . 849
II. Analyse des melanges sans dispersion des radiations . . . . 871
G. Intensites absolues des bandesoudes raies d'absorption infrarouges . 875 I. Generalites et mesures des intensites 875
II. Resultats experimentaux . . . . . . . 887 III. Mesure de la largeur des raies . . . . . 895 IV. Moments electriques et leurs variations . 898
H. Dispersion dans le spectre infrarouge . . . . . 908 I. Principales methodes pour la mesure des indices de refraction 908
II. Resultats sur les indices de refraction 917 III. Formnies de dispersion . 925
Bibliographie . . . . . . . . . . . . . . . 934 Ouvrages generaux . . . . . . . . . . . . 934 Articles generaux de revues scientifiques et numeros speciaux 935 Collections de spectres et fiches bibliographiques 937
Sachverzeichnis (Deutsch-Englisch) 938
Luminescence. By
I. Introduction. a) Luminescence and its associated phenomena.
1. The definition of luminescence. Luminescence has not always been properly defined in previous literature, as has been pointed out in a recent book by ADIRO­
VICH1. The definition given by WIEDEMANN 2 and adopted by the author pre­ viously3 states that luminescence is a general term for the emission of light from a substance during or following the absorption of energy such as that of ultra violet radiation or high energy particles. However, luminescence must be distinguished from thermal radiation since it does not follow KrRCHHOFF's law. The energy density of luminescence emission per unit wavelength interval is always !arger than that which would result from thermal radiation at the par­ ticular temperature of the luminescent material. lt is fairly easy to distinguish luminescence from neighbouring physical processes, such as the RAMAN and CaMPTON effects, because of the time delay in the luminescence emission after excitation which is greater than 10-9 sec. The RAMAN and CaMPTON effects are completed in a time of about 1 o-14 sec or less.
2. Fluorescence, phosphorescence and other phenomena. Two of the most important aspects of luminescence are designated by the terms fluorescence and phosphorescence. Originally fluorescence was the name given to the emission from natural fluorites, while phosphorescence or afterglow was a general descrip­ tion of emission persisting for long times after removal of exciting radiation. In earlier investigations of luminescence in gas discharges at low pressures and of liquid solutions fluorescence was specifically applied to the emission character­ istics determined solely by optical transitions, while phosphorescence was re­ served for the temperature dependent decay of luminescence due to recombina­ tion processes of a bimolecular character, as discussed later in Sect. 5. Such definitions in terms of physical processes are unfortunate since under excitation and afterwards emission may be a combination of both processes. For example, in diamond forbidden optical transitions determine the form of the luminescence decay with a half-life of ~ 105 sec, while in luminescent zinc and cadmium sul­ phides strongly temperature dependent recombination processes predominate from a fraction of a microsecond up to hours or days after cessation of excitation. The afterglow in diamond would thus, according to PRINGSHEIM 4, be called slow fluorescence. The writer has suggested previously 3 that it is much more satis­ factory to define fluorescence and phosphorescence in terms of physical conditions
1 E. I. AmRovrcH: Einige Fragen zur Theorie der Lumineszenz der Kristalle. Akad. Wiss., Berlin 1954.
2 E. WIEDEMANN: Wied. Ann. 37, 177 (1889). 3 G. F. J. GARLICK: Luminescent Materials. Oxford 1949. 4 P. PRINGSHEIM: Fluorescence and Phosphorescence. New York 1950.
Handbuch der Physik, Bd. XXVI.
2 G. F. J. GARLICK: Luminescence. Sect. 3.
of experiment in view of the complex nature of decay processes revealed in modern researches. We thus define fluorescence as the luminescence emitted during excitation and phosphorescence as that emitted after excitation is removed. For nearly all practical purposes the division between the two may be fixed at 10-8 sec after excitation ceases, this being about the mean life-time of excited atoms in the gaseous state. In this article we shall be concerned only with luminescence in liquids and solids. Phosphorescence is rarely observed in the former unless viscosities are high or a polymerisation occurs1. In solids phosphorescence of very long duration can occur. Closely related to this is the phenomenon of thermoluminescence. If some long-afterglow solids are cooled to a low enough temperature, excited and then warmed in the dark they emit luminescence as the temperature rises. This effect is known as thermoluminescence. The de­ signation is not a very logical one since the prefix suggests that heat is the primary excitant. LEVERENZ 2 prefers the term thermostimulation but this purist defini­ tion has not had much popularity and thermoluminescence or more simply thermal glow is mostly used. Phosphorescence and thermoluminescence are due to one and the same process, as shown later, the only difference being the fixed and rising temperature respectively of the emitter during the observation of emission. Many phosphorescent and thermoluminescent solids can have their emission enhanced or stimulated by long wavelength visible or infra-red radiation the effect being known as optical stimulation (German: Ausleuchtung). Diminution in afterglow due to such irradiation can sometimes occur and is simply termed optical quenching ( German: Tilgung). Stimulation or queuehing can sometimes be produced by application of a strong electric field as shown in Chap. IV e. The primary excitation of emission by an applied electric field is called electrolumin­ escence and in recent years a simple form of electric lamp has been devised by making the luminescent material the dielectric of a condenser with one or both electrodes of transparent conducting glass.
Luminescence may be accompanied by other phenomena such as photo­ conductivity, semiconduction or photochemical reactions.
To obtain a more specific definition luminescence may have a descriptive prefix as in the following cases :
(i) Photoluminescence-that produced by absorption of photons of several electron volts or less (e.g. ultra violet radiation).
(ii) Radioluminescence-that produced by bombardment of a phosphor with high energy particles or radiation (e.g. y-rays).
(iii) Cathodoluminescence-a specific case of radioluminescence, produced by cathode rays.
(iv) Sonoluminescence-emission produced by sound waves usually of the ultrasonic variety.
(v) Cherniluminescence-occurs as a product of a chemical reaction. (vi) Bioluminescence-chemiluminescence in living organisms. (vii) Triboluminescence-emission resulting from mechanical strain and frac­
ture of certain phosphor crystals. Most noticeable in solids containing traces of manganese impurity. An effect not very amenable to precise study.
b) Early theories of luminescence mechanisms. 3. Synthesis of phosphors. The growth of interest in the mechanisms of
luminescence made rapid pace with the development of synthesised phosphors of relatively high efficiency, in particular the sulphides of zinc and cadmium and
1 G. F. J. GARLICK: Luminescent Materials. Oxford 1949. 2 H. W. LEVERENZ: Luminescence of Solids. New York 1956.
Sects. 4, s. Bimolecular processes. 3
of the alkaline earths with various impurities which "activated" the materials to give luminescence. Details of phosphor development and the study of lu­ minescence mechanism prior to 1930 may be found in the articles on luminescence in previous Handbuch volumes1 andin the books by PRINGSHEIM 2. As the general ideas on phosphor preparation and constitution in relation to luminescence pro­ perlies have undergone such radical changes in recent years, this earlier work will not be reviewed here. Some indication of the transition in ideas on the sulphide phosphors may be found in the author's book 3.
Although there has been a corresponding development in the understanding of luminescence mechanisms some of the early theories are still basic to modern hypotheses and so we consider them briefly below. In his studies with a phos­ phoroscope wheel BECQUEREL 4 distinguished two main types of decay of lumin­ escence with time analogaus to mono- and bimolecular chemical reactions re­ spectively.
4. The monomolecular decay of luminescence. In the siruplest case the finite delay in emission of absorbed energy is due to the life-time of the excited state of the emitting atom, ion or molecule, the latter being called the luminescence centre. Thus if the transition probability is p for return of the centre to its ground state and n centres are excited at any instant, J being excited per sec by the incident radiation, we have for the kinetic equation:
dn -=J-pn dt . (4.1)
The luminescence intensity L is given by pn and so, on commencing excitation, the emission rises according to the following equation which is the solution of Eq. (4.1):
(4.2)
where L0 is the emission intensity at t = oo under steady excitation. The decay of luminescence is given by the solution of Eq. ( 4.1) with J = 0 which is:
L = L 0 e-P 1• (4.3)
The important characteristic of the decay is its exponential form, the decay rate depending little on temperature (however, see below) and being given by the spectroscopic transition probability between the two electronic energy states involved. For electric dipole transitions p is :2:: 106/sec, for electric quadripole or magnetic dipole >::;; 104/sec and for higher multipale transitions of even lower value (e.g. diamond with p >::;; 10-5/sec).
5. Bimolecular processes. In many synthesised phosphors it was observed by early workers that the phosphorescence decay was of hyperbolic form and that luminescence was closely associated with the occurrence of photoconduct­ ivity in the material. From these facts it was inferred that the excitation of luminescence involved the liberation of charge carriers within the phosphor and that luminescence emission was conditional on the return of these carriers to their normal states by recombination with the vacated luminescence centres.
I P. LENARD, F. ScHMIDT and R. ToMASCHEK: Handbuch der Experimentalphysik (Editors: WIEN und HARMs), vol. 23, parts 1 and 2, 1928.
2 P. PRINGSHEIM: Fluorescence and Phosphorescence. N ew Y ork 19 50. 3 G. F. J. GARLICK: Luminescent Materials. Oxford 1949. 4 E. BECQUEREL: La Lumiere. Paris 1867.
1*
4 G. F. J. GARLicK: Luminescence. Sect. 5·
If no other rate determining process than that of the carrier recombination with an empty emission centre is involved, the reaction kinetics follow the form for a simple bimolecular chemical reaction as follows:
If n carriers are separated from centres at any instant, then n empty centres exist. If J centres are emptied per second by excitation the process is given by:
!!!__ = J- ßn2 dt
(5.1)
where ß is the recombination coefficient fo.r the free carriers and empty centres. L is given now by the term ß n2 this being assumed to be the rate determining step. Solution of this equation for the rise of fluorescence with time from the initial onset of radiation gives:
L = J(Tan V ß J · t) 2 , (5.2)
while the solution giving decay of emission is:
(5.3)
(5.4)
where oc =V ß L 0 • Derivations can be made for the photoconductivity, this depend­ ing on n and not on n2 (for the treatment of photoconductivity in general see the author's contribution to Vol. XIX of this Encyclopedia). The rapidity of the decay in this case depends on the initial intensity of luminescence and on the coefficient ß. The latter determines the temperature dependence of the decay process since it is given by the product of the capture cross-section of an empty centre and the mean thermal velocity of the liberated carrier, the latter being proportional to the square-root of the absolute temperature. Thus the decay speeds up as the temperature is raised.
It is very difficult to find phosphors with such simple kinetics. In any case the long duration of phosphorescence in sulphides (hours, days or weeks) could hardly be explained as due to a long life-time of free carriers and a small recom­ bination coefficient, as previously discussed by the writer1• 2• In addition the usual form of the decay in these phosphors is as follows:
L = Lo (1 +r:xW· (5.5)
where x is rarely equal to two and is usually between one and two. It is possible to construct other models for the decay due to recombination. If there are in the unexcited phosphor a nurober of empty luminescence centres v, then the kinetic equations become:
dn dt = J - ß (n + v) n (5.6)
as proposed by VAVILOV 3. When n is large compared to v (that is, high excita­ tion densities) then the bimolecular form holds, but at long decay times where n ~ v or at very low excitation intensities the kinetics become monomolecular in form.
1 G. F. J. GARLieK: Luminescent Materials. Oxford 1949. 2 G. F. J. GARLICK and M. H. F. WILKINS: Proc. Roy. Soc. Lond., Ser. A 184,408 (1945). 3 I. VAVILov: Phys. Z. Sowjet 5. 369 (1934).
Sect. 6. The energy band theory of crystalline phosphors. 5
Values of x less than 2 in Eq. (5.5) could be explained if polymolecular reac­ tion kinetics were assumed, e.g.:
dn -=J -ßnY dt
(5.7)
where y=xf(x-1). However, LENARD1 proposed that the basic kinetics of luminescence in the alkaline earth sulphides (LENARD phosphors) were mono­ molecular and that the hyperbolic form observed was due to the summation of centre processes with a range of p values. It is interesting to note that this idea appears again, with much more experimental F
justification in the modern theory of phosphores- w \1 cence proposed by RANDALL and WILKINS 2 and described in Chap. III. '---+--f-..)L_--+--:H
An important step in the theory of phosphores­ cence was that made in 193 5 by J ABLONSKI 3 for the decay of emission from organic molecules in rigid media (e.g. fluorescein dispersed in boric acid). He proposed an energy level scheme for the "emitting centre" as shown in Fig. 1. The fluorescence is given by the transitions between the excited state F and the ground state G, but excited molecules or centres may relax to the metastable state M, transitions M-E-+ G being forbidden. To return to the normal state the transition M-+ F must be effected by thermal activation. Thus phosphorescence emission due to F -+G transitions is conditioned in rate by
--~-~--~--,0
Fig. 1. Energy states in a lumines­ cence centre. G ground state; F ex­ cited state; M metastable state (after
}ABLONSK1 8).
the thermal process M -+F. If the energy required for the latter is W, then the probability per second that it occurs is p, given by:
p = sexp (- WfkT). (5.8)
Thus the decay kinetics are as in Eq. (4.3) but with p given by Eq. (5.8), i.e. the decay rate is an exponential function of temperature. Such a form of decay is found in organic systems (see Chap. III) and was found for the thallous ion centres in potassium chloride containing thallium as an impurity by BüNGER and FLECHSIG 4• Furthertreatment of this form of decay is given later in Chap. III.
c) Growth of the modern theory of luminescence. 6. The energy band theory of crystalline phosphors. Many important aspects
of luminescence remained inadequately interpreted until the development of the modern wave mechanical theory of solids. One of these was the temperature dependent decay of phosphorescence, another the exact nature of many lumin­ escence emission centres and another the transfer of energy through the phosphor crystals without the use of liberated charge carriers. In 1928 BLOCH 5 published what is often known as the collective electron model for the energy states of a perfect crystal lattice. The further extension of this model to semiconductors,
1 P. LENARD, F. SCHMIDT and R. TOMASCHEK: Handbuch der Experimentalphysik, (Editors WIEN und HARMs), vol. 23, parts 1 and 2, 1928.
2 J. T. RANDALL and M. H. F. WILKINS: Proc. Roy. Soc. Lond., Ser. A 184, 366 (1945). 3 A. JABLONSKI: Z. Physik 94, 38 (1935). 'W. BUNGERand W. FLECHSIG: Z. Physik 67, 42 (1931). 6 F. BLocH: Z. Physik 52, 555 (1928).
6 G. F. J. GARLICK: Luminescence. Sect. 6.
phosphors, the latent image process in silver halides etc. has been reviewed by Morr and GURNEY 1 and by SEITz 2 who have made notable contributions them­ selves.
For a perfect crystallattice of simple ionic or homopolar binding (e.g. KCl or diamond respectively) the energy states of the electrons of the constituent ions or atoms are not discrete and localised at the latter but extend throughout the crystal falling into quasi continuous bands of energy levels separated by zones of forbidden energy states. In phosphors we are concerned with the valence electrons of the original constituents, and these fill the highest occupied energy band as shown in Fig. 2. Impurities introduced to produce emission centres for luminescence will usually give rise to discrete localised states (L) in the forbidden zone above the filled band (F) with the ground state of each centre occupied.
T
A
Fig. 2. Energy band model for a photo­ conducting phosphor. A lattice absorption; L luminescence centre; T electron trap; F va-
lence or filled band; C conduction band.
Other impurities and the presence of vacant lattice sites or other lattice defects and irregu-
r: larities will provide unoccupied levels in the forbidden zone (T) in which electrons excited by energy absorption can be trapped. The ener­ gy band scheme thus built up and shown in Fig. 2 was first proposed by RIEHL and ScHöN 3
and independently by J OHNSON 4 to explain the long duration phosphorescence of the sulphide
f
and silicate phosphors. We first consider the model qualitatively
leaving recent quantitative treatment to later sections. Excitation may take place by photon absorption in the matrix lattice resulting in the transition A of Fig. 2. By this means, electrons are raised into the lowest unoccu- pied band of lattice energies, known as the
conduction band, in which they can migrate freely and give a current under the action of an applied electric field. The states in the filled band vacated by the electrons are also mobile and behave like positively charged electrons with a mass equal to or greater than the conduction electron mass which usually is about that of a free electron. These entities are known as positive holes. They migrate to emission centres and capture the ground state electrons thus empty­ ing the centres and rendering them available for capture of and recombination with conduction electrons. Thus, although excitation is in the matrix lattice the subsequent emission is characteristic of the emission centres. Some electrons excited into conduction levels will be captured by the defect or impurity states T and will only return to their normal states via emission centres by being first raised into the conduction band by thermal activation, optical activation (i.e. stimulation) or by application of a strong electric field. However, should there be mobile positive holes in the filled band then these can migrate and become captured in the attractive CoULOMB fields of trapped electrons. The electrons and holes then recombine the excess energy being given up to the crystal lattice as vibrational quanta or, at low enough temperatures, by radiative emission char­ acteristic of the trapping state.
1 N. F. MaTT and R. W. GuRNEY: Electronic Processes in Ionic Crystals. Oxford 1940. 2 F. SEITZ: Modem Theory of Solids. New York 1940. 3 N. RIEHL and M. ScHöN: Z. Physik 114, 682 (1939). ' R. P. JoHNSaN: J. Opt. Soc. Amer. 29, 387 (1939).
Sect. 7- The absorption spectrum of a crystalline phosphor. 7
7. The absorption spectrum of a crystalline phosphor. As shown above, lu­ minescence excitation can be effected by absorption of photons in the matrix Iattice. The absorption limit should thus be set by the width of the forbidden zone. However, even in a perfect lattice absorp- tion of lower energy photons may create bound electron-positive hole pairs which .l!l are mobile. The energy states for such ~ pdirs of excitons, as they are called, form ~ A a hydrogen like series below the conduc- ~ tion levels1 . Thus, if excitons can be pro- duced then the lattice absorption edge may not correspond to band-to-band transitions. For the present neglecting exciton forma­ tion, the energy states of Fig. 2 will give rise to an absorption spectrum for a phos­ phor as shown in Fig. 3. At Ionger wave­ lengths than that of the absorption edge excitation will raise electrons from emis-
r:
Fig. 3. Absorption spectrum of a ZnS-MnS (2%) phosphor. A !attice absorption; B absorption in manganese centres giving conduction electrons; C ab-
sorption confined to manganE'se centres (alter KRÖGER ')
sion centres into the conduction band (B). If the centres have excited states, then absorption bands due to transitions to these states will occur in the same regi0n (C). However, such transitions will confine the luminescence process to
f()
/Otl(/{}A f.;(J{JIJ
Fig. 4. Phosphorescence decay curves for ZnS-MnS (2%) at room temperature. A excited by 0.365 IJ.-relative intensityt; B cxcited by 0.365 11--re!ative intensity 9; C excited by 0.4361J.-relative intensity 9; D excited by 0.4361J.-relative
intensity t (alter GARLieK and WILKINS, ref. 2, p. 4).
Fig. s. Change in absorption spectrum ol a SrS-Eu-Sm phosphor due to excitation. A unexcited phosphor; B excited phosphor (alter URBACH et coll., rel. 3, p. 8).
the emission centres, and trapping of the electrons in the states of Fig. 2 will not occur. Fig. 3 is the absorption spectrum of a zinc sulphide phosphor with Mn2+ ions as the emission centres 2• To illustrate the effect of absorption on the luminescence process, we give in Fig. 4 the decay of luminescence of such a
1 J. FRENKEL: Phys. Rev. 37, 17 (1931). 2 F. A. KRöGER: Physica, Haag 6, 764 (1939).
8 G. F. J. GARLICK: Luminescence. Sect. 8.
phosphor for different exciting wavelengths1. When excitation is confined to the luminescence centres (Äexc=4358 A) the afterglow (curves C and D) is mainly exponential in form and characteristic of the transition probability in the Mn2+
ion centre. When excitation raises electrons from centres into the conduction band (Äexc = 3650 A) then the electrons are trapped and phosphorescence is deter­ mined by their thermal activation from traps and subsequent retum to empty emission centres (curves A and B).
The absorption spectrum may also contain bands representing transitions of electrons froni the filled band directly into trapping states, the holes left behind migrating to emission centres and thus emptyingthe latter. In practice it is difficult to distinguish such an excitation from that involving absorption in emission centres which raises electrons into the conduction band and from there into trap­ ping states 2• A further set of absorption bands may occur in excited phosphors due to transitions of trapped electrons to higher states. As an example of this absorption Fig. 5 gives the absorption spectrum of a strontium sulphide phosphor activated by europium and samarium impurities 3• The europium gives the emis­ sion centres and the samarium the trapping states. Trapped electrons show an absorption band at 1 !L· Absorption of radiation in this band liberates the trapped electrons which retum to emission centres to give luminescence (i.e. optically stimulated). Note that in excitation luminescence centres are emptied and their characteristic absorption band is decreased in intensity. Other transitions become possible when a phosphor is excited. Electrons can be raised into emptied emission centres, thus giving a long wavelength absorptionband similar to that for trapped electrons. Centres filled in such a way will no Ionger be available for the recombination with conduction electrons, and so such absorption df ra­ diation leads to a queuehing of emission.
8. Electron trapping and luminescence. There has been sufficient discussion in Sect. 7 to show that the characteristics of luminescence will be determined by the way in which trapping states function. These will affect both fluorescence and phosphorescence. During excitation the efficiency of emission will be deter­ mined by the ability of mobile positive holes to reach and capture trapped electrons and also by the thermal activation of electrons into empty emission centres from the filled band or, to look at it in a different way, by thermal activa­ tion of the positive holes in empty centres into the filled band in which they can migrate to trapped electrons. It becomes clear, therefore, that in general, positive hole migration has a deleterious effect on luminescence efficiency. As shown below in Chap. 111 it results in a dependence of the latter on intensity of excita­ tion giving rise to the so called "superlinear" fluorescence. Thermal activation of holes from empty centres leading to removal of trapped electrons by non­ radiative recombination will also affect the decay of phosphorescence.
An important step forward in the modern theory of phosphorescence due to electron trapping was the development of an experimental method for deter­ mining the distribution in "depth" of the trapping states below the bottom of the conduction band. The method was originally used by U RB ACH 4 for alkali halide phosphors and later extended to studies of trapping states in sulphide phosphors by RANDALL and WrLKINs 5 who also developed a more comprehensive
1 G. F. J. GARLICK and M. H. F. WILKINS: Proc. Roy. Soc. Lond., Ser. A 184, 408 (1945). 2 G. F. J. GARLicK: Brit. J. Appl. Phys. Suppl. 4, S 85 (1955). 3 F. URBACH, H. HEMMENDINGER and D. PEARLMAN: Luminescent Materials, p. 279.
New York: John Wiley 1948. 4 F. URBACH: Wien. Ber., IIA 135, 149 (1926); 139, 353 (1930). 5 J. T. RANDALL and M. H. F. WILKINS: Proc. Roy. Soc. Lond., Ser. A 184, 366 (1945).
Sects. 9, 10. The configurational coordinate model for emission centres. 9
theoretical analysis. The experimental method is to measure the variation of thermoluminescence intensity with temperature obtained by warming a phosphor in the dark at a uniform rate after excitation at a low temperature. It is obvious that thermal activation will first remove electrons from shallow traps and then from deeper ones. The light emission vs. temperature curve should thus represent the distribution of trap depths in the phosphor. More recent investigations, reviewed in later sections, show that generally the thermoluminescence experi­ ment does not give the true or complete dis­ tribution of trapping states in a phosphor. However, the method remains one of the most powerful tools for study of metastable electron states in crystals.
For convenience in dealing with phosphor characteristics in Chap. II, we may anticipate the derivations of trap depths from thermolumi­ nescence curves. The basic equation involved is (5.8) and depths corresponding to various thermoluminescence peak temperatures and for various s values are given in Table 1 for a
Table 1. A pproximate relations between thermoluminescence, tem­
perature and trap depth. (Warming rate 2.5°/sec.)
Tem·1 W I W I W perature (s=107fsec) 1
(s= 108/sec) 1
100 I 0.16 200 0.32 300 0.48 400 0.64
0.18 I 0.36
I 0.54 0.72
0.2 0.4 0.6 0.8
warming rate of r::::>2.5°fsec. These values are based on the assumption that escape of trapped electrons rather than recombination with empty emission centres is the rate determining step.
d) The nature and characteristics of luminescence centres. 9. Conditions for luminescence. The occurrence of luminescence in solids
raises the questions of the nature of the emission sites in various phosphors and the conditions for luminescence emission rather than dissipation of absorbed energy in other ways, such as thermal degradation or photochemical change. An important qualitative condition suggested by PEIERLS1 isthat electron transi­ tions responsible for emission should be well shielded by the surrounding atomic configuration from interactions with the crystallattice. However, in some phos­ phors, such as zinc and cadmium sulphides, the centres for luminescence emission are not well defined and shielding conditions would appear to be absent. In other cases the centres can be clearly designated. For example, in the uranyl salts the emission centres are formed by the uo2 coordination group; in the platinocyanides the Pt (CN)~- group is responsible. When the centre is well defined the spectrum of emission is characteristic of the centre and shows fine structure, but in other cases, such as copper activated zinc sulphide, the emission is in a broad band which shows no structure even at very low temperatures.
10. The configurational coordinate model for emission centres. The general energetic conditions for the luminescence centre were first considered for inorganic solids by VON HIPPEL 2 and later by SEITZ 3 in terms of an energy vs. configura­ tional coordinate diagram for the emission centre as shown in Fig. 6. A similar diagram was available for describing the behaviour of organic molecules on excitation, and we can best consider the two cases tagether inserting also the vibrational states as shown by the horizontal energy levels. The curves repre­ sent the energy of the centre in relation to the variation of one of its Coordinates, e.g. distance between atoms in a molecule or distance between an impurity
1 R. PEIERLS: Ann. Phys., Lpz. 13, 905 (1932). 2 A. V. HIPPEL: Z. Physik 101, 680 ( 1936). 3 F. SEITZ: Trans. Faraday Soc. 35, 74 (1939).
10 G. F. J. GARLICK: Luminescence. Sect. 10.
atom or ion and next nearest neighbours in an inorganic crystallattice. Normal and first excited state curves are shown and the curve minima A and B are shown at different coordinate positions in order to take the general case. Optical transitions will be represented by verticallines since they occur before the system has time to readjust itself. Thus absorption in the centre results in the transition AA' after which the centreis in a non-equilibrium, high vibrational state and so by vibrational interchange with its surroundings collapses to the equilibrium state B. From this state jt may return to the ground state with emission of luminescence (B B') and finalloss of more energy by vibrational interchange. At CC' the two curves approach each other very closely, this point being at a height rp
Fig. 6. Energy-configurational diagram for a luminescence centre. AA' absorption transition; BB' luminescence transition; BCC'A non radiative transition; hv vibrational level separation; cp acti-
vation energy for non-radiative transition.
(energy) above B. If the point A' reached on excitation is high enough, the system may go over to the ground state via CC' with completely non-radiative dissipation of absorbed energy. Even after B is reached from A ', there is a finite probability q per unit time of the point C being reached by
,;w .KlO Temperolure
Fig. 7. Variation of luminescence efficiency with temperature [see Eq. (10.1)]. A theoretical curve v/P=107, op=0.4 eV; B theoretical curve P/P = 104 , rp=0.4 eV; C experimental curve for a ZnS-MnS phosphor (0.436 !L excitation) (alter GARLICK 1).
thermal activation, given by v exp (- rpfkT), v being the vibrational frequency (~1012 or 1013/sec). Thus the probability of emission (p as above) is offset by the probability q of a non-radiative transition (BCC'A) especially at higher tem­ peratures. Thus for luminescence processes confined to the emission centres, the efficiency of luminescence is given by r; = pf(p + q) which gives:
p r;= P+vexp(-rp/kT) (10.1)
the form of which is shown graphically in Fig. 71 . This efficiency we shall call the intrinsic efficiency of the luminescence centre and the non-radiative process (q) as internal conversion of the absorbed energy. The latter will also affect the decay kinetics since Eq. (4.1) becomes:
dn di=J-(P+q)n. (10.2)
Thus the decay of luminescence is given by:
L = L 0 exp {- (p + q) t}, (10.3)
1 G. F. J. GARLICK: Luminescent Materials. Cxford 1949.
Sects. 11, 12. Diamond. 11
i.e. non-radiative transitions speed up the decay. This was realised by KRÖGER
et coll. 1, and Fig. 8 shows their decay measurements for a magnesium titanate phosphor with Mn4+ ions as the impurity luminescence centres.
In practice, it is not easy to determine energy-configurational coordinate diagrams for luminescence centres, but successful attempts have been made by WrLLIAMS 2 for thallous ion centres in potassium chloride, by KLicKs for Mn2+ centres in zinc silicate and by VLAM 4 for centres in the silicates and tungstates. Their treatments are discussed later in Chap. III. The types of experimental data necessary to construct such curves include the measurement of absorption and emission spectra, of decay constants and luminescence efficiency, and their depend­ ence on temperature over a wide range. In practice absorption measurements con­ stitute the greatest difficulty unless single crystal phosphors are available.
~ 11. Discussion of phosphors and their '--Arl+-\,-~!--'........---3o,:-+-~---4'~-~
characteristics. The above introduction will ~ i:l enable us to review in the next Chap. II ~ the imporlant groups of inorganic and ~ organic phosphors, while in later sections ~ we shall consider selected aspects of their ~ properlies. Luminescence has become a i /OI---I-+--1tf---'\t-----'\:-t---t very vast field during recent years, and ~ within the scope of the present text it is impossible to treat all of its aspects in de­ tail. F or this reason selection of topics has been inevitable, but it is hoped that the general references given in the text
5
~~-~~~--~M~---q~m-~--~3D D11C'qy llin11
and at its COllclusion Will serve as intra- Fig.S. Decayofphosphorescencefor aMg,TiO,-Mn<+ phosphor excited by 3650 A radiation (after KaöGER ductions to classes and properlies of phos- et coll.').
phors not discussed here. It should also be pointed out that emphasis has been laid on physical characteristics of phosphors and in particular on the underlying physical processes. For this reason the applications of luminescence can only be given bare mention in the text.
11. Classes and characteristics of phosphors. a) Luminescence in monatomic solids.
Luminescence has been observed in relatively few monatomic solids, for ex­ ample diamond, germanium and the solid halogens.
12. Diamond. The first extensive studies on the luminescence of diamond were made by the RAMAN school, and their findings have been collectively pre­ sented in symposia of 1944 and 1946 5• 6• The generalproperlies of diamonds were
1 F. A. KRöGER, W. HooGENSTRAATEN, M. BoTTEMA and TH. P. J. BoTDEN: Physica, Haag 14, 81 (1948j.
2 F. E. WILLIAMS: Brit. J. Appl. Phys. Suppl. 4, 97 (1955). 3 C. C. KLICK: J. Opt. Soc. Amer. 42, 910 (1952). 4 C. C. VLAM: Structure of emission bands of luminescent solids. Diss. Haarlern 1953. 5 Symposia on Diamond: Proc. Indian Acad. Sie. A 19 (1944). 6 Symposia on Diamond: Proc. Indian Acad. Sei. A 24 ( 1946).
12 G. F. J. GARLieK: Lumineseenee. Seet. 12.
investigated earlier by RoBERTSON, Fox and MARTIN1 who grouped diamonds into two main types (I and II) according to physical properties. Type I diamonds are opaque to ultraviolet radiation below 3000 A, while type II diamonds are transparent to 2250 A and, in cantrast to type I, are photoconducting. The typesarealso distinguished by their different birefringence and infra-red absorp­ tion charaCteristics. BLACKWELL and SuTHERLAND 2 have given a useful compari-
•!'1: "<t son of the properties of ~lllt~ ~ ~~~ ~ ~ fl!l11 ~ ~ the two groups while the
l'Riission .specfrum of ,Biue' rliomond
.a.- Fig. 9a. Emission spectrum of a "blue" luminescent diamond (numbers other than wavelengths in Adenote wave nurober differences from principal
· line 4152 A) (after MANI ').
Wllission sp~m of • tll't'l'n • rliomono'
Ä.- Fig. 9b. Emission spectrum of a "green" luminescent diamond (numbers other than wavelengths in A denote wave number differences from principal
line 5032 A) (after MANI ').
luminescence characteris­ tics have been investigated quantitatively by BuLL and GARLICK 3 and by MANI 4•
Photoconduction studies on diamond have been treated by the writer in the article on photoconductivity in vol. XIX of this Encyclo­ pedia.
Type I diamonds show a blue fluorescence and a feeble green-yellow phos­ phorescence when excited by near ultraviolet radia­ tion at room temperature. Both fluorescence and phos­ phorescence become more intense with increase in crystal imperfection and in specimens intermediate be­ tween types I and II having some transparency in the 2250 to 3000 A region. In­ tense thermo-luminescence is observed in these dia­ monds when heated to 5 50° K after excitation. Ty­ pe II diamonds, originally thought to be non-lumines­ cent by the RAMAN group
can, however, show a yellow-orange emission 2• Until recently, it was generally agreed that luminescence in diamond was a characteristic of crystal imperfection rather than of impurity inclusions. From the emission spectra, as measured by MANI4
and shown in Fig. 9, RAMACHANDRAN and CHANDRESEKHARAN 5 have suggested that the doubletat 4152Ä is due to transitions 1S0~ 3Jl and 150~ 3~ in a carbon atom. BULL and GARLieK have suggested that the difference between the mean
1 R. RoBERTSON, J. J. Fox and A. E. MARTIN: Phil. Trans. Roy. Soe. Lond., Ser. A 232, 463 (1934).
2 D. E. BLACKWELL and G. B. B. M. SUTHERLAND: J. Chem. Phys. 46, 9 (1946). 3 C. BULL and G. F. J. GARLICK: Proe. Phys. Soe. Lond. A 63, 1283 (1950). ' A. MANr: Proe. Indian Aead. Sei. A 19, 231 (1944). 5 G. N. RAMACHANDRAN and V. CHANDRESEKHARAN: Proe. Indian Aead. Sei. A 24, 176
(1946).
Sect. 13. Germanium. 13
wave number of the doublet and that of the transitions in an isolated carbon atom is most probably due to distortion produced by an adjacent vacancy in the diamond lattice1. For yellow fluorescent diamonds there is also a sharp line at 5032 A. In both cases the sharp lines are accompanied by a continuum to the long wavelength side of the lines associated with vibrational states of the lattice 2• The thermo-luminescence curves of all diamonds showing the effect at all are similar in form 3, and trapping states appear to be metastable states of the emission centres. BuLL and GARLICK have given the energy level scheme shown in Fig. 10. The transitions P provide the green-yellow phosphorescence
.. at room temperature and are of a forbidden character. Thus their increased 'intensity in imperfect diamonds suggests that distortion due to defects makes them possible. Absorption in the 1 !L spectral region causes stimulation and may be associ­ ated with the M -+F transitions of Fig. 10.
A topic of considerable interest in recent years has been the change in the optical pro­ perties of diamonds produced by bombardment t with nuclear particles. The most interesting Observationsare those of DuGDALE 4 and more ~ recently those of DITCHBURN et coll. 6 for elec- ~ tron bombardment (1.0 MeV) 1. Fig. 11 shows
I / ---T-.,.--.11
I lp 1 /
I I
the changes in the absorption spectrum of a type II diamond on electron bombardment and after various conditions of subsequent heat treatment. High temperatures bleach out the =======(] band system (i) producing a new system (ii) Fig.10. Energylevelschemeforemissioncentre
in diamond. G ground state; F excited state possibly due to Centres formed by pairs of M metastable states (alter BuLL and GARLicK')
interstitial carbon atoms 6• The emission result- ing from absorption in the region B closely resembles that for natural, green fluorescing diamonds in Fig. 9 due to Miss MANI. The detailed nature of the centres produced by bombardment is still not clear.
13. Germanium. Photoexcited luminescence has not yet been reported for germanium, but emission has been observed on injection of minority charge carriers into germanium crystals. Observations have been made by HAYNES and BRIGGS 7 and by NEWMAN 8 and more recently by AIGRAIN 9• The emission spectrum measured by NEWMAN is given in Fig. 12 together with the theoretical curve for band-to-band recombination of holes and electrons. The formula for the emission is :
I. oc v3 exp (- hvjkT){ 1 - exp (- oc. · d)} (13.1)
1 DITCHBURN et coll. (private communication of unpublished studies) have recently found that the doublet at 4152 A arises from self-absorption of the radiation in this reso­ nance line and is not therefore a real doublet. In polarisation studies they have found that the lines at 4152 and 5032 A are characteristic of oscillators lying in the 111 planes of the crystal.
2 H. SMITH: Phil. Trans. Roy. Soc. Lond., Ser. A 241, 105 (1949). 3 C. BuLL and G. F. J. GARLICK: Proc. Phys. Soc. Lond. A 63, 1283 (1950). 4 R. A. DUGDALE: Brit. J. Appl. Phys. 4, 334 (1953). 5 R. W. DITCHBURN et coll.: Defect in crystalline solids, p. 92. Bristol 19 55. 6 J. ÜWEN, J. H. E. GRIFFITHS and I.M. WARD: Nature, Lond. 173, 439 (1954). 7 J. R. HAYNES and H. B. BRIGGS: Phys. Rev. 86, 647 (1952). 8 R. NEWMAN: Phys. Rev. 91, 1313 (1953). 9 P. AIGRAIN: Physica, Haag 20, 1010 (1954).
14 G. F. J. GARLICK: Luminescence.
~------.-----------.-----------.-----------,
a
A
Sect 13.
Fig. 11 a and b. Absorption spectra of a diamond after 1 MeV electron bombardment and subsequent heat treatments. A unirradiated crystal; B-+G after electron bombardment; B no heating; C after heating for 2 hours at 480° C; D alter heating for further hour at 580° C; E after heating for further half hour at 630° C; F afterfurther heating for half hour
at 680° C; G after heating for 18 hours at 930° C (after DITCHBURN et coll., ref. 5, p. 13).
t
30
\
Quontum Pnf?IYY Fig.13
Fig. 12. Emission spectruni of a germanium crystal due to minority Carrier injection (after NRWMAN, ref. 8, p. 13).
Fig. 13. Emission spectrum of germanium due to minority carrier injection as measured by AIGRAIN (ref. 9, p. 13).
Sect. 13. Germanium. 15
where I. is the intensity of emission per unit frequency interval at frequency v, oc.. is the absorption coefficient at that thickness, and. d the specimen thickness. There is a discrepancy in peak position between experimental and theoretical values rather !arger than that due to possible spectrometer errors. The dif­ ference may be due to incorrect absorption data, but there is also the problern of the dependence of escape of radiation on critical angle ::j.nd therefore on re­ fractive index in the absorption edge region. Altematively, the electrons or holes
exC'ifofion JEfTsr
b
l/.5
Fig. t4 a and b. Excitation and emission spectra of the solid halogens and interhalogen compounds (after DUMBLETON, ref. 2, p. 16).
may be captured in shallow traps before recombination can take place and so giving a transition energy slightly less than that of the gap between energy bands. AIGRAIN's measurements reveal emission up to the limit of his detector (R:~6 !L) as shown in Fig. 13 and this may be due to transitions in impurity or defect levels 1 • The quantum efficiency of the emission is very small. A moreextensive theoretical treatment by VAN RooSBROECK and SHOCKLEY 2 deals with the relation
1 In a recent discussion (International Colloquium on Luminescence, Paris, May 1956) AIGRAIN has stated that the emission from germanium shown in Fig. 13 is most likely thermal emission from minority carriers which have an effective temperature higher than that of the matrix Iattice.
2 W. VAN RoosBROECK and W. SHOCKLEY: Phys. Rev. 94, 1558 (1954)
16 G. F. J. GARLICK: Luminescence. Sects. 14-16.
of recombination emission to carrier lifetime. The latter is short compared with that predicted for the emission.
14. The solid halogens. Of the solid halogens iodine has been investigated as a photoconductor by Moss1 and reviewed by the writer in Vol. XIX of this
Encyclopedia. I t has been found by DUMBLETON 2 that, with the exception of fluorine, which could not be investigated be­ cause of its low solidification point, the solid halogens and inter-halogen compounds are luminescent in the near infra­ red region. Fig. 14 shows their excitation and emission spectra, while Fig. 15 gives additional data on absorption and photo­ conduction in iodine. The ex­ citation and emission spectra are at Ionger wavelengths than the normal molecular absorption regions but may be associated with normally forbidden transi­
15 Wovl'll'nglll
Fig. 15. Absorption, excitation, luminescence and photoconduction response spectra for solid iodine.
tions of the molecule. From a private discussion with Professor R. S. MuL­ LIREN of Chicago, the suggested transitions would appear to be:
Absorption: 1E;-+ 31I1 , Emission 31I1 -+ 1E; or 3li2 -+ 1E;.
However, further analysis and measurement appear to be necessary.
b) Alkali halide and other halide phosphors.
15. The pure alkali halides. The luminescence and phosphorescence emission from alkali halide crystals excited by X-rays, y-rays or high energy particles has been known for many years. Emission spectra are not intense and studies by various workers do not often show correlation for such spectra, or for other characteristics. Considerable interest lies in the relation of the luminescence characteristics to those of the colour centres, particularly the so called F centres due to electrons trapped in anion vacancies in the crystallattice. From the many studies made on pure alkali halides we select one or two aspects of importance, firstly the search for the luminescence from F centres, secondly the attempts to relate the thermoluminescence characteristics of the various pure halides to lattice vacancies and defects etc. For a general review of colour centre chan.c­ teristics and inclusive of references to luminescence in alkali halides up to 1953 the recent work of SEITZ 3 may be consulted.
16. The luminescence emission of F centres. The F centre in alkali halides is formed by trapping of electrons at vacant anion sites in the crystal lattice. The ground and excited levels in this centre are shown in Fig. 16 with reference to the conduction band. It was suggested by MoTT and GuRNEY that at low enough temperatures F centre absorption should be followed by F centre emission at
1 T. S. Moss: Photoconductivity. London 1952. 2 M. J. DuMBLETON: Proc. Phys. Soc. Lond. B 68, 53 (1955). 3 F. SEITZ: Rev. Mod. Phys. 26, 7 (1954).
Sect. 16. The luminescence emission of F centres. 17
Ionger wavelengths than those for the absorption band. Later theoretical investi­ gations suggested that a quantum efficiency of unity might be expected for the process 1• 2• 3 • However, KLICK 4 was unable to find emission, his experimental sensitivity limit corresponding to about 3 % quantum efficiency for the process
Fig. t6. Energy band scheme for an alkali halide containing F centres.
Fig. t 7. Thermoluminescence emission spectra of X rayed LiF crystal (after GHORMLEY and LEVY 1).
at 4 °K in X-ray colonred LiF and additively colonred KCl. GHORMLEY and LEVY 5 found infra-red emission from y-ray irradiated LiF on excitation in the F band andin both y-ray and electrolytically colonred KCl crystals. Their emission spectra for LiF are given in Fig. 17. Recently BüTDEN, VAN DOORN and HAVEN 6
have been able to find and measure the F centre emission ~ characteristics in a number Jl of alkali halides additively ·~ colonred by heating in the -~ alkali metal vapour and by ~ electrolysis, the crystals con­ taining from 1016 to 1 o1s F centres per cm3• An essential feature of successful Observa­ tions was found to be the re­ tention of specimens at liquid nitrogert temperature following
Fig. 18. Infra-red emission spectra of various alkali halides measured at 77 °K after F coloration (after BoTDEN et coll.').
additive colouration and a short period of heating in air. Emission spectra of some alkali halides obtained by BooTEN et coll. are given in Fig. 18. The temperature
I K. HuANG and A. RHvs: Proc. Roy. Soc. Lond., Ser. A 204, 406 (1950). 2 S. I. PEKAR: Z. eksp. teor. Fiz. 22, 641 (1952). a H. ]. G. MEYER: Physica, Haag 20, 1016 (1954). 4 C. C. KLICK: Phys. Rev. 79, 894 (1950); 94, 1541 (1954). 5 J. A. GHORMLEY and H. A. LEvY: J. Phys. Chem. 56, 546 (1952). 6 T. P. ]. BoTDEN, C. Z. VAN DooRN and Y. HAVEN: Philips Res. Rep. 9, 469 (1954).
Handbuch der Physik, Bd. XXVI. 2
18 G. F. J. GARLICK: Luminescence. Sect. 17.
dependence of emission is given in Fig. 19. Return to low temperatures after F band irradiation at room temperature is not accompanied by a return of the emission. Retention in the dark at room temperature for may hours produces changes in the emission spectra measured after recooling. Fig. 20 shows the effect for RbCl and KBr.
From these experiments the quantum efficiency for the luminescence is found tobe only about 1% or les. The reason for this low value is not clear at present. The temperature dependence of the emission can of course be explained by a
17
Fig. 19. Effect of warming RbC! and KBr crystals to room temperature on the infra-red emission spectra measured after cooling again to 77 °K. --before warming; ---- after retaining at 290 °K in dark for 15 bours and tben recooling
to 77 •K (after BoTDEN et coll., ref. 6, p. 17).
radiationless transition from excited to ground states. However, the ther­ mal quenching occurs in the same tem­ perature rangeasthat in which photo­ conduction increases and in which F band irradiation leads to the forma­ tion of F' centres (F centres containing two trapperl electrons).
It has been found recently by FEOFILOV 1 that the F centre lumi­ nescence of lithium and sodium fluo­ rides is polarised when exciting radia­ tion is polarised. The degree of polar­ isation is dependent on the orientation of the electric vector of the incident radiation relative to the crystal axes (cf. CaF2 below).
17. Thermoluminescence in the pure alkali halides. Thermoluminescence curves of alkali halide crystals after X-ray or y-ray irradiation have been measured by
16 P-
Fig. 20. Temperature dependence of emission spectrum of KCI (after BoTDEN et coll., ref. 6, p. 17).
various workers. The pioneer work of U RB ACH et coll. 2• 3 in devising lhe ther­ moluminescence experiment included sturlies of rotk salt and fluorite. More recent work has shown that the characteristics of thermoluminescence in the alkali halides are in general complex although in some cases it is possible to identify peaks in the ther­ mal glow curves with the thermal bleaching temperatures of colour cen­ tres. In this respect we may note the experiments of DurroN and MAURER 4
and of HrLL and SCHWED 5• The for­ mer measured both electrical con- ductivity and thermal glow peaks in
relation to colour centre bleaching. Their results are summarised in Table 2 which indicates the probable type of centre associated with each peak of ther­ moluminescence.
1 P. P. FEOFILov: Dokl. Akad. Nauk. SSSR. 92, 743 (1953). 2 F. URBACH: Wien. Ber. IIA 139, 353 (1930). 8 F. URBACH and A. ScHWARz: Wien. Ber. IIA 139, 483 (1930). 4 D. DuTToN and R. J. MAURER: Phys. Rev. 90, 126 (1953). 5 J. J. HILL and P. ScHWED: J. Chem. Phys. 23, 652 (1955).
Sect. 18. Impurity activated alkali halide phosphors. 19
Thermal glow curves for NaCI crystals obtained recently by HILL and ScHWED for different amounts of irradiation are shown in Fig. 21. As the irradiation dose is increased, the glow curve simplifies to a single peak at 560 °K. By use of an experimental method due to GHORMLEY and LEVY1 the peaks of the glow curves are analysed, each giving an activation energy or trap depth of ab out 1.23 e V, i.e. the frequency constant is very differ- ent for each peak [see Eq. (5.8)]. These results might be due to the particular method of glow curve analysis but if not then, as suggested by the authors, the ener- gy of 1.25 eV might be associated with the F centres of the crystal lattice (rff :=:::; 1 e V for thermal bleaching). In this case the frequencyconstant cannot be characteristic of the F centre but of subsequent proces- t' ses. The precise mechanism is still in ~ doubt. Other studies of thermolumines- ~ cence in pure alkali halides have been ~
d ~ made by SHARMA 2 an by BoNANOMI 3 • -~ ~ 1!
Table 2. Conductivity and thermal glow peaks for ~ KBr and KCI after irradiation at 77°K. ~
Peak Colour I
stance perature bleached depth region (oC) A
-
0.49 -
Fig. 21. Thermoluminescence curves for NaCl after different X-ray doses at room temperature (after HILL
and ScHWED, ref. 5, p. 18).
With respect to optical stimulation of luminescence in the alkali halides the only effective wavelengths causing ejection of trapped electrons are those within the F centre absorption band 4 • 5 . As mentioned below, this is also true for silver activated sodium chloride. The nature of the luminescence centres in the pure alkali halides (with the exception of F centres giving the infra-red emission des­ cribed above) is still unknown.
18. Impurity adivated alkali halide phosphors. A review of the known impurity activators for the alkali halides has been given by PRINGSHEIM 6• The important activators and those which have been of most interest are thallium, tin, manga­ nese, lead, antimony and silver. Thallium activated iodides have become of considerable importance as scintillation counter crystals. We consider first the activation by thallium.
1 J. A. GHORMLEY and H. A. LEvY: J. Chem. Phys. 56, 546 (1952). 2 J. SHARMA: Phys. Rev. 85, 692 (1952). 3 J. BONANOMI: Helv. phys. Acta 25, 725 (1952). 4 C. BuLL: Ph. D., thesis, Birmingham University 1951. 5 M. FURST and H. KALLMANN: Phys. Rev. 91, 1356 (1953). 6 P. PRINGSHEIM: Fluorescence and Phosphorescence. New York 1950.
2*
rx) Thallium activated alkali halides. The fluorescence and phosphorescence of thallium activated potassium chloride were investigated quantitatively by BuNGER and FLECHSIG in 1931 1 . Later the thermoluminescence characteristics were studied by RANDALL and WILRINS 2 •
Introduction of Tl+ ions into alkali halide crystals produces new absorption bands in the ultraviolet region of the spectrum. There are usually three bands as shown schematically in Fig. 22 for KCl.
Table 3 gives the various peak positions in electron volts for a number of alkali halides. The bands are characteristic of the Tl+ ions which substitute for alkali metal ions in the lattice to form the luminescence centres. A detailed theoretical analysis of the phosphor KCl-Tl has been made by WILLIAMS and his co-workers and he has recently summarised the knowledge of this phosphor 3• 4•
Fig. 22. Absorptionbands for a KCl-Tl phosphor. (For position of peaks A, Band C in other alkali halides see Table 3.)
Table3. Absorptionbandsinthe alkali kalides due to thallium and position of
the fir st fundamental lattice peak. (Band positions in eV.)
Alkali Lattice Peak A PeakB PeakC halide peak
Na Cl 7.82 4.87 5.8 6.2 KCl 7.6 4.92 5.9 6.3 Rb Cl 7.39 4.98 5.94 6.4 CsCl 7-63 4.90 5.9 6.3 NaBr 6.49 4.63 - 5.72 KBr 6.58 4.73 - 5.88 RbBr 6.43 4.77 - 5.82 CsBr 6.61 4.69 - 5.76 Nai 5.38 4.22 - 5.28 KI 5.63 4.3 - 5.23 Rb I 5.55 4.32 - 5.15
The absorptionband at 2460 A and the emissionband at 3050 Aare ascribed to the transitions 150~ 3P 1°. The absorption at 1960 A and emission at 4750 A are ascribed to 1S0~ 1P1° transitions. The band model for the phosphor due to jOHNSON and WILLIAMS 4 is given in Fig. 23. They have also investigated the metastable states of the Tl+ ion centre responsible for thermoluminescence. Thermal glow curves obtained by them are shown in Fig. 24. The peaks at 205 and 300 °K appear to be characteristic of single TI+ ions and are selectively "bleached" by 1. 5 e V radiation. These peaks vary in intensity with thallium content in the same way as the emission spectrum for TI+ ions, while the peak at 250 °K follows the variation shown by the emission characteristic of pairs of adjacent TI+ ions. The configurational coordinate diagram for the TI+ ion centre arrived at by ]OHNSON and WrLLIAMS 4 is shown in Fig. 25. The metastable states giving the thermal glow peaks at 205 and 300 °K are ascribed to 3 Pg and 3pg states of the ion as suggested by selection rules and the energies involved.
With respect to the thermal glow curve experiment a relatively small number of metastable states can be created by excitation (1010 -+1011 per cm3). Excita­ tion is of course to 3 P~ or 1 P~ states followed by transfer to the metastable levels 3 P~ and 3 Pg. The vibrational frequency for the 3 P~ state is 1012Jsec while the
1 W. BUNGERand W. FLECHSIG: Z. Physik 67, 42 (1931). 2 J. T. RANDALL and M. H. F. WILI<:INs: Proc. Roy. Soc. Lond., Ser. A 184, 366 (1945). 3 F. E. WILLIAMS: Brit. J. Appl. Phys. Suppl. 4, S 97 (1955). 4 P. D. JoHNSON and F. E. WILL!AMS: J. Chem. Phys. 21, 125 (1953).
Sect. 18. Impurity activated alkali halide phosphors. 21
frequency factors for the spg~sp~ and spg~s~ transitions are R=~ 108jsec. Thus, if transfer is only possible while polarisation of the centre system remains, the relanve probability of reaching one of the metastable states is R=~ 10-4•
The above detailed analysis for KCI- Tl can be adapted to explain the absorp­ tion, emission and phosphorescence characteristics of similar halides such as KBr, activated by TI+ ions.
ß) Silver activated sodium chloride. It was found by MANDEVILLE et coll. 1
that silver activated sodium chloride provided a very efficient ultraviolet emit­ ting phosphor for detection of nuclear particles. A little later on KALLMANN and FuRST 2 showed that the phosphor could store large amounts of energy by electron trapping .after particle excitation and later made more quantitative
10
eV
]:_. t ·~
Temperalure ~n ~~
Fig. 23. Energy band model for KCl-Tl phosphor (after ]OHNSON and WILLIAMS, ref. 4, p. 20).
Fig. 24. Thermoluminescence curves for KCl-Tl phosphors after excitation by a hydrogen discharge lamp (short u. v excitation) (after ]OHNSON and WILLIAMS, ref. 4, p. 20).
measurements on the stimulation of the luminescence by long wavelength ra­ diation 3. The luminescence occurs in two bands, an intense one from 2300 to 2600 A with a peak at about 2500 A, and a weaker one with range 3200 to 4500 A and a peak at about 4000 A. The emission could be stimulated after primary excitation by light lying in the F centre band of NaCl at 3000 to 5000 A. They also found that the maximum stimulability and thus optimum numbers of trap­ ping states occurred for silver concentrations of the order of 1%. However, these authors concluded that the trapping states were not determined solely by the activator inclusion but also by the state of the matrix lattice. This con­ clusion has been reached in a more quantitative way by recent experiments of DoBRINSKI and HINRICHS 4 who have measured the thermoluminescence curves of NaCl-Ag phosphors with various amounts of activator. Some of the peaks in these curves can be attributed to the host lattice while others are obviously dependent on silver content. A typical set of curves for 1% Ag content are given
1 C. E. MANDEVILLE et colt.: Phys. Rev. 79, 1010 (1950); 80, 299, 300 (1950); 81, 163 (1951).
2 H. KALLMANN and M. FURST: Phys. Rev. 82, 964 (1951). 3 M. FuRST and H. KALLMANN: Phys. Rev. 91, 1356 (1953). 4 P. DoBRINSKI and H. HrNRICIIS: Z. Naturforsch. 10a, 620 (1955).
22 G. F. ]. GARLICK: Luminescence. Sect. 18.
in Fig. 26. It will be noticed that the low temperature peak is relatively weak
from the curve measured for the 2500 A wavelength. DOBRINSKI and HINRICHS
have provided a tentative energy scheme for their phosphors and tentative models for the various trapping states and luminescence centres involved. Both the energy
Fig. 25. Energy-configurational coordinate diagram for a KCI-Tl phosphor (after ]OHNSON and WILLIAMS,
ref. 4, p. 20).
I \L) ..j 1/:i :t(",
,, ~I _/
0
ld) l\J \~,, ~'
T/mD Fig. 26. Thermoluminescence curves of NaCI-Ag phosphors
(after DOBRINSKI and HINRICHS, ref. 4, p. 21 ).
Fig. 27. Energy band model for NaCI-Ag phosphor (after DoBRINSKI and HINRICHS, ref. 4, p. 21).
band model and the centre configurations are given in Fig. 27. Following ETZEL,
ScHULMAN, GINTHER and CLAFFY 1, they associate the emission at 2500 A with
1 H. W. ETZEL, ]. H. ScHULMAN, R. ]. GINTHER and E. W. CLAFFY: Phys. Rev. 85, 1063
(1952).
Sect. 19. Luminescence in the silver halides. 23
single silver ions and the 4000 A emission with pairs of silver ions, in both cases, the centre including an anion vacancy. Ag+ ions without associated vacancies are assumed to form trapping sites for electrons. It is also assumed that F and F' centres function as traps.
Calculations such as those on KCl-Tl phosphors might be extended to Ag+ ions in N aCl and a quantitative configurational coordinate diagram constructed for this phosphor.
y) Other impurity activated alkali halides. MuRATA and SMITH 1 have made NaCl-Mn-Pb phosphors byslowevaporation fromsolution. Unless leadis present no ultraviolet absorption occurs and no emission in the red Mn2+ band is observed. The Pb2+ absorption is in a band centred on 2740 A. A smaller band to the shorter wavelength side may be due to the manganese as it occurs in a concentrated aqueous solutions of MnCl2 • This system has been discussed in some detail by PRINGSHEIM 2• WILLIAMS 3 has given a brief discussion of the Mn2+ activation of the alkali halides. He considers that Mn2+ replaces a cation at the centre of the unit cell tagether with the creation of a remote cation vacancy. Absorption and emission are attributed to the 6 S ~ 4 P transitions which are of forbidden character and involve a change of electron spin direction in the 3d shell.
A further system of practical interest is uranium activated sodium fluoride which provides a very sensitive method for uranium estimation 4 : RuNeiMAN 5
has shown that the resulting yellow-green emission cannot be due to a .UF 6 group but, since the samples are usually melted in air, a U06 group is the mostprobable luminescence centre. The extra negative charge resulting can be compensated by an adjacent divalent positive ion. Addition of a small trace of calcium causes changes in the line intensities in the emission spectrum which suggests a close proximity of the Ca2+ ion to the U06 centre.
19. Luminescence in the silver halides. In 1939 RANDALL 6 reported the low temperature fluorescence of silver chloride. Later on, a more extensive investiga­ tion was made by FARNELL and BuRTON 7• They found that at 77 °K silver chloride gives a strong blue-green emission between 4300 and 5800 A with a peak at 5000 A. Silver bromide gives at 20 °K a green emission with a peak at 5050 A. A red emission band is also present which is due to excess silver. Silver iodide has a relatively sharp emission band at 4200 A with another in the region 4400 to 4600 A. The relative band positions in this case depend on the crystal form which can be of the zinc blende (cubic) or wurtzite (hexagonal) structure or intermediate between the two. In all cases F ARNELL and BuRTON assume that the emission (with the exception of the red band in AgBr) is characteristic of band-to-band transitions in the halide lattice. The red band of AgBr is enhanced by exposure to light suggesting its association with excess silver. There appears to be some relation between the low temperature fluorescence of bromo-iodide emulsions and the photographic latent image process the recombination giving fluorescence competing with electron trapping at latent image sites. FARNELL and BuRTON did not observe any thermoluminescence effects in their halldes and found a
1 K. J. MuRATA and R. L. SMITH: Amer. Mineral. 31, 527 (1946). 2 P. PRINGSHEIM: Acta Phys. Austriaca 3, 396 (1950). 3 F. E. WILLIAMS: Brit. J. Appl. Phys. Suppt. 4, S 97 (1955). 4 G. R. PRICE, B. J. FERRETTI and S. SCHWARZ: Analyt. Chem. 25, 322 (1953). 5 W. A. RUNCIMAN: Nature, Lond. 175, 1082 (1955). 6 J. T. RANDALL: Trans. Faraday Soc. 35, 2 (1939). 7 G. C. FARNELL and P. C. BuRTON: Mechanism of Photographie Sensitivity, edit. J. W.
MITCHELL, p. 61, London 1951.
24 G. F. J. GARLieK: Luminescence. Sects. 20, 21.
very rapid decay of fluorescence. In more recent studies the kinetics of lumines­ cence decay have been investigated by ARKHANGELSAYA and FEOFILOV1 who have given a brief review of previous work. They find that in AgCl and AgBr the build up · and decay of luminescence occur within 10-3 sec, but are not usually of exponential form.
DORFNER 2 has extended investigations to silver bromide crystals activated by 0.2 mol-% silver sulphide, the crystals being of three kinds, cold worked, annealed and "formed" crystals. The latter were obtained by successive irra­ diations of crystals at low temperatures with intermediate warming to room temperature. The "formed" crystals exhibited the strongest afterglow and thermoluminescence. Luminescence in these sulphide activated crystals is in
the yellow region (maximum 5800 A) and per­ sists to higher temperatures than that of the
~ pure bromide. The form of thermolumines- 1:; ~ cence obtained is shown in Fig. 28. The author ~·~ uses a formula W = 23 k TG for the relation ~~ between trap depth and temperature of maxi- ~ ~ mum glow giving trap depths of 0.21 and 0.27 e V ~ ""' from the above glow curve. An extensive dis-
cussion of the nature of traps and emission -lßll -1{1{} "i#() "C -ED centres is given but the conclusions demand
Temperolvrt> Fi~. 28. Thermoluminescence curve for a silver further and more specific verification. The bromide crystal activated by silver sulphide crystals show Stimulation and queuehing of
{afterDoRFNER'). afterglow by near infra red radiation.
20. Other monovalent halides. rx.) Thallous halides. Salutions of thallous halides show fluorescence which is enhanced by presence of alkali halides. In solid form at -180° C thallous chloride shows luminescence if water is present 3•
ß) Cupraus halides. The luminescence of cupraus chloride at low temperature was reported by RANDALL 4 • Recently it has been suggested by SHALIMOVA and MENDAKOV 5 that the emission is connected with excess copper in the halides. The transition from red luminescence at low temperature to green luminescence at higher temperatures (- 85° C) noted by RANDALL has been further studied by TSUJIKAWA and KANDA 6•
21. Fluorite phosphors. An extensive survey of literature up to 1954 on fluorite phosphors and their many different activators has been given by PRZI­ BRAM7. We shall consider some recent studies of some aspects of luminescence in fluorite. Synthesised fluorite with uranium as activator has received attention from KRÖGER 8 • The emission of the uranium centres is enhanced by the presence of traces of calcium oxide. A detailed discussion by RuNeiMAN 9 considers the various ways in which uranium can be incorporated into the fluorite lattice and gives the low temperature emission spectra for specimens also containing oxygen or lithium.
1 V. A. ARKHANGELSAYA and P. FEOFILov; DokL Akad. Nauk. SSSR, 91, 1055 (1953). 2 K. R. DORFNER: Ann. Physik 16, 331 (1953). 3 H. GoBRECHT and F. BECKER: Z. Physik 135, 553 (1953). 4 J. T. RANDALL: Trans. Faraday Soc. 35, 2 (1939). 5 K. V. SHALIMOVA and H. C. MENDAKOV: J. exp. theor. Phys. URSS. 26, 248 (1954). 6 I. TSUJIKAWA and E. KANDA: Sei. Rep. Res. lnst. Tohoku Univ. A 6, 220 (1954}. 7 K. PRZiliRAM: Verfärbung und Lumineszenz. Berlin: Springer 1953. 8 F. A. KRÖGER: Physica, Haag 14, 488 (1948). 9 W. A. RUNCIMAN: Brit. J. Appl. Phys. Suppl. 4, S 78 (1955).
Sect. 21. Fluorite phosphors. 25
Perhaps the most interesting study of fluoriteisthat of GINTHER 1 on CaF2-
Ce-Mn phosphors (powder form). The fluorite lattice is transparent to radiation from the vacuum ultraviolet region out to many microns of wavelength. Addi­ tion of manganese in synthesis produces a cathodoluminescent phosphor having a green emission band (shown in Fig. 29) characteristic of the manganese ion Mn2+ (see Sect. 53 below) but not excited by 2537 A radiation. Introduction of cerium (Ce3+ ion) sensitizes the phosphor and it can now be excited by 2537 A and other ultraviolet radiation. The mechanism of sensitisation
{/j5
()/
Fig. 29. Cathodoluminescence spectra forCaF3-Mn phosphors. A 0.5 to 1.0% Mn; B 5% Mn; C 10% Mn (alter GINTHER 1).
Fig. JOa-c. (a) Reflexion spectra of CaF3 phosphors with various Ce activator contents. (b) Excitation spectra for CaF3 phosphors with various Ce activator contents. (c} Emission spectra for CaF3 phosphors with various Ce activator
contents and for different excitation wavelengths. (After GINTHER 1.)
is discussed in Chap. III, Sect. 73 below, but the experimental characteristics are given here as a typical indication of the properlies of sensitised phosphors. Addition of cerium produces absorption bands as indicated by the reflexion spectra of Fig. 30 a while cerium emission (Fig. 30c) is produced by the excitation regions shown in Fig. }Ob. Manganese emission results from a similar excitation spectrum and has the emission spectrum of Fig. 29. There is a slight indication of a direct absorption in the Mn2+ ions at 4000 A when large manganese concentrations are
1 R. J. GrNTHER: J. Electrochem. Soc. 101, 248 (1954).
26 G. F. J. GARLieK: Luminescence. Sect. 22.
present (5% ). This absorption has recently been confirmed in the writer's laboratory by R. LEACH (unpublished work) using activated single crystals of fluorite. In another investigation FEOFILOV 1 has found an intense red lu­ minescence produced by F centres in the fluorite. The emission is produced by all wavelengths lying in the absorption spectrum shown in Fig. 31 a and is in a broad band overlapping the tail of the absorption. The decay of the red emission is very rapid and has a mean lifetime less than 10-5 sec. The einission shows strong
JO
15
10
\ V
0
""
polarisation when excited by plane polarised light and Fig. 31 b shows the dependence of the degree of polarisation on the excitation wavelength. The values exceed those for isotropic fluorescent solu­ tions and suggest a strong anisotropy in the F centre system. This is confirmed by the depend­ ence of polarisation on the angle between the exciting light beam and the crystallographic axes. FEOFILOV 1 considers the long wavelength absorp­
a tion and the emission to be due to dipole "oscil­ M Of lators" with axes parallel to the edges of the ~ elementary cube of the lattice form. Negative
polarisation for shorter wavelength excitation results from "circular oscillators" lying in the 100 cube surface planes.
I I I
Äexc~~!,-
Studies of thermoluminescence in X-ray colo­ ured synthetic fluorites were made by HILL and ARON 2• A large nurober of spectral emission bands were found. The thermoluminescence peaks appear to fall into two groups and two types of lattice defects giving rise to electron traps were assumed.
b However, the large nurober of emission bands 02 OJ J-:W;wJ/, 06 ..U. 07 suggests the presence of trace impurities. It is
Fig. 31 a and b, (a) Absorption spectrum quite clear from this and from the Russian work ofCaF3 containing F centres. (b) Variation that very pure fluorite is worthy of extensive in­ of degree of polarisation with excitation wavelength for F centre fluorescence in vestigation in Single crystaJ form as structurally
CaF3• (After FEOFILOV 1.)
it is one of the siruplest phosphor systems.
22. Zinc and magnesium fluorides. Zinc and magnesium fluorides activated by manganese were developed in the writer's laboratory from 1942 onwards as cathode ray tube phosphors for radar. The main characteristic of practical importance is the relatively long lived transition in the Mn2+ ion (,..."10-1 sec) giving an exponential decay with trapping states giving a Ionger lived tail of phosphorescence. The zinc fluoride has an emission peak at 5870 A while the magnesium fluoride emission shows a peak at 5900 A. In the latter case prepara­ tion of the phosphor by precipitation from potassium fluoride produces a KMgF3
phosphor. Small additions of zinc fluoride to this together with manganese give a long afterglow specimen. jOHNSON and WrLLIAMS 3 have found a change in magnetic susceptibility of fluoride phosphors on excitation which was attributed to changes in multiplicity in the manganese ions. A recent report by HERSH­ BERGER 4 on attempts to detect changes in the Mn2+ paramagnetic resonance
1 P. P. FEOFILov: Dokl. Akad. Nauk SSSR. 92, 545 (1953). 2 J. J. HILL and J. ARON: J. Chem. Phys. 21, 223 (1953). 3 P. D. JoHNSON and F. E. WILLIAMs: J. Chem. Phys. 17, 435 (1945). 4 W. D. HERSHBERGER: J. Chem. Phys. 24, 168 (1956).
Sects. 23, 24. Beryllia and magnesia. 27
spectrum with excitation suggests that the effects observed by JoHNSON and WrLLIAMS were associated rather with photoconduction processes.
23. Cadmium iodide. Manganese activated cadmium iodide was investigated by KuTZELNIGG 1 . The variation of luminescence with temperature was found by GARLICK and WrLKINS 2 tobe dependent on the excitation wavelength. Since then lead activated cadmium iodide precipitated from solution · has been found tobe an efficient scintillator and the same workers have now studied the phosphor obtained from the melt 3.
c) Luminescence in some oxide phosphors. 24. Beryllia and magnesia. HEAD 4 reports a rather inefficient whitish cathodo­
luminescence for beryllia. The latter is more important as a component in the formation of the zinc beryllium silicate phosphors.
Magnesium oxide has become of some importance in single crystal form since
ouodvclioo bood o-1rT-r~-.~------------------7s
I I
the observations by HIBBEN 5 of the colouration of the crystals by short wavelength ultraviolet radiation and t !1.7--
the occurrence of thermoluminescence .Y---- - -- -----
t I
--------f..j
on thermal bleaching of the colour. Since then, DAv 6 has made a study of the photoconductivity in neutron irradiated crystals and found it to be P-type. The photoconductivity charac­ teristics have been dealt with by the writer elsewhere in this Encyclopedia (Vol. XIX). Luminescence spectra under discharge excitation have been meas­ ured by SAKSENA and PANT 7 who have also proposed an energy level scheme for the system 8 • More precise studies
.1.11---- - - ----- --- - -------,'17
.7.?------- ----- - - - -- ----3./
.:c- Fig. 32. Tentative energy Ievel scheme for MgO crystals
containing defect levels.
of cathodoluminescence have been made by EISENSTEIN 9• It is convenient here to summarise the data on optical transitions found in absorption photoconduction response and luminescence emission obtained in the various studies above.
The beginning of strong absorption characteristic of the lattice occurs at energies greater than 7 e V, but whether this is due to exciton or to free carrier production is not yet clear. CLARKE 10 has suggested a possible energy scheme for MgO which is more satisfactory than that of SAKSENA and PANT who assume an energy gap of only 5.9 eV. His scheme is shown in Fig. 32. The assigned values assume a tentative gap width of about 7.5 eV which is more consistent with optical data on MgO.
1 A. KuTZELNIGG: Angew. Chem. 49,267 (1936); 50,-366 (1937). 2 G. F. ]. GARLICK and M. H. F. WrLKINS: Proc. Roy. Soc., Lond., Ser. A 184, 408
(1945). 3 S. ScHLIVITCH and G. MoNOD-HERZEN: C. R. Acad. Sei. Paris 238, 2071 (1954). 4 R. B. HEAD: Electronic Eng. 20, 219 ( 1948). 5 J. HIEBEN: Phys. Rev. 51, 530 (1937). 6 H. R. DAY: Phys. Rev. 91, 822 (1953). 7 B. D. SAKSENA and L. M. FANT: Proc. Phys. Soc. Lond. B 67, 811 (1954). 8 B. D. SAKSENA and L. M. FANT: J. Chem. Phys. 23, 989 (1955). 9 A. ErsENSTEIN: USA. ONR, report N 7 onr 292, 1953.-- Phys. Rev. 93, 1017 (1954).
10 F. CLARKE: Private communication.
28 G. F. J. GARLICK: Luminescence. Sect. 25.
Table 4. Absorption, excitation and emission bands in MgO.
Absorption (WEBER) . 5.6 4.3 I (Excess 0 2)
4.8 3. 7 2.1 (Excess Mg)
Photoconduction (DAY) 4.8 3-7 2.1 (Virgin crystal) 4.8 3-7 2.1 1.2 (Neutron irradiated)
Emission (EISENSTEIN)
:I 5.4 3.61 2.72 2.42
Emission (SAKSENA and PANT). 3-7 2.75 2.2 Stimulation bands (EISENSTEIN) 3.4 2.3
EISENSTEIN has found that the 3.6 e V emission has a long afterglow of hyper­ bolic form, while Y AMAKA 3 has measured thermoluminescence vs. temperature curves for various specimens with and without additional coloration. He asso­ ciates the various peaks with oxygen or magnesium excess, but his data are not adequate enough to give thermal activation energies. The detailed know­ ledge of the energy scheme for MgO crystals will be much advanced by a more careful study of luminescence and photoconduction inclusive of exact decay measurements for each transition and of the detailed excitation spectra for the emission. In a recent publication Woons and WRIGHT 4 report a field-enhanced cathodoluminescence in MgO which is discussed in Chap. IV, Sect. 89 bdow.
Solid solutions of MgO-NiO studied by KRÖGER et coll. 5 show a number of discrete absorption bands between 2500 and 8000 A. MgO-NiO (1o-3 per mol. MgO) phosphors show a green luminescence at 90 °K when excited by cathode rays, the emission band showing eight sub-bands with separation L1 v = 194 cm-1
and attributed to transitions in the Ni2+ ions broadened by vibrational coupling. MgO-Bi phosphors 6 show such a fine structure but with different spacing. Vi­ brational terms are thus most likely modes of the activator and neighbours rather than of the crystal lattice.
25. The luminescence of zinc oxide. Zinc oxide has been extensively studied in the past with respect to its semiconduction properlies 7• Luminescent zinc oxide has been investigated by LEVERENZ and his associates 8, in thin film form by MoLLWO and STÖCKMANN 9 and by VERGUNAS and GAVRILov 10• The charac­ teristics of zinc oxide phosphors are closely related to those of zinc sulphide phosphors and the model for "self-activated" zinc sulphide has been applied also to zinc oxide by KRÖGER and VINK 11• Fig. 33 shows the variation of the cathodoluminescence spectrum of zinc oxide with temperature 12• The band at 3900 A is the effective edge emis.sion spectrum of zinc oxide (absorption edge at 3800 A) due do band-to-band transitions, while the broad emission band in the 5000 A region is associated with excess of zinc, i.e. a presence of oxygen vacancies. RANDALL 13 found a fine structure in the edge emission spectrum of
1 Shows long decay. 2 Enhanced by excess Mg. 3 E. YAMAKA: Phys. Rev. 96, 293 (1954). 4 J. WOODS and D. A. WRIGHT: Proc. Phys. Soc. Lond. B 68, 566 (1955). • F. A. KRÖGER, H. J. VINK and J. VAN DEN BooMGAARD: Physica, Haag 18, 77 (1952). 6 J. EwLEs and C. CURRY: Proc. Ph