Physica status solidi: Volume 7, Number 1 Oktober 1 [Reprint 2021 ed.] 9783112497043, 9783112497036

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physica status solidi B o a r d of E d i t o r s P. A I G R A I N , Paris, S. A M E L I N C K X , Mol-Donk, W. D E K E Y S E R , Gent, W. F R A N Z , Münster, P. GÖRLICH, Jena, E. G R I L L O T , Paris, R. K A I S C H E W , Sofia, P. T. L A N D S B E R G , Cardiff, L. N É E L , Grenoble, A. P I E K A R A , Poznan, A. S E E G E R , Stuttgart, O. S T A S I W , Berlin, M. S T E E N B E C K , Jena, F. STÖCKMANN, Karlsruhe, G. S Z I G E T I , Budapest, J . TAUC, Praha Editor-in-Chief P. GÖRLICH Advisory Board M. B A L K A N S K I , Paris, P. C. B A N B U R Y , Reading, M. B E R N A R D , Paris, W. B R A U E R , Berlin, W. COCHRAN, Cambridge, R. COELHO, Fontenay-aux-Roses, H.-D. DIETZE, Aachen, J . D. E S H E L B Y , Birmingham, G. J A C O B S , Gent, J. J A U M A N N , Köln, E. K L I E R , Praha, E. K R O E N E R , Clausthal-Zellerfeld, M. M A T Y A S , Praha, H. D. MEGAW, Cambridge, T. S. MOSS, Camberley, E. NAGY, Budapest, E. A. N I E K I S C H , Jülich, L. P A L , Budapest, M. RODOT, Bellevue/Seine, B. V. R O L L I N , Oxford, H. M. R O S E N B E R G , Oxford, R. V A U T I E R , Bellevue/Seine

Volume 7 1964

A K A D E M I E - V E R L A G . B E R L I N

Subscriptions and orders for single copies should be addressed to AKADEMIE-VERLAG GmbH, 108 Berlin, Leipziger Straße 3 - 4 or to Buchhandlung K U N S T U N D WISSEN, Erich Bieber, 7 Stuttgart S, Wilhelmstr. 4 - 6 or to Deutsche Buch-Export und -Import GmbH, 701 Leipzig, Postschließfach 276

S c h r i f t l e i t e r u n d v e r a n t w o r t l i c h f ü r d e n I n h a l t : P r o f e s s o r D r . D r . h . c. P . G ö r l i c h , 102 B e r l i n , N e u e S c h ö n h a u s e r S i r . 20 b z w . 69 J e n a , H u m b o l d t s t r . 26. R e d a k t i o n s k o l l e g i u m : D r . S. O b e r l ä n d e r , D r . £ . G u t s c h e , D r . W . B o r c h a r d t . A n s c h r i f t d e r S c h r i f t l e i t u n g : 102 B e r l i n , N e u e S c h ö n h a u s e r S t r . 20, F e r n r u f : 42 67 88. V e r l a g : A k a d e m i e - V e r l a g G m b H , 108 B e r l i n , L e i p z i g e r S t r . 3 - 4 , F e r n r u f : 2 2 0 4 4 1 , T e l e x - N r . 0 1 1 7 7 3 , P o s t s c h e c k k o n t o : B e r l i n 3 5 0 2 1 . — D i e Z e i t s c h r i f t „ p h y s i c a s t a t u s Bolidi" e r s c h e i n t j e w e i l s a m 1. d e s M o n a t s . B e z u g s p r e i s eines B a n d e s M D N 6 0 , — . B e s t e l l n u m m e r dieses B a n d e s 1068/7. G e s a m t h e r s t e l l u n g : V E B D r u c k e r e i „ T h o m a s M ü n t z e r ' * B a d L a n g e n s a l z a . — V e r ö f f e n t l i c h t u n t e r d e r L i z e n z n u r a m e r 1310 d e s P r e s s e a m t e s beim Vorsitzenden des Ministerräte? der Deutschen Deniokratischen Republik.

Contents Pa c

Review Articles A.

SCHMID

B . T . KOLOMIETS B . T . KOLOMIETS

«

Der Tunneleffekt bei Supraleitern Vitreous Semiconductors ( I ) Vitreous Semiconductors ( I I )

3 359 713

Original Papers J. E.

A . ALDERSON

Electron Transfer Transitions in KC1:T1

21

V . GALLINA a n d M . OMINI

Vacancy as a Phonon Field Perturbation (III)

29

B . SHARAN a n d L . M . TIWARI

Coupling Coefficients and Elastic Constants of Caesium Halides .

39

R . SUNDARAM a n d M . S . I I . CHARI

The Concentration Dependence of the Anomalous Electronic Transport Properties of Noble Metals Containing Small Amounts of Transition Metals

53

N . VAN H U O N G a n d K . O L B R Y C H S K I

Symmetrized Plane Waves for the Groups —Z>JjJ a t Four Symmetry Points of the Brillouin Zone N. K . HINDLEY Cyclotron Resonance and the Free-Carrier Magneto-Optical Properties of a Semiconductor 0 . HENKEL Ein Beitrag zum Problem der anhysteretischen Suszeptibilität von Dauermagnetwerkstoffen H . HOFFMANN Magnetische Feinstruktur und magnetische Eigenschaften einachsiger ferromagnetischer Schichten (I) A . 0).

Diese

n

tragen zu einem Ladungsstroni aus folgt

Pn a(P) D(En) bei (e Elementarladung). Dar-

VI

E pnna(p)D(En). «)

(2)

p (Pr>> 0)

Jb-^a erhält man aus dieser Gleichung dadurch, daß man na(p) durch die Verteilung nb(p) des Metalls b ersetzt und die Summation mit der Einschränkung pn < 0 durchführt. Bei kleiner Durchlässigkeit ist die Verteilung tij(p), j = a, b, sicher die des isolierten Metalls, also eine Fermiverteilung. Diese wird bestimmt durch die absolute Temperatur T und das elektrochemische Potential (Fermienergie)

fjLj

( I P r , - - \Pn 0

f(x)

— (er'k

r

+ l)" 1 . Somit ist

'

(3)

Jh^n.= (1>*>0)

Die Spannungsdifferenz U zwischen Metall a und b ist nun proportional der Differenz der elektrochemischen Potentiale: U = ~

(jua — /¿b):3) Diese ist bei

den dieser Betrachtung zugrunde liegenden Experimenten (|C7| < 1/10 V) klein 2 ) Man mag gegen diesen Ausdruck einwenden, daß auf Grund des Pauliverbots keine Elektronen aus a in besetzte Zustände von 6 gelangen können und folglich der Summand noch mit dem Faktor (1 — n ^ p ) ) zu multiplizieren ist; nb(p) = Zahl der Elektronen/cm 3 des Metalls 6 mit dem Impuls p. Dieser Faktor ändert das Endresultat (3) jedoch nicht und ist somit ohne Belang. 3 ) Die Spannungsdifferenz hängt nur von der Differenz der elektrochemischen Potentiale ab. Die Differenz der potentiellen Energie der Elektronen in den beiden Metallen (für das Modell nach Fig. 2 ist diese Differenz Null) ist, wie man nachprüfen kann, ohne Belang, wenn diese innerhalb vernünftiger Grenzen bleibt. Dies bestätigt die wohlbekannte Erfahrung, daß die Eigenschaften und das Verhalten eines entarteten Fermigases nur von den Elektronen in einer gewissen Umgebung der Fermikante bestimmt werden und daß die Tiefe des Fermisees praktisch bedeutungslos ist.

A. Schmid

6 gegen— fi a « — ( c a . e

e

10 V). Somit ist der Faktor {/„ — fb} in Gleichung (3)

»2

nur in einem kleinen Bereich - — « u„ wesentlich von Null verschieden. 2

to

r bewirkt beim normalen Tunnelstrom Js s eine Stufenstruktur der Strom-Spannungs-Charakteristik, wobei die Stufenbreite AU =

ist [51, 52]. Diese Charakteristik ist ähnlich der von J c , wo die Stufen-

breite, wie in Abschnitt 9 gezeigt, nur halb so groß ist. 11. Schlußbemerkungen Der Tunneleffekt zwischen Supraleitern bietet sowohl für das Experiment wie auch für die Theorie interessante Probleme, die es lohnen, sich mit ihm zu befassen. Seine Hauptbedeutung liegt jedoch darin, daß er die effektive Zustandsdichte eines Supraleiters zu bestimmen gestattet. Diese hängt unmittelbar von der f ü r die Theorie v o n BARDEEN, COOPER und SCHRTEFFER charakteristischen

Energielücke A ab. Die bei Tunnelexperimenten erreichte Genauigkeit bei der Bestimmung von A wird von keinem anderen Experiment übertroffen. Die Entdeckung einer Feinstruktur in der Strom-Spannungs-Charakteristik des Tunnelstroms ließ überdies die Wechselwirkung der Elektronen mit den Gitterschwingungen „sichtbar" werden, was die Theorie ermutigt hat, realistischere Modelle für die den supraleitenden Zustand wesentlich bestimmende Wechselwirkung in Betracht zu ziehen. Die Experimente schließlich, welche den von JOSEPHSOK geforderten anomalen Tunnelstrom bestätigten, bewiesen aufs neue die reale Existenz der Cooper-Paare, welche ihrerseits die wesentliche Voraussetzung der T h e o r i e v o n B A R D E E N , COOPER u n d SCHRIEFFER ist.

Herrn Professor G. FALK danke ich sehr für wertvolle Kritik und Hilfe, die er dieser Arbeit zuteil werden ließ. Literatur [ 1 ] J . FRENKEL, P h y s . R e v . 36, 1604 ( 1 9 3 0 ) ;

siehe auch W. EHRENBERG und H. H Ö N L , Z. Phys. 68, 289 (1931). [2] I. GIAEVER, Phys. Rev. Letters 5, 147 (1960); 5, 464 (1960). [3] J. NICOL, S. SHAPIRO und P. H. SMITH, Phys. Rev. Letters 5, 461 (1960). [4] J. BARDEEN, Phys. Rev. Letters 6, 57 (1961). [ 5 ] A . SCHMID, Z . P h y s . 178, 26 (1964).

[6] M. H. 23)

COHEN,

L.M.

FALICOV

und J. C.

PHILLIPS,

Phys. Rev. Letters 8, 316 (1962).

Ein Supraleiter vom Typ I I verhält sich bei Tunnelexperimenten anders [49].

Der Tunneleffekt bei Supraleitern

19

[7] W. HOLZAPPEL, Unveröffentlichte Diplomarbeit; Institut f. Mathematische Physik, Techn. Hochschule Karlsruhe, 1962. [8] R . E . PRANGE, Phys. Rev. 131, 1083 (1963). [9] L. P. KADANOFF und G. BAYM, Quantum Statistical Mechanics, W. A. Benjamin, Inc., New York 1962 (siehe insbesondere Abschn. 2). [10] L . D . LANDAU, Zh. exper. theor. Fiz. 30, 1058 (1956); übersetzt in: Soviet Physics J E T P 3, 920 (1957). [11] M. GELL-MANN, Phys. Rev. 106, 369 (1957). [ 1 2 ] J . BARDEEN, L . N . COOPER u n d J . R . SCHRIEFFER, P h y s . R e v . 1 0 8 , 1 1 7 5 ( 1 9 5 7 ) .

[13] N. N. BOGOLJUBOV, Zh.expsr. theor. Fiz. 34, 58 (1958); übersetzt in: Soviet Physics J E T P 7, 41 (1958).

N. N. BOGOLJUBOV, Uspekhi fiz. Nauk 67, 549 (1959); übersetzt in: Soviet Phys. üspekhi 2, 236 (1959). [14] L . P . GORKOV, Zh. exper. theor. Fiz. 34, 735 (1958); übersetzt in: Soviet Physics J E T P 7, 505 (1958). [ 1 5 ] I . GIAEVER, H . R . HART, j r . u n d K . MEGERLE, P h y s . R e v . 1 2 6 , 9 4 1 ( 1 9 6 2 ) . [ 1 6 ] J . M . ROWELL, A . G . CHYNOWETH u n d J . C . PHILLIPS, P h y s . R e v . L e t t e r s 9 , 5 9 ( 1 9 6 2 ) .

[17] J . M. ROWELL, P . W . ANDERSON u n d D . E . THOMAS, P h y s . R e v . L e t t e r s 10, 334 (1962). [ 1 8 ] L . VAN HOVE, P h y s . R e v . 8 9 , 1 1 8 9 ( 1 9 5 3 ) .

[19] G. M. ELIASCHBERG, Zh.expsr. theor. Fiz. 38, 966 (1960); übersetzt in: Soviet Physics J E T P 11, 696 (1960). [ 2 0 ] Y . NAMBU, P h y s . R e v . 1 1 7 , 6 4 8 ( 1 9 6 0 ) . [ 2 1 ] J . R . SCHRIEFFER, D . J . SCALAPINO u n d J . W . WILKINS, P h y s . R e v . L e t t e r s 1 0 , 3 3 6

(1963). J . R. SCHRIEFFER, Rev. mod. Phys. 36, 200 (1964). [22] D . J . SCALAPINO u n d P . W . ANDERSON, P h y s . R e v . 133 A, 921 (1964).

[23] W. A. HARRISON, Phys. Rev. 123, 85 (1961). [ 2 4 ] W . FRANZ, Z . N a t u r f . 1 6 a , 4 3 6 ( 1 9 6 1 ) .

[25] J . BARDEEN, P h y s . R e v . L e t t e r s 9, 147 (1962). [ 2 6 ] I . GIAEVER u n d K . MEGERLE, P h y s . R e v . 1 2 2 , 1 1 0 1 ( 1 9 6 1 ) .

[27] N. V. ZAVARITSKH, Zh. expsr. theor. Fiz. 41, 657 (1961); übersetzt in: Soviet Physics J E T P 14, 470 (1962). [ 2 8 ] S . SHAPIRO, P . H . SMITH, J . NICOL, J . L . MILES u n d P . F . STRONG, I B M J . R e s . D e -

velopm. 6, 34 (1962). [29] I. DIETRICH, Z. Naturf. 17a, 94 (1962). [30] P . TOWNSEND u n d J . SUTTON, P h y s . R e v . 128, 591 (1962).

[31] P. W. ANDERSON, J . Phys. Chem. Solids 11, 26 (1959). [32] N. V. ZAVARITSKII, Zh. exper. theor. Fiz. 43, 1123 (.1962); übersetzt in: Soviet Physics J E T P 16, 793 (1963). [ 3 3 ] B . N . TAYLOR u n d E . BURSTEIN, P h y s . R e v . L e t t e r s 1 0 , 14 ( 1 9 6 3 ) .

[34] J . R. SCHRIEFFER und J . W. WILKINS, Phys. Rev. Letters 10, 17 (1963). [35] F. REIF und M. A. WOOLF, Phys. Rev. Letters 9, 315 (1962). [36] A. A. ABRIKOSOV und L. P. GORKOV, Zh.expsr.theor.Fiz. 39, 1781 (1960); übersetzt in: Soviet Physics J E T P 12, 1243 (1961). [37] B. D. JOSEPHSON, Phys. Letters (Netherlands) 1, 251 (1962). B . D . JOSEPHSON, R e v . m o d . P h y s . 3 6 , 2 1 6 ( 1 9 6 4 ) .

[38] [39] [40] [41] [42]

P. W. ANDERSON und J . M. ROWELL, Phys. Rev. Letters 10, 230 (1963). S. SHAPIRO, Phys. Rev. Letters 11, 80 (1963). R. A. FERRELL und R. E. PRANGE, Phys. Rev. Letters 10, 479 (1963) . P. W. ANDERSON, Preprint: Weak Superconductivity. The Josephson Tunneling Effect. P. G. DE GENNES, Phys. Letters 5, 22 (1963).

[43] J . M. ROWELL, P h y s . R e v . L e t t e r s 11, 200 (1963).

[44] M. D. FISKE, Rev. mod. Phys. 36, 221 (1964). [45] A . A . ABRIKOSOV, Zh. exper. theor. Fiz. 32, 1442 (1957); übersetzt in: Soviet Physics J E T P 5, 1174 (1957). 2*

20

A. SCHMID: Der Tunneleffekt bei Supraleitern

[46] V . AMBEGAOKAR u n d A . BARATOFF, P h y s . R e v . L e t t e r s 1 0 , 4 8 6 ( 1 9 6 3 ) ;

Erratum: Phys. Rev. Letters 11, 104 (1963). [47] B. RIEDEL, Unveröffentlichte Diplomarbeit; Institut f. Theoretische Physik der Univ. zu Köln, 1964. [48] D . H . DOUGLASS, P h y s . R e v . L e t t e r s 7, 14 (1961).

[49] W. J . TOMASCH, Phys. Lstters (Netherlands) 9, 104 (1964). [50] V. L. GINZBURG und L. D. LANDAU, Zh. exper. theor. Fiz. 20, 1064 (1950). L . P . GORKOV, Zh. expsr. theor. Fiz. 36, 1918 (1959): übersetzt in: Soviet Physics J E T P 9, 1364 (1959). [51] A . H . DAYEM u n d R . H . MARTIN, P h y s . R e v . L e t t e r s 8, 2 4 6 (1962). [52] P . K . TIEN u n d I . P . GORDON, P h y s . R e v . 1 2 9 , 647 ( 1 9 6 3 ) . (Received

June

8,

1964)

Original Papers phys. stat. sol. 7, 21 (19C4) Department

of Physics,

University of Western

Australia

Electron Transfer Transitions in KC1:T1 By J . E . A . ALDERSON Evidence is presented for the occurrence of electron transfer transitions in KC1:TI, the return to the unexcited state producing the previously observed but unexplained 4750 A emission. Recognition of this electron transfer process leads to an explanation of KC1:T1 thermoluminescence, and a tentative explanation of phosphorescence. Es wird gezeigt, daß in KC1:T1 Übergänge zwischen Tl-Ionen und den benachbarten Cl-Ionen auftreten; die Rückkehr des Elektrons in den nichtangeregten Zustand führt zu der bisher nicht erklärten Emissionsbande bei 4750 A. Auf dieser Grundlage läßt sich die Thermolumineszenz von KCl :T1 erklären. Außerdem wird eine Deutungsmöglichkeit für die Phosphoreszenz angegeben.

1. Introduction A basis for the luminescence process in activated alkali halides involving transitions contained within the impurity ion was proposed by SETTZ [ 2 2 ] . The possibility of electron transfer transitions i.e. from halogen host crystal ions to activator ions, was rejected bj' him because the expected doublet structure from the halogen ion was not observed in the impurity absorption spectrum. KNOX [15] has since shown that this doublet structure is not necessarily expected. Recently there has been an accumulation of experimental observations which are difficult to explain by the Seitz model alone. These include thermoluminescence and the complex absorption and emission spectra observed for the thallium A-band (EWLES a n d JOSHI [10], PATTERSON [18], BUTLER [4], JOHNSON a n d WILLIAMS [13].

In order to explain these various results several previously discarded theories of luminescence have been reconsidered by various authors. No single type of luminescence centre has been found that can account for all the experimental observations. This has led to explanations involving at least two different types of luminescence centre. One of the centres is usually that used in the Seitz model because this satisfactorily explains many of the experimental results (JOHNSON and WILLIAMS

[13]).

Other types of centres which have been considered include: 1. Two different T l + luminescence centres in which the host lattice in the vicinity of the thallium activator has two possible structures, i.e. CsCl or NaCl structure (PATTERSON [18]).

2. Centres in which the thallium activator and the host crystal ions form complexes with molecular binding. This suggestion was made by PRINGSHEIM [19] to explain the similarities between the luminescence of liquid and solid solutions of activator and alkali halides. 3. Association of activator ions with crystal defects (TEEGARDEN and WEEKS [24]) or with crystalline interfaces (GINDINA [11]). 4. Aggregates of activator ions. 5. An activator centre differing from the Seitz model by allowing electron transfer transitions in addition to the electron excitation transitions confined to the impurity ion (KNOX [15]).

22

J . E . A. ANDERSON Fig. X. Lumìnescence variation with temperature. Excitation energy 10 eV a) b) e) d)

-750

-100

-50

0 50 Temperature (°C)

KC1 "visible" emission KC1:T1 (7 x 10- s mf) "visible" emission KC1:T1 (7 x 10-* mf) "uv" emission Thermoluminescence "visible" emission

100

2. Experimental Consideration of many properties of thallium activated KC1 suggests that a single centre of the fifth type listed above can provide a picture of the luminescence mechanism which does not conflict with those results explained by the Seitz model, and which can include many of the previously unexplained results. Fig. 1 curves a), b) and c) show the variation of luminescence efficiency for samples of KC1 and KC1:T1 excited in the band-to-band absorption region at 9.5 eV. The observations are separated into two emission components as described in a previous publication (ALDERSON and DIMOND [ 1 ] ) . The " u v " consists of 3 0 5 0 A emission characteristic of the T l + ion. The low temperature dependence of the "visible" luminescence from KC1:T1 is similar to the luminescence from pure KC1, except for the peak at about — 50 °C. This can be related to the glow peak at — 70 °C. If the glow peak is caused by thermal release of carriers then these same carriers will be free at temperatures in excess of — 70 °C i.e. in the range of the — 50 °C peak appearing in the curve for luminescence efficiency change with temperature. Although the emission spectrum has not been measured for band-to-band excitation at — 50 °C it is expected to be the same 4750 A emission as observed from the — 70 °C glow peak. The correlation between thermoluminescence emission and the luminescence emission resulting from band-to-band excitation at various temperatures is further indicated in Fig. 2 where the ratio of " u v " to "visible" emission is plotted for the two types of luminescence. In either case a similar temperature dependence is observed. The same decrease is not observed for excitation in the impurity absorption bands where the ratio of " u v " to "visible" fluorescence shows very little variation with temperature. The thermoluminescence emission, however, shows the same variation as observed for band-to-band excitation.

Fig. 2. Variation of the ratio

-150

-W0

-50

0 50 Temperature (°C)

no

/Uv

" u v " emission

lyis

"visible" emission

with temperature for sample KCliTl (7 x 10- 5 mf), excitation energy 10 eV

Electron Transfer Transitions in KCl:TI

23

I t appears that the same carriers as those produced by band-to-band transitions are responsible for the thermoluminescence. The problem of producing these carriers by excitation in the impurity absorption bands is discussed below. If KC1:T1 is excited by photons with energy greater than 8.1 eV free electrons a n d holes are p r o d u c e d (KUWABARA a n d AOYAGI [ 1 6 ] , TAFT a n d PHILLIP [ 2 3 ] )

by ionisation of CI" ions. At room temperature the electrons are presumably captured by the T1+ ions to form Tl° centres. The Tl° atoms then act as hole traps, and characteristic thallium 3050 A emission results from this later recombination. At low temperatures the holes do not immediately recombine at Tl° centres because there is a competing self-trapping process which produces CL~ centres (CASTNER a n d KANZIG [6]). A t t e m p e r a t u r e s in e x c e s s of — 1 2 0 °C t h e Cl;r c e n t r e s b e c o m e m o b i l (DELBECQ, SMALLER, a n d YUSTER [7]) a n d m i g r a t e t o t h e a t o m i c

Tl° centres. The capture of mobile CI,7 is presumably observed in the glow peak a t — 7 0 °C w h i c h produces 4 7 5 0 A emission (VITOL [ 2 5 ] , Y A E K a n d OKK [ 2 6 ] ) .

At lower temperatures where the CI7 are not mobile excitation of band-toband transitions in KC1: T1 does not produce either 3050 or 4750 A emission.

A n emission b e t w e e n 4 0 0 0 a n d 4 2 0 0 A

h a s b e e n o b s e r v e d b y AOYAGI a n d

KUWABARA [2] which they suggested was the result of electron capture at defect centres. The same emission was observed from pure KCl, and is assumed to be that shown in the dependence of luminescence with temperature for pure KCl in Fig. l a . This emission is presumably also the same as that observed in the glow peak at about — 100 °C reported for both KCl :T1 and pure KCl by YAEK and OKK [26] and by DUTTA and GHOSH [8] when electrons thermally released from shallow traps are captured at defects. The emission spectrum from this thermoluminesc e n c e is in t h e r a n g e f r o m 4 0 0 0 t o 4 2 0 0 A.

The "visible" emission from KCl at — 160 °C (under band-to-band excitation) is increased considerably by a small amount of thallium activation. This may be due to an increase in the crystal defects near the surface, but seems more likely to result from electron capture at thallium ions to produce thallium atoms. Because of the breadth of the emission spectra it is not necessarily separable from the emission produced by electron capture at defects. From band-to-band excitation of pure KCl at low temperature ( — 1 6 0 °C) a " u v " emission was detected (ALDERSON and DIMOND [1]) which may be caused by electron capture at immobile CI 7 centres. This was only observed in the purest vacuum grown samples where the competing process of capture at defect centres was expected to be a minimum. In the vicinity of room temperature the luminescence emission becomes predominantly that characteristic of the thallium centre (3050 A). After band-to-band excitation this presumably results from successive capture of electrons and mobile CljT at thallium ions as indicated above. In the final stage prior to recombination there is a thallium atom Tl° with a neighbouring CI.7. This configuration is the same as would be expected from an electron transfer transition by which an electron is excited to a Tl + ion from a neighbouring CI" ion. This produces Tl° and Cl° in neighbouring lattice sites and the Cl° may then share an electron with a neighbouring CI" to form CI7, provided that the CI 7 is stable at the temperature of observation. If this transition is possible then it should be observed in the absorption spectrum of KC1:T1, and the return to the unexcited state should produce an emission at 4750 A as in the case of the — 70 °C glow peak. This mechanism would account for the 4750 A emission observed when KCl: T1 is excited in t h e i m p u r i t y a b s o r p t i o n b a n d s (JOHNSON a n d WILLIAMS [ 1 3 ] ) .

24

J. E . A . ALDERSON

PATTERSON [18] has shown that the thallium A-absorption band is a doublet with maxima at 5.0 and 4.9 eV (the peak at 4.7 eV is of no interest here as it was shown to be due to thallium aggregates). Excitation in the two peaks of the A-band produces both 3050 and 4750 A emission. One of these peaks could result from a transition within the thallium ion corresponding to the 1*S'0 to 3P1 transition as in the Seitz model, and the other could be due to an electron transfer transition as anticipated above. Because of the broadness of the absorption bands both transitions will normally be excited throughout the energy spread of the composite A-band. At low temperatures ( — 196 °C) and for excitation on the high energy edge of the thallium A-band at 5.06 eV, JOHNSON and WILLIAMS [13] have observed 3050 A emission not accompanied by 4750 A emission, which associates the 3050 A emission with the higher energy component of the A-band. The mixture of 4750 A and 3050 A emission in the 30 °C glow peak and the room temperature fluorescence from band-to-band excitation is attributed to two recombination processes for Tl° and CI7 i.e. firstly, recombination by a direct electron transfer transition with 4750 A emission, and secondly, recombination producing a thallium ion in the 3PL excited state which subsequently decays to the ground state with characteristic 3050 A emission as in the Seitz model. 3. Energy Level Diagram A schematic energy level diagram for KC1:T1 which includes the electron transfer transition, and which is based on the total energy of the KC1:T1 system, is shown in Fig. 3. A single configuration coordinate diagram cannot be simply devised to accommodate the processes involved because a single coordinate cannot be used where several centres are involved viz. Tl°, Tl + , Cl°, CI", Ci~. Commas separating the named constituents of an energy state signify physical separation great enough to prevent interaction. Diagonal broken lines indicate energy loss in Stokes shift. For the case of the transition corresponding to a thallium 1 , w,j H h