efforts of Riga and Tartu physicists (Lushchik, Vitol,. Valbis, Zirap, Belkind, Yaek, Denks, Khaldre, and. Pung), who used a wide variety of methods based on.
LUMINESCENCE O F IONIC CRYSTALS* F . D. K l e m e n t and Ch. B. L u s h c h i k Zhurnal P r i k l a d n o i S p e k t r o s k o p i i , Vol. 8, No. 4, pp. 545-552, 1968 UDC 535.37 Systematic research on the luminescence of ionic crystals has been carried out in the Soviet Union for more than thirty years. The pioneers of this line of research-Terenin and Kliment in Leningrad [I], Kudryavtsev in Tomsk [2], Antonov-Romanovskii in Moscow [3], Parfianovich in Irkutsk [4], and Kats in Saratov [5]-correctly assessed, in the middle 'thirties, the immense opportunities opened up by research on the optical characteristics of large single crystals and thin sublimed layers. Even before World War If. the great prospects of the technical application of large luminescent crystals and the thin luminescent layers developed by Klement [6] and called "sublimate phosphors ~ had become clear. Since that time, alkali halide crystals (AHC)-crystals with ionic bonds-have become the object of thorough investigation by several generations of Soviet physicists. Since 1948, AHC have been successfully used for the detection and spectrometry of nuclear radiations. After 1960, lasers were developed on the basis of ionic crystals. In recent years, attractive prospects of using crystals as largecapacity memory cells, dielectric electronic components, etc. have been envisaged. A l l this gives great practical significance to research on the luminescence of ionic crystals. Nevertheless, the mainspring of the development of our field of solid-state physics is probably the great cognitive-scientificvalue of research on ionic crystals, which constitute a special and relatively simple class of condensed systems with pronounced maximum and minimum characteristics. Many large and small scientific teams in the Soviet Union are now working in the field of luminescence of ionic crystals. Research in this line is being carried out in Moscow, Leningrad, Kiev, Khar'kov, Lvov, Tartu, Riga, Irkutsk, Saratov, Tomsk, Tbilisi, Tashkent, Baku, Odessa, Alma-Ata, Osh, and other towns. Research on the physics of luminescent ionic crystals in the Soviet Union is fairly well coordinated, as is indicated, in particular, by the regular convention of conferences on various aspects of the physics of ionic crystals. In the last eight years there have been six of these conferences (Tartu, 1959 [7]; Riga, ]961 [8]; Lvov, 1964 [9]; Riga, 1965 [10]; Tbilisi, 1966 [11]; Tomsk, 1967). One of the main centers of research on the luminescence of ionic c r y s t a l s - T a r t u - h a s regularly published its work in the last 12 years in the form of a series of periodicals [12]. The Riga physicists also regularly report their results in a special publication [13]. Below we discuss very briefly some of the most important scientific results obtained in the Soviet
Union on the physics of luminescent ionic crystals. These results can be grouped in three sections. Luminescence centers in ionic crystals. Discussion of the nature of the luminescence centers has been a regular feature in the topic of luminescence. In the case of ionic crystals, this discussion has led to a fairly clear concept of the structure of the main luminescence centers. Electronic vibrational processes within these centers have also been carefully investigated. The research of El'yashevich []4] and Feofilov [15] on the luminescence of rare earths in ionic crystals, which laid the groundwork for the development of man} types of lasers, is well known. The use of Feofilov's polarization method []6] of investigating the latent anisotropy of luminescence centers in cubic crystals and of Kaplyanskii's piezospectroscopic method [17] has led to a thorough investigation of the symmetry and structure of impurity centers in ionic crystals colored and activated by rare earths. An important contribution to this research has been made by the magnetooptic method, widely used by Zakharchenya and her colleagues [18]. Reasearch on luminescence centers in AHC activated by "mercurylike" ions (Ga-, In~, T] +, Sn2+, Pb2~-, Bia~) has also been successfully developed. A program of X-ray structural, elcetron-diffrtlction,physicochemical, and optical investigations led Klement [19, 20] to the important concept of crystal phosphors as solid solutions of the activator in the main substance. In thallium-activated AHC, for instance, the main luminescence centers are T] + ions at the regular cationic points of the crystal lattice. The correctness of this view was disputed by Shamovskii [21]~ but recently it was rigorously proved by the technique of electron paramagnetic resonance. A program of investigations by Klement, Ivanova, Kirs, Gindina, Malysheva, Pae, and Khaav et ah has been devoted to the interrelationship between the luminescence characteristics and the crystal structure of phosphors (see [22]. for instance). Of particular interest is the work of Ivanova's ~roup [23] on ~complex" luminescence centers in AHC and that of Kers' group [24] on the effect of high hydrostatic pressures on luminescent ionic crystals.
*Paper read at the Jubilee Session of the Scientific Committee on Luminescence of the Division of General and Applied Physics of the Academy of Sciences, USSR, 17 October 1967. 329
Using AHC activated by mercurylike ions, Shalimova [25] in Moscow, Lushchik et al. [26] in Tartu, and Plyavin and Trinkler [27] in Riga successfully developed a ~quasi-atomic spectroscopy" of crystals. They managed to establish a genetic relationship between the energy levels of luminescence centers in the crystals and the free activating ions. They determined the nature (multipolarity) and orientation of the elementary emitters and absorbers, investigated the mechanisms and probability of radiative and nonradiative transitions, and devised methods of predicting approximately the spectra of new crystal phosphors from the characteristics of the a c t i v a t i n g ions. A p r o g r a m of o p t i c a l i n v e s t i g a t i o n s on a c t i v a t e d ionic c r y s t a l s has been c a r r i e d out in T a r t u and Riga. Such w o r k was conducted e a r l i e r by Vavilov, the found e r of the Soviet school of l u m i n e s c e n c e , and a l s o by L e v s h i n , Stepanov, and N e p o r e n t on s o l u t i o n s of c o m p l e x m o l e c u l e s . C o m p r e h e n s i v e r e s e a r c h on the a b s o r p t i o n , e m i s s i o n , quantum y i e l d , d u r a t i o n of l u m i n e s c e n c e , and v a r i o u s p o l a r i z a t i o n c h a r a c t e r i s t i c s has p r o v i d e d abundant i n f o r m a t i o n about e l e c t r o n i c v i b r a t i o n a l p r o c e s s e s in a c t i v a t e d ionic c r y s t a l s . At r o o m and h i g h e r t e m p e r a t u r e s , the r e l a t i o n s h i p s a r e the s a m e as for c o m p l e x m o l e c u l e s , and in a c t i v a t e d ionic c r y s t a l s at high t e m p e r a t u r e s , an e q u i l i b r i u m d i s t r i b u t i o n of e x c i t e d l u m i n e s c e n c e c e n t e r s o v e r the v a r i o u s v i b r a t i o n a l and e l e c t r o n i c s t a t e s is e s t a b l i s h e d . In a c t i v a t e d AHC at low t e m p e r a t u r e s , the s i t u a t i o n is much m o r e c o m p l i c a t e d . I n s t e a d of the y i e l d and l i f e t i m e being independent of the f r e q u e n c y of e x c i t i n g r a d i a t i o n , t h e r e is a c h a r a c t e r i s t i c p o w e r r e l a t i o n s h i p between t h e s e q u a n t i t i e s and the f r e q u e n c y . At low t e m p e r a t u r e s , the link between the d i f f e r e n t e l e c t r o n i c s t a t e s is w e a k e r , and the e q u i l i b r i u m d i s t r i bution can be e s t a b l i s h e d only f o r the v i b r a t i o n a l s t a t e s of each e l e c t r o n i c s t a t e [28]. S p e c i a l m e n t i o n m u s t be m a d e of the d e v e l o p m e n t of a c o n s i s t e n t q u a n t u m - m e c h a n i c a l t h e o r y of l u m i n e s c e n c e c e n t e r s in c r y s t a l s . Such a t h e o r y has b e e n d e v e l o p e d f o r c e n t e r s of l a r g e r a d i u s by P e k a r and K r i v o g l a z [29], Davydov and Lubchenko [30], P e r l i n [31], and o t h e r s . A t h e o r y f o r c e n t e r s of s m a l l r a d i u s (of the m e r c u r y l i k e ion type) has b e e n d e v e l o p e d by K r i s t o f e l , P e r a s h e n , and A r a b e n o v [32]. L i v s h i n , Kagan, and Zaft~s s y s t e m a t i c c o n s i d e r a t i o n of l o c a l and q u a s i - l o c a l v i b r a t i o n s in s o l i d s t a t e p h y s i c s is also worth mentioning. Of g r e a t i n t e r e s t is Rebane, Trffonov, and Khizhnyakov~s t h e o r y [33] of q u a s i - l i n e s p e c t r a of i m p u r i t y c e n t e r s in c r y s t a l s . On the b a s i s of this t h e o r y Z a k i s , L~Aissar; R e b a n e , Siltd, and o t h e r s d e v e l o p e d " q u a s i m o l e c u l a r s p e c t r o s c o p y " of ionic c r y s t a l s a c t i v a t e d by molecular impurities. L u m i n e s c e n c e c e n t e r s have now b e e n i n v e s t i g a t e d not only in AHC, but a l s o in o t h e r ionic c o m p o u n d s , p a r t i c u l a r l y s i l v e r h a l i d e s (Golub, Belous), a m m o n i u m h a l i d e c r y s t a l s ( P a e , Uibo), e t c . M i g r a t i o n and a c c u m u l a t i o n of e n e r g y in l u m i n e s c e n t ionic c r y s t a l s ~ The i n v e s t i g a t i o n of c o l l e c t i v e e f f e c t s is at the f o r e f r o n t of m o d e r n s o l i d - s t a t e p h y s i c s . The m e t h o d of q u a s i - p a r t i c l e s is e x t r e m e l y f r u i t f u l f o r this p u r p o s e . Q u a n t u m - m e c h a n i c a l t h e o r y is c o n s i s t e n t 330
with e x p e r i m e n t in p r e d i c t i n g the e x i s t e n c e in ionic c r y s t a l s of such q u a s i - p a r t i c l e s as conductivity e l e c t r o n s , h o l e s , e x c i t o n s , e t c . Soviet p h y s i c i s t s have s u c c e s s f u l l y i n v e s t i g a t e d the r o l e of v a r i o u s q u a s i p a r t i c l e s in the m i g r a t i o n and a c c u m u l a t i o n of e n e r g y in l u m i n e s c e n t ionic c r y s t a l s . This has given i m p o r t a n t i n f o r m a t i o n about the m e c h a n i s m s of l u m i n e s c e n c e of s o l i d s , and about new p r o p e r t i e s of d i f f e r e n t q u a s i p a r t i c l e s in ionic c r y s t a l s . The c l a s s i c a l w o r k s of A n t o n o v - R o m a n o v s k i i [3], Kats [5], and P a r f i a n o v i c h [4, 35], and the d i s c o v e r y of the photoconductivity of a c t i v a t e d AHC by K y a e m b r e and T i i s l e r , left no doubt that the long l u m i n e s c e n c e of a c t i v a t e d AHC is r e c o m b i n a t i o n l u m i n e s c e n c e . T h e r e a r e two t y p e s of r e c o m b i n a t i o n l u m i n e s c e n c e : the l u m i n e s c e n c e can be p r o d u c e d e i t h e r by r e c o m b i n a t i o n of mobile e l e c t r o n s with l o c a l i z e d h o l e s o r by the r e c o m b i n a t i o n of m o b i l e holes with l o c a l i z e d e l e c t r o n s . The d i f f e r e n t i a t i o n of t h e s e t y p e s n e c e s s i t a t e s d e t e r m i n a t i o n of the sign of the c u r r e n t c a r r i e r s , which is not e a s y even in the c a s e of s e m i c o n d u c t o r s , and is e x c e p t i o n a l ly difficult in the c a s e of d i e l e c t r i c c r y s t a l s . This has been a c c o m p l i s h e d in r e c e n t y e a r s by the c o l l e c t i v e e f f o r t s of Riga and T a r t u p h y s i c i s t s (Lushchik, Vitol, V a l b i s , Z i r a p , Belkind, Yaek, Denks, K h a l d r e , and Pung), who u s e d a wide v a r i e t y of m e t h o d s b a s e d on m e a s u r e m e n t of the " n o n i s o t h e r m a l r e l a x a t i o n of e l e c t r o n - h o l e p r o c e s s e s " [ 3 7 - 4 3 ] . They m a n a g e d to show that the a c t i v a t o r l u m i n e s c e n c e is u s u a l l y due to r e c o m b i n a t i o n of h o l e s with a t o m i c a c t i v a t e d c e n t e r s [ 3 9 - 4 3 ] . The e l e c t r o n - h o l e p r o c e s s e s l e a d i n g to r e c o m b i n a t i o n l u m i n e s c e n c e a r e c o m p l i c a t e d by p r i o r l o c a l i z a t i o n of e l e c t r o n s and h o l e s in v a r i o u s t r a p s . E l e c t r o n and hole t r a p s in ionic c r y s t a l s have been t h o r o u g h l y i n v e s t i g a t ed by the m e t h o d of t h e r m a l l u m i n e s c e n c e , the o r i g i n and d e v e l o p m e n t of which owes much to the w o r k s of M o r g e n s h t e r n and K a t s , to the m e t h o d s of n o n i s o t h e r m a l r e l a x a t i o n of o p t i c a l and p a r a m a g n e t i c a b s o r p t i o n d e v i s e d by L u s h c h i k , Z a i t o v , Yaek, K h a l d r e , and o t h e r s t37, 38, 42] and to the e l e c t r i c a l m e t h o d s d e v e l o p e d in R i g a by Vitol, Z i r a p , Betkind, and o t h e r s . D i v e r s e r e s u l t s r e l a t i n g to t r a p p i n g c e n t e r s in AHC have been obtained in Baku b y K h a l i l o v , M a m e d o v , and o t h e r s [44] and in Kiev by Shatalov, Kalabukhov, G o r l i e n k o , and o t h e r s . S p e c i a l m e n t i o n m u s t be m a d e of the s e r i e s of w o r k s of Vitol and his c o l l e a g u e s using an u l t r a m o d e r n r e l a x a t i o n d e v i c e with an e l e c t r o n i c d i g i t a l c o m p u t e r at the output. ~'Industrial p h y s i c s n has a r r i v ed in the l u m i n e s c e n c e l a b o r a t o r y . E l e c t r o n - h o l e p r o c e s s e s in AHC and s e m i c o n d u c t i n g s y s t e m s d i f f e r q u a n t i t a t i v e l y and q u a l i t a t i v e l y . The q u a n t i t a t i v e d i f f e r e n c e c o n s i s t s , in p a r t i c u l a r , in the fact that i n t e r b a n d t r a n s i t i o n s in AHC a r e e f f e c t e d in the f a r v a c u u m u l t r a v i o l e t . Hence, f o r a long t i m e the c o m m o n n a t u r e of m a n y p h y s i c a l p h e n o m e n a in AHC and, s a y , zinc sulfide p h o s p h o r s , could not be e s t a b l i s h e d . Yet, as f a r back as 1935 T e r e n i n and K l e m e n t [1] d i s c o v e r e d e n e r g y m i g r a t i o n f r o m the m a i n s u b s t a n c e to the l u m i n e s c e n c e c e n t e r s in a l k a l i i o d i d e s - an effect t y p i c a l of sulfide s y s t e m s . The v a c u u m u l t r a v i o l e t r e g i o n has now been well i n v e s t i g a t e d . T a r t u p h y s i c i s t s ( I I ' m a s , Kink, L i i d ' y a , and o t h e r s ) have m a n a g e d to show m a n y e l e c t r o n - h o l e e f f e c t s in AHC
which appear to be peculiar to semiconducting systems [45]. The qualitative differences between electronhole processes in AHC and semiconductors lie in the fact that, in AHC, electron and hole processes are asymmetrical. At low temperatures the holes (as distinct from electrons) lose their mobility and are autolocalized (in Landau's sense). This effect, discovered by Kanzig (U. S. A. ) plays an important role in many luminescence effects, as Zolotarev, Pung, Yaek, and others have shown [45]. The current-free electronic excitations (excitons) in AHC also differ greatly from the Wannier-Gross excitons in semiconducting systems. At room temperature, excitons in AHC are mobile and, as Liid'ya [46] showed, interact with the many structural defects of the crystal lattice. Kink, Liid'ya, Lushchik, and Soovik have shown that at low temperatures the excitons (like the holes) in AHC lose their mobility [47]. Under these conditions, their annihilation is often accompanied by luminescence, the discovery of which owes a great deal to the work of Morgenshtern on the luminescence of pure AHC [48] and Valbis on the luminescence of AHC containing "foreign" alkali and halide ions [491. In the course of more than a quarter of a century, physicists have mainly investigated structure-sensitive effects in ionic crystals. The presence of structural defects in real crystals has made it extremely difficult to investigate the fundamental effects resulting from the interaction of electronic excitations with the regular defect-free (before treatment) regions of the lattice. The discovery of autolocalization of holes and excitons in AHC has opened up interesting perspectives for the investigation of fundamental effects even in crystals with defects. The investigation of fundamental phenomena in AHC will probably be one of the main lines of research on the physics of AHC in the years immediately ahead. A satisfactory theoretical foundation has been laid for this. The theory of crystal electronic excitations in AHC h a s b e e n v e r y s u c c e s s f u t l y d o v e l o p e d by P e k a r [50], Tolpygo [51], G l a u b e r m a n [52], and o t h e r g r e a t Soviet t h e o r e t i c a l p h y s i c i s t s . Radiation physics of luminescent ionic crystals. In recent years, rapid progress has been made in the development of the radiation physics of ionic crystals. Particuhtr attention has been given to the development and investigation of luminescent detectors and dosimeters for nuclear radiations, and to radiation coloration of ionic crystals. A large program of investigations in this field has been carried out in Khar'kov, Moscow, Tartu, Riga, Leningrad, and Lvov. Panova and o t h e r s [53] have d e v e l o p e d a new s c i n t i l l a t o r b a s e d on a C s I - N a s y s t e m : S h a m o v s k i i , L y s k o v i c h , and t h e i r c o l l e a g u e s have p e r f e c t e d a method of g r o w ing e f f i c i e n t N a I - T l s c i n t i l l a t o r s [54,55]. Soovik et al. have d e s i g n e d a new type of s c i n t i l l a t o r b a s e d on s u r f a c e - a c t i v a t e d ionic c r y s t a l s [56]. M i k h a l ' c h e n k o , S h v a r t s , and A l u k e r have thoroughly i n v e s t i g a t e d the e n e r g y y i e l d of r a d i o l u m i n e s e e n c e in AHC [57]. F r o m the s c i e n t i f i c viewpoint, the p r o g r e s s in r e s e a r c h on the m i e r o m e c h a n i s m s of r a d i o l u m i n e s e e n c e in AHC owes a g r e a t d e a l to the d e v e l o p m e n t of the
technique of spectral-kinetic investigations in a wide range of photon energy--from I to 1000 eV (IPmas, Liid~ya, Lukirskii, Elango} and the development of the slow-electron technique for AHC (Fridrikhov, ShuPman, Trofimova, Kuusman). This covers a broad t i m e r a n g e down to 10 -9 s e e (Galanin, P l y a v i n , and Soovik). S p e c i a l a t t e n t i o n has been given to i n v e s t i g a t i o n of the m u l t i p l i c a t i o n of e l e c t r o n i c e x c i t a t i o n in ionic c r y s t a l s . Many y e a r s ago, V a v i l o v and F a b r i k a n t s u g g e s t e d that p h o t o l u m i n e s c e n c e with a quantum y i e l d ~ > 1 was p o s s i b l e . This effect was d i s c o v e r e d in ionic c r y s t a l s by I I ' m a s , L i i d ' y a , and L u s h c h i k [58]. In "photon m u l t i p l i c a t i o n . " one quantum of u l t r a v i o l e t r a d i a t i o n g e n e r a t e s two o r t h r e e quanta of v i s i b l e light. T a r t u p h y s i c i s t s have m a n a g e d to show that t h e r e a r e t h r e e m a i n m e c h a n i s m s of photon m u l t i p l i c a t i o n - - g e n e r a t i o n of s e c o n d a r y e l e c t r o n - h o l e p a i r s and s e c o n d a r y e x c i t o n s by f a s t p h o t o e l e c t r o n s , and the d i r e c t e x c i t a t i o n of l u m i n e s c e n c e c e n t e r s by e l e c t r o n i n : p a c t [59]. S e n s i t i z a t i o n p r o c e s s e s involving photon m u l t i p l i c a t i o n a r e also of g r e a t i n t e r e s t , but have not y e t been d i s c o v e r e d . T a r t u p h y s i c i s t s have a l s o d i s c o v e r e d the d e c a y of m a n y e l e c t r o n i c e x c i t a t i o n s in AHC into e l e c t r o n - h o l e p a i r s and e x c i t o n s [601. The r a p i d d e c a y of r e s o n a n c e e x c i t a t i o n s g r e a t l y s i m p l i f i e s all r a d i a t i o n e f f e c t s . Of p a r t i c u l a r i n t e r e s t is the d e c a y of c o l l e c t i v e e l e c t r o n i c p t a s m o n excitations. S u c c e s s f u l w o r k on the d e v e l o p m e n t of l u m i n e s c e n t d o s i m e t e r s f o r n u c l e a r r a d i a t i o n s was c a r r i e d out in the Soviet Union in the p o s t w a r y e a r s . Using the phen o m e n a of t h e r m o - and p h o t o s t i m u l a t e d l u m i n e s c e n c e , A n t o n o v - R o m a n o v s k i i and K e i r i m - M a r k u s [61], A r k h a n g e l ' s k a y a , S h v a r t s , and o t h e r s d e v e l o p e d s e v e r a l t y p e s of individual T - r a y , X - r a y , and s l o w - n e u t r o n d o s i m e t e r s . L u s h c h i k , Sokolov, Yaek, et a I . , u s i n g c r y s t a l p h o s p h o r s with n o n l i n e a r e l e c t r o n - h o l e p r o c e s s e s , d e s i g n e d f a s t - n e u t r o n d o s i r n e t e r s [62]. Ionic c r y s t a l s a r e v e r y s u i t a b l e m o d e l s y s t e m s f o r i n v e s t i g a t i o n of the n a t u r e and m e c h a n i s m s of f o r m a tion of r a d i a t i o n d e f e c t s in s o l i d s . A n d r o n i k a s h v i l i , P o l i t o v , and o t h e r T b i l i s i p h y s i c i s t s c a r r i e d out a v a r i e t y of e x p e r i m e n t s , including" the u s e of o p t i c a l t e c h n i q u e s , which led to a f u n d a m e n t a l r e s u l t r e g a r d ing the r a d i a t i o n - i n d u c e d f o r m a t i o n of d i s l o c a t i o n s in ionic c r y s t a l s [63]. S h v a r t s et al. [13] have i n v e s t i g a t e d r a d i a t i o n d e f e c t s on the s u r f a c e of ionic c r y s t a l s and in p a r t i c l e t r a c k s . V o r o b ' e v , Z a v a d o v s k a y a , M e l i k - G a i k a z y a n , V a i s b u r d , and o t h e r T o m s k p h y s i c i s t s have carried out an extensive program of research on the mechanisms of formation of radiation defects in AHC by protons, electrons, and X-rays [64, 65]. The role of intrinsic and impurity defects in radiation coloration has been particularly well investigated. Zaitov, Lukantsever, and others [66] have investigated the role of ionic processes in the radiation coloration and deceleration of ionic crystals. L u m i n e s c e n c e and e m i s s i o n t e c h n i q u e s e n a b l e d the T a r t u p h y s i c i s t s ( L i i d ' y a , I I ' m a s , Ehmgo, K y a e n : b r e , and o t h e r s ) to m e a s u r e ' t h e s p e c t r a of the f o r m a t i o n of c o l o r c e n t e r s by u l t r a v i o l e t r a d i a t i o n " in a wide 331
r a n g e and to show that p a r t i c u l a r l y i m p o r t a n t p r o c e s s e s in the f o r m a t i o n of r a d i a t i o n defects a r e the n o n r a d i a tive a n n i h i l a t i o n of excitons in the r e g u l a r p a r t s of the c r y s t a l l a t t i c e and the e l e c t r o n - h o l e m e c h a n i s m s of f o r m a t i o n of F - c e n t e r s f r o m c o a g u l a t e s of point defects [67]. S u r v e y i n g the d e v e l o p m e n t of p h y s i c s in the last c e n t u r y it is quite a p p a r e n t that the l u r n i n e s c e n c e method, which is e x c e p t i o n a l l y s e n s i t i v e , v e r s a t i l e , and g r a p h i c , has played an i m m e n s e role in m a n y f u n d a m e n t a l d i s c o v e r i e s . The d i s c o v e r y of r a d i o a c t i v ity of the atomic n u c l e u s , V a v i l o v - C e r e n k o v e m i s s i o n , and the a n t i p r o t o n - - a l l these outstanding a c h i e v e m e n t s of s c i e n c e owe t h e i r a c c o m p l i s h m e n t to the use of l u m i n e s c e n c e m e t h o d s . The role of l u m i n e s c e n c e m e t h ods i n t h e d e v e l o p m e n t of l a s e r s is also an i m m e n s e one. As we have s e e n , the l u m i n e s c e n c e m e t h o d h a s e n a b l e d r e s e a r c h w o r k e r s to p e n e t r a t e into the highly c o m p l i c a t ed and i n t e r e s t i n g m i c r o w o r l d of ionic c r y s t a l s . In a s h o r t paper it is difficult even to list what has been done by many s c i e n t i f i c t e a m s in the Soviet Union over m a n y y e a r s , This accounts for the i n c o m p l e t e n e s s of the p r e s e n t r e v i e w . Soviet p h y s i c i s t s engaged in the field of l u m i n e s c e n c e of ionic c r y s t a l s have c e r t a i n l y much to be proud of. Some of the r e s e a r c h c a r r i e d out in this field will undoubtedly b e c o m e p a r t of the t r e a s u r e house of s o l i d - s t a t e p h y s i c s . [;nfortunatcly, the level of much r e s e a r c h is low. It should be noted that A m e r i c a n p h y s i c i s t s , as well as using l u m i n e s c e n c e m e t h o d s , make much w i d e r use of such powerful m e a n s of i n v e s t i g a t i n g solids as e l e c t r o n p a r a m a g n e t i c r e s o n a n c e , double c l c e t r o n - n u c l e a r r e s o n a n c e , the M 5 s s b a e r effect, magnet6optic methods, ultralow t e m p e r a t u r e t e c h n i q u e s , l a s e r s , etc. The r e s u l t is that much of the r e s e a r c h on the p h y s i c s of ionic c r y s t a l s , begun e a r l i e r in the Soviet Union than in the U. S. A . , is somewhat i n f e r i o r in e x e c u t i o n to that of the A m e r i c a n s . We m u s t also note, u n f o r t u n a t e l y , the t r a d i t i o n a l lag in the technical r e a l i z a t i o n of s c i e n t i f i c ideas in the f o r m of c r y s t a l d e v i c e s . As in other fields of p h y s i c s , we have our s u c c e s s e s and s h o r t c o m i n g s . F u r t h e r d i r e c t e d and coordin'ated work on the p r i n c i p a l t h e o r e t i c a l and p r a c t i c a l s c i e n rifle p r o b l e m s of the p h y s i c s of l u m i n e s c e n t ionic c r y s t a l s is n e c e s s a r y .
REFERENCES 1. A. N. T e r e n i n a n d F. D. K l e m e n t , Acta Phys. C h i m . , S S S R , 1, 941, 1935. 2. V. N. K u d r y a v t s e v a , Z h E T F , 4, 709, 1934. 3. V. V. A n t o n o v - R o m a n o v s k i i , T r u d y F I A N , no. 2 - 3 , 1942. 4. I . A. P a r f i a n o v i e h , Z h E T F , 9, 169, 1939. 5. M. L. Kats, Z h E T F , 8 , 100, 1938. 6. F. D. K l e m e n t , Izv. ANSSSR, 9, 411, 1945. 7. F. D. K l e m e n t and Ch. B. Lushchik, UFN, 70, 733, 1959. 8. C o l l e c t i o n : P h y s i c s of Alkali Halide C r y s t a l s [in R u s s i a n ] , Riga, 1962. 9. Izv. ANSSSR, s e r . f i z . , 29, no. 1, no. 5,1965. 332
10. Izv. ANSSSR, s e r . f i z . , 30, no. 4, no. 9, 1966. 11. Izv. AN SSSR, s e t . f i z . , 31, no. 5, no. 12, 1967. 12. T r u d y Instituta fiziki i a s t r o n o m i i AN ESSR, no. 1 - 3 7 , 1955-1968. 13. Radiation P h y s i c s , vols. 1 - V [in R u s s i a n ] , Riga, 1962-1967. 14. M. A. E l ' y a s h e v i c h , S p e c t r a of R a r e E a r t h s [in R u s s i a n ] , GITTL, Moscow, 1953. 15. P. P. Feofilov, Acta P h y s i c a Polonica, 26, no. 3 - 4 , 1964. I6. P. P. FeofiIov, P o l a r i z e d L u m i n e s c e n c e of Atoms, M o l e c u l e s , and C r y s t a l s [in R u s s i a n ] , GIFML, Moscow, 1959. 17. A. A. KapLyanskij, P r o c . Inter. Confer. S e m i cond. P h y s . , P r a g u e , p. 356, 1960. 18. B. P. Z a k h a r c h e n y a and I. B. Rusanov, F T T , 8 , 41, 1966. 19. F. D. K l e m e n t , T r u d y IFA AN ESSR, no. 1, 3, 1955. 20. F. D. K l e m e n t , Izv. ANSSSR, s e t . f i z . , 29, 86, 1965. 21. L. M. Shamovskii and A. A. Dunina, P r o c e e d ings of Se~'enth C o n f e r e n c e on L u m i n e s c e n c e ; C r y s t a l Phosphors [in R u s s i a n ] , T a r t u , p. 13, 1959. 22. F. D. K l e m e n t , T r u d y IFA AN ESSR, no. 7, 3, 1958. 23. N. I. Ivanova, L. I. T a r a s o v a , and A. P. Zhukovskii, P h y s i c s of Alkali Htdide C r y s t a l s [in R u s s i a n ] , Riga, p. 149, 1962. 24. Ya. Ya. K i r s and A. I. L a i s a a r , T r u d y IFA AN ESSR, no. 10, 51, 1959. 25. K. V. Shalimova, DANSSSR, 97, 437, 651, 1954. 26. N. E. Lushchik, Ch. B. Lushchik, S. G. Z a z u bovich, and I. A. Meriloo, Izv. AN SSSR, s e r . f i z . , 29, 373, 1965. 27. M. F. T r i n k l e r and I. K. P l y a v i n , P h y s . - S t a t . S o L , 11, 227, 1965. 28. N. Lushchik and T. Lushehik, Acta P h y s i e a Poloniea, 26, no. 3 - 4 , 711, 1964. 29. S. I. P e k a r , Izv. AN SSSR, s e r . f i z . , 18,712, 1954. 30. A. S. Davydov, Izv. ANSSSR~ s e r . f i z . , 17, 523, 1953. 31. Yu. E. P e r l i n , Uch. zap. Kishinevskogo gos. u n - t a , 80, 3, 1965. 32. N. N. K r i s t o f e l , M. I. P c t r a s h e n , and I. V. Abarenkov, Opt. i s p e k t r . , 10, 487, 1961. 33. I. K. Rebane, N. N. K r i s t o f e l , E. D . Trifonov, and V. V. Khizhnyakov, Izv. AN ESSR, s e r . f i z . - m a t . i t e k h n . , 13, no. 8, 87, 1964. 34. M. L. Kats, L u m i n e s c e n c e and E l e c t r o n - H o l e P r o c e s s e s in Akali Halide Compounds [in R u s s i a n ] , Saratov, 1960. 35. V. G. Krongauz, and N. A. P a r f i a n o v i c h , Izv. ANSSSR, s e t . fizo, 31, 839, 1967. 36. Kh. F. K y a e m b r e and E. S. T i i s l e r , T r u d y IFA AN ESSR, no. 21, 139, 1962. 37. Ch. B. Lushchik, T r u d y IFA AN ESSR, no. 3, 3, 1955.
38. I. K. Vitol, Ch. B. Lushehik, and I. V. Yaek, Trudy IFA AN ESSR, no. 12, 175, 1960. 39. I. K. Vitol, Ya. A. Valbis, and V. E. Zirap, Izv. ANSSSR, ser. f i z . , 29, 446, 475, 1965. 40. A. I. Belkind and A. I. Kalendarev, Radiation Physics, vol. 1 [in Russian], Riga, p. 69, 1964. 41. V. P. Denks, Trudy IFA AN ESSR, no. 26, 160, 1964. 42. Yu. Khaldre and L. Pung, Izv. AN SSSR, ser. f i z . , 30, 1443, 1966. 43. I. K. Vitol, Izv. ANSSSR, s e t . f i z . , 30, 564, 1966. 44. A. Kh. Khalilov, Izv. AN AzSSR, 7, 181, 1965. 45. Ch. B. Lushehik, Trudy IFA AN ESSR, no. 31, 19, 1966. 46. G. G. Liid~ya, Trudy IFA AN ESSR, no. 12, 149, 1960. 47~ R. A. Kink, G. G. Liid~ya, Ch. B. Lushchik, and T. A. Soovik, Izv. ANSSSR, s e t . f i z . , 31, 581, 1967. 48. Z. L. Morgenshtern, Opt. i spektr., 7, 231, 1959; 8, 672, 1960. 49. Ya. A. Valbis, V. E. Graveris, and Z. Z. Rachko, Izv. ANSSSR, set. f i z . , 30, 661, 1966. 50. S. I. Pekar, Investigations on the Electronic Theory of Crystals [in Russian], GITTL, Moscow, 1951. 51. K. B. Tolpygo, collection: Physies of Alkali ttalide Crystals [in Russian], Riga, p. 15, 1961. 52. A. E. Glauberman, Izv. AN SSSR, ser. f i z . , 29, 5, 1965. 53~ A. N. Panova, Izv. ANSSSR, ser. f i z . , 31, 859, 1967. 54. L. M. Shamovskii, L. M. Rodionova, and A. S.
Glushkova, Izv. ANSSSR, ser. f i z . , 22, 3, 1958. 55. A. B. Lyshkovich, Z. P. Chornyi, and N. K. Guseva, Izv. ANSSSR, ser. f i z . , 29, 423, 1965. 56. N. E. Lushchik, Ch. B. LushcMk, L. A. Pinogorova, T. A. Soovik, and N. Ya. Yansoon, Trudy IFA AN ESSR, no. 31, 160, 1966. 57. G. A. Mikhal:chenko, Radiation Physics, vol. 5 [in Russian], Riga, p. 73, 1967. 58. E. R. II:mas, G. G. Liid~ya, andCh. B. Lushchik, Opt. i spektr., 18,453, 631, 1965. 59. Collection: Photon Multiplication [in Russian], Trudy IFA AN ESSR, no. 34, I966. 60~ E. R. II'mas, V. A. Kink, G. G. Liidlya, and Ch. B. Lushchik, Izv. ANSSSR, ser. f i z . , 29, 27, 1966. 61. V. V. Antonov-Romanovskii, I. B. KeirimMarkus, and Z. A. Trapeznikova, Proceedings of Session on Peaceful Uses of Atomic Energy[in Russian], Izv. ANSSSR, Moscow, p. 342, 1966. 62. V. V. Kuz~min, Ch. B. Lushchik, F. I. Savikhim, A. D. Sokolov, and I. V. Yaek, Atomnaya energiya, 22, 482, !967. 6 3 . E . L . Andronikashvili, I. G. Politov, and M. Sh. Getiya, Physics of Alkali Halide Crystals [in Russian], Riga, p. 284, 1962. 64. A. A. Vorob~ev and A. D. Buylin, Effect of Radiations on Matter [in Russian], Tomsk, 1965. 65. I. Ya. Melik-Gaikazyan and D. I. Vaisburd, DANSSSR, 165, 1029, 1965. 66. E. S. Dudarev, F. I. Zaitov, and Yu. L. Lukantsevcr, lzv. VUZ. Fizika [Soviet Physics Journal], no. 5, 136, 1966. 67. G. K. Vale, Ch. B. Lushehik, and M. A. Elango, Izv. ANSSSR, set. f i z . , 31, 820, 1967.
333
