EXCITON MIGRATION AND LOCALISATION IN PURE INORGANIC SOLIDS

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EXCITON MIGRATION AND LOCALISATION IN

PURE INORGANIC SOLIDS

J. Thorne, R. Denning, T. Barker, D. Grimley

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au nOIO, Tome 46, octobre 1985 page C7-125

EXCITON MIGRATION A N D LOCALISATION IN P U R E INORGANIC SOLIDS

J.R.G. Thorne, R.G. Denning, T.J. Barker and D . I . Grimley

I n o r g a n i c Chemistry Laboratory, S o u t h P a r k s Road, Oxford OX1 3QR, U.K.

A b s t r a c t : Energy T r a n s f e r i n Uranyl Compounds f o r T>0.4K i s d i s c u s s e d .

It i s n o t easy t o d e v i s e experiments which w i l l measure r a t e s of resonant energy t r a n s p o r t i n pure i n o r g a n i c s o l i d s , and d i s t i n g u i s h between t h e m i g r a t i o n and c a p t u r e p r o c e s s e s

[

I]. We have t a k e n two approaches t o t h i s problem which a r e unusual i n t h a t they e x p l o i t i s o t o p i c e f f e c t s i n t h e s e m a t e r i a l s . In t h e f i r s t c a s e we use i s o t o p i c s u b s t i t u t i o n i n a mode which i s so weakly coupled t o t h e e l e c t r o n i c t r a n s i t i o n , t h a t t h e i s o t o p e s h i f t i n t h e p u r e e x c i t o n energy i s s m a l l compared w i t h t h e e x c i t o n i c c o u p l i n g , and use t h e emission p r o p e r t i e s t o c h a r a c t e r i s e t h e energy t r a n s f e r r a t e . In t h e second c a s e we c h a r a c t e r i s e t h e t r a p p i n g p r o c e s s , by s t u d y i n g t h e p r o p e r t i e s of analagous ' a n t i - t r a p s ' . We a r e a b l e t o show t h a t , a t r a t h e r low temperatures t r a n s p o r t o v e r t h e ' l i p ' of t h e t r a p i s r a t e determining, and t h a t t h e presence of b i e x c i t o n a n n i h i l a t i o n a t 0.4K, i m p l i e s a d e l o c a l i s e d e x c i t o n i n t h e bulk m a t e r i a l .

In s i n g l e c r y s t a l s of C S U O ~ ( N O ~ ) ~ , both t h e a b s o r p t i o n and emission spectrum show f e a t u r e s a s s o c i a t e d with t h e i n t e r n a l modes of t h e n i t r a t e ions. n r e e of t h e s e i o n s a c t a s b i d e n t a t e l i g a n d s , occupying t h e e q u a t o r i a l r e g i o n surrounding t h e l i n e a r ~ 0 ion. ~ ~These modes show l a r g e i s o t o p e s h i f t s on nitrogen-15 s u b s t i t u t i o n ' but t h e d i f f e r e n c e i n t h e i r frequency i n t h e ground and e x c i t e d s t a t e s i s such t h a t t h e d i f f e r e n c e i n zero-point v i b r a t i o n a l energy i n <0.1cm-l. There a r e two

symmetry-related molecules p e r u n i t c e l l , and t h e p o l a r i s a t i o n i n a b s o r p t i o n can be used t o a s s e s s t h e magnitude of t h e Davydov coupling. This measurement must be c a r e f u l l y analysed because t h e e f f e c t of inhomogenous broadening i s t o amplify t h e a p p a r e n t Davydov s p l i t t i n g [2]. We f i n d ~=0.05cm-~. a t h o u g h J i s comparable t o

14-N EXCITATION

20280

FREPUEMY (WAVEH$~%S>

F i g u r e 1. 'Nme r e s o l v e d emission spectrum of C S U O ~ ( N O ~ ) ~ , 14-N e x c i t a t i o n , a t 4.2K. t h e i s o t o p e s h i f t t h e time r e s o l v e d emission ( F i g u r e 1 ) shows a t r a n s f e r r a t e c o n s t a n t of loss-'. We i n t e r p r e t t h i s t o mean t h a t t h e much l a r g e r s p e c t r a l

C7-126 JOURNAL DE PHYSIQUE

inhomogeneity ( ~ ~ 2 c m - l ) i s manifest a s d i s o r d e r with a range which i s comparable t o t h e i n t e r m o l e c u l a r spacing. This d i s o r d e r e f f e c t i v e l y l o c a l i s e s t h e e x c i t o n , and may be a consequence of t h e scope f o r s m a l l d e v i a t i o n s i n t h e n i t r a t e geometry. We have a l s o made a s t u d y of t r a p p i n g mechanisms i n Cs2U02C14. We u s e d a t a from t h e r a t e of t r a n s f e r between i s o t o p i c ' a n t i - t r a p s ' and t h e bulk m a t e r i a l , u s i n g b o t h oxygen-18 and oxygen-17 [3]. 'Ihese r e s u l t s g i v e a t r a p p i n g r a t e a t a b s o l u t e z e r o given by

where AElp i s t h e energy d e f e c t expressed i n wavenumbers. A t h i g h e r t e m p e r a t u r e s t h e r a t e i s c o n t r o l l e d by a two-phonon, one-site, Raman process w i t h a r a t e given by

where P is a constant i n c o r p o r a t i n g t h e electron-phonon coupling, and Y(w,b,T) d e s c r i b e s t h e temperature dependence a t t r i b u t a b l e t o a n i n t e g r a t i o n over t h e d e n s i t y of s t a t e s , i n t h e Debye approximation, of two phonons d i f f e r i n g i n frequency by b.

We have i n v e s t i g a t e d t h e r a t e of t r a p p i n g i n c r y s t a l s c o n t a i n i n g s m a l l con-

c e n t r a t i o n s of bromide ion. This t r a p i s e a s i l y observed s p e c t r o s c p i c a l l y and i s 30cm-' deep [4]. I h e primary t r a p p i n g r a t e can be a s s e s s e d d i r e c t l y because t h e

' l i p ' s t i e s a r e s p e c t r a l l y r e s o l v e d , having a t y p i c a l depth of 3cm-ye There i s c o n s i d e r a b l e a n i s o t r o p y i n t h e t r a p p i n g process a s shown by t h e time-resolved e x c i t a t i o n spectrum of t r a p emission i n t h e r e g i o n where t h e ' l i p ' s i t e s absorb

( F i g u r e 3). I h e f a s t e s t r a t e i s 1.5 x lo6 s", a t 5K which i s c o n s i s t e n t w i t h t h e r a t e expected from Figure 2 f o r t h i s t r a p depth.Moreover t h e r a t e of t r a p p i n g above 5K from t h e bulk i s c o n s i s t e n t w i t h r a p i d migration t o t h e t r a p s i t e s , followed by a r a t e determining c a p t u r e . Below 5K Figure 2 p r e d i c t s t h a t i f t h e 30cm-' t r a p p i n g p r o c e s s i s r a t e determining t h e r a t e should s t a y e f f e c t i v e l y constant w i t h temperature. I n f a c t t h e t r a p p i n g r a t e f a l l s by over an o r d e r of magnitude i n t h e range between 1.2K and 0.4K, and shows i n c r e a s i n g d e v i a t i o n from e x p o n e n t i a l decay. These lowtemperature d a t a a r e included i n Figure 2 where t h e e x t r a p o l a t i o n t o zero temperature s u g g e s t s a r a t e determining s t e p of t h e o r d e r of 5cm-l. l h i s i s c l o s e t o t h e 3 . 9 ~ ~ ' i n t e r v a l which d e s c r i b e s t h e d e p t h of t h e * " l i p W s i t e .

To summarize, t h e d a t a i s c o n s i s t e n t with f a s t resonant bulk migration followed by a r a t e determining s t e p which, above 5K, i s t h e primary c a p t u r e a t t h e t r a p , b u t below 5K i s t h e t r a n s f e r t o t h e l i p s i t e . I n s u p p o r t of t h e h y p o t h e s i s t h a t r e s o n a n t m i g r a t i o n i s s t i l l r a p i d a t 0.4K, F i g u r e 4 shows t h a t b i e x c i t o n a n n i h i l a t i o n i s

s t i l l s i g n i f i c a n t a t t h i s temperature. Because t h e s p e c t r a l inhomogeneity i s l e s s t h a n 0*3cm-', t h e ' r e s o n a n t ' p r o c e s s cannot proceed by a s e r i e s of non-resonant s t e p s with r a t e s p r e d i c t e d by Fig. 2 and we i n f e r t h a t t h e e x c i t a t i o n i s d e l o c a l i s e d , d e s p i t e o u r c o n c l u s i o n t h a t i n t h i s m a t e r i a l ~--0.02cm-~[3].

4906 : 4905.4 : 4984.8

'

4984.2 : 4903.6 '4903

cM1cROSECONOs)

WAVELENGTH ( ANGSTROM )

F i g u r e 3. Time Resolved E x c i t a t i o n S p e c t r a of Trap Emission i n

JOURNAL

DE

PHYSIQUE

TIME

iMSEC) F i g u r e 4. E f f e c t of l a s e r i n t e n s i t y o n decay of Cs2U02C1,, e m i s s i o n

a t 0.4K.

REFERENCES

[ l ] V.M. Kenkre and P.E. P a r r i s , J. h m i n 31/32, 612, (1984). [ 2 ] J.R.G. 'Ihorne and R.G. Denning, M o l . Phys. 54, 701. (1985).

[ 3 ] J.R.G. Thorne, R.G. Denning, T.J. Barker and D.I. Grimley, J. Lumin; ( i n p r e s s ) . [ 4 ] R.G. Denning, C.N. I r o n s i d e , T.R. S n e l l g r o v e and J.R.G. Thorne, Mol. Phys. 47,