PHONON RELAXATION IN CRYSTALS STUDIED BY HOT LUMINESCENCE

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PHONON RELAXATION IN CRYSTALS STUDIED BY HOT LUMINESCENCE

P. Saari, K. Rebane

To cite this version:

P. Saari, K. Rebane. PHONON RELAXATION IN CRYSTALS STUDIED BY HOT LUMINESCENCE. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-502-C6-504.

�10.1051/jphyscol:19816147�. �jpa-00221212�

JOURNAL DE PHYSIQUE

CoZZoque C6, suppZdment a u n o 12, Tome 42, dgoernbre 1981 page C6-502

PHONON R E L A X A T I O N I N CRYSTALS S T U D I E D B Y HOT LUMINESCENCE

P .

and K .

I n s t i t u t e of P h y s i c s , E s t o n i a n SSR Academy of S c i e n c e s , T a r t u 202400, U.S.S.R.

A b s t r a c t . - We have developed a method f o r studying phonon i n t e r - a c t i o n s i n picosecond t i m e s c a l e , which m a n i f e s t themselves i n op- t i c a l emission s p e c t r a . I n sodium n i t r i t e t r a n s i t i o n r a t e s b e t - ween high-frequency i n t e r n a l and l a t t i c e phonon s t a t e s have been e v a l u a t e d and t h e i r c o r r e l a t i o n w i t h t h e o r d e r of anharmonicity has been observed. For a n t h r a c e n e t h e k i n e t i c s of v i b r o n r e l a x a - t i o n up t o a c o u s t i c phonons h a s been revealed.

1. I n t r o d u c t i o n . - Phonons d i s p l a y themselves i n o p t i c a l s p e c t r a of c r y s t a l s through electron(exciton)-phonon i n t e r a c t i o n . For example, t h e luminescence p u r e - e l e c t r o n i c s p e c t r a l l i n e may p o s s e s s a phonon s i d e - band which r e f l e c t s t h e phonon d e n s i t y - o f - s t a t e s spectrum r a t h e r p r e c i -

s e l y [ l ] . The same i n t e r a c t i o n e n a b l e s one t o e x c i t e o p t i c a l l y v a r i o u s phonon s t a t e s through t h e a b s o r p t i o n of photons whose energy exceeds t h a t of t h e e l e c t r o n i c e x c i t a t i o n by t h e energy of t h e phonon s t a t e chosen. Although phonon s t a t e s decay much f a s t e r t h a n e l e c t r o n i c e x c i - t a t i o n s a r e r e - e m i t t e d a s luminescence t h e r e i s a small p a r t of t h e emission, h o t luminescence (HL) [ 2 1 , which o c c u r s i n t h e c o u r s e of t h e phonon s t a t e transformation/decay process. A s s p e c t r a l bands of HL can be a s c r i b e d t o phonon s t a t e s involved i n t h e p r o c e s s , t h e s t a t e l i f e - times d i s p l a y themselves i n t h e temporal behaviour of t h e HL bands o r , i n t h e c a s e of s t e a d y - s t a t e e x c i t a t i o n - i n band i n t e n s i t i e s . Making u s e of t h e s e circumstances an e x t e n s i v e study of u l t r a f a s t phonon r e - l a x a t i o n i n luminescence c e n t r e s of v a r i o u s impurity c r y s t a l s h a s been c a r r i e d o u t [ 3 ] . Here we demonstrate t h e p o s s i b i l i t i e s of t h e approach on pure monocrystals possessing i n t r a m o l e c u l a r phonon modes, where we study, i n f a c t , t h e decay of exciton-phonon complexes c r e a t e d i n i t i a l - l y by l i g h t a b s o r p t i o n .

2. R e s u l t s on sodium n i t r t i t e . - T h i s c r y s t a l r e p r e s e n t s an example of s t r o n g binding between i n t r a m o l e c u l a r phonons and t h e (Frenkel) e x c i - ton: t h e f r e q u e n c i e s of ^0-N-0 s t r e t c h i n g and bending modes, v l = 1330 and v 2 = 830 cm-' d e c r e a s e t o v; = 1020 and v; = 630 cm-', i f t h e e x c i t o n i s c r e a t e d i n t h e c e l l . Thus, t h e exciton-phonon complex ( v i b r o n ) can

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816147

3. R e s u l t s on anthracene. - This c r y s t a l r e p r e s e n t s an example of va- n i s h i n g binding of i n t e r n a l phonons t o t h e e x c i t o n b u t of s t r o n g i n t e r - a c t i o n between e x c i t o n s and photons. The f i r s t p e c u l i a r i t y means t h a t h e r e HL from e x c i t e d s t a t e s of i n t e r n a l v i b r a t i o n s should d i s p l a y t h e

.-

S

iz

+

B

a 0

-8

I

phonon

Fig. 1 : On t h e r i g h t - emission spec- ' energy, cm' NaN02

trum around t h e pure e x c i t o n i c lumi-

nescence l i n e

i t s p$onon sidebanci) n';

a t cw e x c i t a t i o n by A r l a s e r 3638 A l i n e ; f o r HL bands i n i t i a l / f i n a l pho- non s t a t e s a r e i n d i c a t e d ; "R" l a b e l s Raman r e p l i c a s (on vl, v2) of t h e l a - s e r l i n e . O n r t h e l e f t - t h e scheme of lowest

r o u t e s ; v i , t r a n s i t i o n v2 v!bron times s t a t e s a r e and g i v e n decay i n

r---

picoseconds and p e r i o d s (T) of t h e decaying phonon, t h e l e n g t h of t h e s h o r t e r wavy l i n e corresponds t o t h e 90T

energy of a n e f f e c t i v e l a t t i c e phonon,

t h e longer one - t o t h a t of v2-phonon.

decay only through anharmonic rsT<-

coupling between i n t e r n a l and l a t t i c e phonons.

The obtained p i c t u r e of r e - l a x a t i o n i s shown i n Fig. 1 141

( s e e a l s o [ 5 1 ) . By v i r t u e of t h e f a c t t h a t t h e maximum energy of l a t t i c e phonons i s a s Low a s

T=4.2K ^-

3; *.?a---

$;-a W -

^- _;

260 cm-', t h e high-energy i n t e r -

n a l v i b r a t i o n s should decay through multiphonon p r o c e s s e s caused by higher-order anharmonic coupling terms ( o r by higher-order t r a n s i t i o n s on lower terms). This i s c l e a r l y expressed i n t h e t r a n s i t i o n t i m e s which allow a l s o some q u a n t i t a t i v e comparison of t h e e f f e c t i v i t i e s of couplings: t h e decay of a ~ i - ~ h o n o n (4-phonon p r o c e s s - a t l e a s t 3 l a t - t i c e phonons should be c r e a t e d ) i s f i v e t i m e s f a s t e r t h a n t h e conver- s i o n of a ~ i - ~ h o n o n i n t o

and 2 l a t t i c e ones, w h i l e t h e d i r e c t decay of a v;-phonon i s s u r p r i s i n g l y f a s t (due t o t h e p o s s i b i l i t y of a 3-pho- non p r o c e s s a r i s i n g through t h e coupling t o u p - v i b r a t i o n s n o t d i s t u r b e d by e x c i t o n ) . The l a t t e r argument i s supported by t h e f a c t t h a t i n NO;- doped a l k a l i h a l i d e s w i t h no high-frequency v i b r a t i o n s i n t h e v i c i n i t y of t h e e x c i t e d molecule t h e d i r e c t decay of a v;-phonon i s n o t observed

[3,41. I n g e n e r a l , we s e e t h a t t h e energy gap law of multiphonon r e l a -

x a t i o n holds a t a g i v e n c a s e o n l y a f t e r being modified by t h e account

of t h e d e t a i l s of anharmonic coupling mechanism.

C6-504

JOURNAL DE PHYSIQUE

phononlpol?riton

energy, ^cm ANTHRACENE 1500;

--

-

- 0 . 2 ~

V)

T = OX

1.6 K

100 E *

Fig. 2

On the right - emission spec- trum in the region of excit~n~energy at cw excitation by the 3638 A laser line. HL band appearinf through anni- hilation of A, 390 cm- internal pho- non is indicated. For the emission in

"bottle-neck" region rise/decay times have been measured at picosecond pul- sed excitation, reflecting polariton relaxation on acoustic phonons. On the left - dispersion curves for polari- tons originating from a pure exciton and its strong interaction with 390 and 1400 cm-' phonons, respectively.

d i s s o c i a t i o n of v i b r o n r a t h e r t h a n t h e decay of t h e phonon.

Indeed, e m i s s i o n s p e c t r a measu- r e d a t v a r i o u s e x c i t a t i o n condi- t i o n s I61 show weak HL back- ground under Raman l i n e s [7] and one weak HL band. These d a t a g i v e f o r t h e d i s s o c i a t i o n / d e c a y t i m e s o f h i g h e r s t a t e s an e s t i m a t i o n 50.1 p s and f o r t h a t of t h e 390 cm-' phonon - 0.2 p s ( F i g . 2 ) . Time r e - s o l v e d measurements l 8 1 l e d t o t h e f o l l o w i n g c o n c l u s i o n s : decay of high- e n e r g y s t a t e s c a n p o p u l a t e w i t h i n 10 p s d i r e c t l y t h e s t a t e s i n p o l a r i - t o n " b o t t l e - n e c k " , where t h e p o p u l a t i o n d i s t r i b u t i o n i s formed w i t h i n 100 ps. From a n i n t r i g u i n g f a c t t h a t t h e spectrum d o e s n o t change w i t h lowering t e m p e r a t u r e t o 0.4 K we deduce t h a t i n t h i s c r y s t a l phonon- s t i m u l a t e d p r o c e s s e s d o n o t t a k e p a r t i n p o l a r i t o n r e l a x a t i o n s and t h e r e l a x a t i o n mechanism c o u l d b e u n d e r s t o o d i f phonon-dressed p o l a r i t o n s a r e i n t r o d u c e d

1. K.K.Rebane, L.A.Rebane, J. Pure Appl. Chem. 37 (1974) 161; see also Fig. 1.

2. P.Saari, K.Rebane, Solid State Comm. 7 (1969) 887.

3. K.Rebane, P.Saari, J. Luminescence, E ^{(1978) 223.}

4. J.U.Aaviksoo, A.O.Anijalg, P.M.Saari, T.A.Soovik, Sov. Phys. Sol. St. 2 (1977) 477.

5. M.Hangyo, H.Yamanaka, R.Kato, J. Phys, Soc. Japan, (1981) 895.

6. J.U.Aaviksoo, P.M.Saari, T.B.Tamm, Pis'ma JETP, 29 (1979) 388.

7. On meaning of HI,-to-Raman ratio see: L.A.Rebane, K.K.Rebane, in present issue and Ref. [31.

8. J.Aaviksoo, P.Saari, T.Tamm, in: Ultrafast Phenomena in Spectroscopy, Pxoc. Sec.

Int. Symp., Reinhardsbrunn, 1980, G;D;R., p. 479.

9. J.Aaviksoo, G.Liidja, P.Saari, Phys. Stat. Sol. (b) (to be published).