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HIGHLY EXCITED CN- MOLECULAR DEFECTS
AND STEPWISE RADIATIVE RELAXATION IN CsCl
Y. Yang, F. Luty
To cite this version:
Colloque C7, suppl6ment au nOIO, Tome 46, octobre 1985 page C7-287
HIGHLY EXCITED CN- MOLECULAR DEFECTS AND STEPWISE RADIATIVE RELAXATION I N C s C l
Y. Yang and F. Luty
Physics Department of t h e U n i v e r s i t y of Utah, S a l t Lake C i t y , Utah 84112, U.S.A.
Abstract: O p t i c a l e x c i t a t i o n o f a new F center/CN- d e f e c t cornplex i n CsCl produces - - i n s t e a d o f any e l e c t r o n i c emission--a h i g h l y e f f i c i e n t 5 band v i b r a t i o n a l emission around -4.8 pm (-0.25 eV), i n v o l v i n g t h e 5 l o w e s t s t a t e s o f t h e CN- anharmonic o s c i l l a t o r . Pulsed 1 aser e x c i t a t i o n shows t h a t t h e energy t r a n s f e r from t h e e x c i t e d F e l e c t r o n occurs mainly a t 1 eV i n t o t h e 4 t h CN- l e v e l producing a sequence o f ~ n = l p o p u l a t i o n i n v e r s i o n s and emissions w i t h g r e a t p o t e n t i a l f o r l a s e r a p p l i c a t i o n .
The s i m p l e s t Oh symmetry e l e c t r o n i c d e f e c t i n i o n i c c r y s t a l s--the F center-- has been associated t o a g r e a t v a r i e t y o f c a t i o n i c , a n i o n i c o r vacancy p o i n t ( o r p a i r ) defects. These F aggregate c e n t e r s o f reduced l o c a l symmetry, changed wave f u n c t i o n s and e l e c t r o n - l a t t i c e c o u ~ l i n q l share w i t h F c e n t e r s one common f e a t u r e : H i g h l y e f f i c i e n t , s p e c t r a l l y broad' andVstokes-shi f t e d e l e c t r o n i c emission bands.
evera o t ese systems i e
,
B,
2'
,
2- c e n t e r s ave gained s i g n i f i c a n c e i s s o l i d :ta:e systems caiib!eif
tLnabFe n e i r i r l a s e r ' e i i s s i o n .A new o p t i c a l p r o p e r t y can be i n t r o d u c e d by a s s o c i a t i n g F c e n t e r s t o s u b s t i - t u t i o n a l diatomic molecular defects. T h i s attempt was o r i g i n a l l y m o t i v a t e d by t r y i n g t o answer two fundamental open questions: ( a ) Can s u b s t i t u t i o n a l molecular d e f e c t s i n i o n i c c r y s t a l s , when e x c i t e d i n t h e i r i n t e r n a l v i b r a t i o n a l mode, under- go r a d i a t i v e d e e x c i t a t i o n processes and e m i t v i b r a t i o n a l fluorescence? ( b ) Can e l e c t r o n i c defects,when o p t i c a l l y pumped i n t o e x c i t e d s t a t e s , t r a n s f e r energy i n t o t h e e x c i t a t i o n o f v i b r a t i o n a l modes o f molecular d e f e c t s ? Both o f these open ques- t i o n s have been answered and demonstrated a f f i r m a t i v e l y by our f i r s t r e a l i z a t i o n 2 o f FH(CN-) center, i .e., an F center/CNe d e f e c t p a i r on (nnn)<llO> p o s i t i o n s , i n KC1.
As we know, CN- molecules as s b s t i t t i o n a d e f e c t s i n most a l k a l i h a l i d e s a r e
4
found t o r e o r i e n t r a p i d l y ( r or-Y>
10 sec-I even a t 2 1 i n KCl) i n a very shal- l o w o r i e n t a t i o n a l p o t e n t i a l w i t E minima i n e i t h e r one o f t h e v a r i o u s p o s s i b l e d i r e c - t i o n s such as <111>,<110> and <loo>. Therefore t h e a b s o r p t i o n due t o t h e CN- s t r e - t c h i n g v i b r a t i o n ( a t-
2080 cm-I o r-
4.8 p m) shows a small s p l i t t i n g due t o t h e t u n n e l i n g among the shallow w e l l s a t low temperature. Towards h i g h temperatures i t develops i n t o a broad v i b r a t i o n a l - r o t a t i o n a l spectrum w i t h double band s t r u c t u r e corresponding c l o s e l y t o t h e P and R branches o f a f r e e r o t o r .C7-288
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DEPHYSIQUE
+ - + - + - + - + - + - +
" ? @ Vibrational
The new developed c o n f i g u r a t i o n , - + + - +
0
+ - +,,
+_
i.e.,the d s s o c i a t i o n o f an F c e n t e r
+ - + - +
-
+-
+ @ s + ~ , - +w i t h t h e CN- d e f e c t i n KC1 shows an
e l e c t r o n i c absorption and emission FCenler + CKDefccI
-
F,(CN-1 band o f s i m i l a r s t r e n g t h and o n l ys l i g h t s p e c t r a l broadening and red- s h i f t compared t o t h e F c e n t e r ( Fig . l )
.
As a new e f f e c t , however, i t d i s p l a y s a very weak energy t r a n s f e r from t h e e x c i t e d F e l e c t r o n t o the CN- i n t e r n a l s t r e t c h i n g mode (Evibr = 0.25 eV),lead- i n g t o n = 0+1 e x c i t a t i o n and subse- quent n = 1+0 r a d i a t i v e r e l a x a t i o n o f t h i s CN- mode2. Though t h i s produced t h e f i r s t discovery o f molecular v i b r a - t i o n a l fluorescence i n i o n i c s o l i d s , t h e weak c o u p l i n g o f t h e CN- t o t h e o p t i c a l F c e n t e r e x c i t a t i o n ( a t Ea 2.3 eV) allowed o n l y l o w quantum e!?iIciency ( q = 0.04) and an energy conver- Wovelength ( ~ m ) Fig
.l
s i o n o f r a t e q6
= q EVjbr/Egbs-
4 x f r o n v i s i b l e pumping ~ n t o i n f r a r e d emission l i g h t .
Ue r e p o r t here - - a f t e r systematic search-- a new FH(CN-) c e n t e r i n CsCl w i t h dramatic increase i n t h e F electron/CN- v i b r a t i o n a l c o u p l i n g and g r e a t a p p l i c a t i o n p o t e n t i a l
.
U n l i k e t h e NaCl s t r u c t u r e o f KC1,
t h e body-centered cesium ha1 i d e s t r u c - t u r e should produce f o r a (n.n.n.) F/CN- p a i r a <loo> o r i e n t e d complex as i l l u s - t r a t e d i n Fig. 2. As f o r FA c e n t e r s i n KC1, one can expect t h a t t h e e x c i t e d F c e n t e r 2p s t a t e c o u l d s p l i t i n t o two s p e c t r a l l y separate absorption t r a n s i t i o n s( F H ( l ) ) and FH(2)), p o l a r i z e d II and 1 t o t h e p a i r a x i s .
Our f i r s t exper'lnents c o n f i r m t h i s ( F i g . 3a): CsCl c r y s t a l s , c o n t a i n i n g low ( 6 x t o 6 x mole r a t i o s o f CN- d e f e c t s , when a d d i t i v e l y colored, quenched and cooled show e s s e n t i a l l y o n l y t h e w e l l known F a b s o r p t i o n band ( w i t h i t s charac- t e r i s t i c s p i n o r b i t s p l i t t i n g s p e c t r a l s t r u c t u r e ) . I r r a d i a t i o n i n t o t h e F band around 170K leads t o conversion i n t o two a b s o r p t i o n bands (0.27eV s e p a r a t i o n ) , which we a t t r i b u t e t o m i g r a t i o n and a s s o c i a t i o n o f t h e F centers w i t h CN- defects,forming the FH(CN-) center as i l l u s t r a t e d i n Fig.3. The s t r o n g i n t e r a c t i o n e x i s t i n g among an F c e n t e r and t h e neighboring CN- molecule i n b o t h d i r e c t i o n s leads us t o observe
t h a t the a s s o c i a t i o n o r aggregation g i v e s r i s e t o an e x t r a , a1 though small, i r absor-
InSb d e t e c t o r . From these c o n d i t i o n s and
5
t h e observed r e l a t i v e emission s t r e n g t h wee s t i m a t e t h a t t h e FH luminescence quantum e f f i c i e n c y q must be considerably h i g h e r
( - 4 - 6 times) than q value o f the F lumine- scence.
Fig. 5 shows the FH (CW-) emis i o n spectra i n CsCl c o n t a i n ~ n g 6 x ID-' and w
5
xl o e 3
mole r a t i o CN-, b o t h measured a ttwo temperdtures. For t h e l o w CN- concen- t r a t i o n case we o b t a i n f i v e alwost e q u a l l y
2 4 2 0 16 2 4 2 o l 6 separated e r ~ i s s i o n bands, a l l s h i f t e d t o
F i g .3 Photon Energy (eV) l o ~ e r energies compared t o t h e i n d i c a t e d p o s i t i o n o f the n=1+0 emission o f t h e i s o l a t e d CN- d e f e c t ( F i g .5a). As t h e band separation Av 3 25 c w - l corresponds t o
t h e ankarmonicity s h i f t among t h e CN- o s c i l l a t o r s t a t e s , (re i n t e r p r e t t h e f i v e emis- s i o n bands ds t h e ~ n = l t r a n s i t i o n s among the s i x l o ~ e s t energy CN- v i b r a t i o n a l s t a t e s o f t h e FH(CN-) c e n t e r . I n f a c t , t h i s can be understood i f we assume t h a t t h e CN- molecular p o t e n t i a l energy curve has t h e form of t h e Morse p o t e n t i a l
V ( R ) = DeC 1 - exp(-a(R-Re))] 2 ( 1 ) where De i s t h e depth o f the p o t e n t i a l minimun, a = (k/2~,)'/' w i t h t h e s p r i n g cons- t a n t k = (d2~/d~2)R,Re, R and Re a r e separations between two n u c l e i of CN- f o r the general and e q i l i b r i u m cases, r e s p e c t i v e l y . The Schrodinger equation can be solved e x a c t l y f o r t h i s p o t e n t i a l , t h e eigenvalues being
E, = ( n
+
1/2)fi w -( n
+ 1/2)' wx ( 2 ) w i t h x = fia2/ 2 pw. The energy s e p a r a t i o n between two consecutive t r a n s i t i o n s d i l l be y i e l d e d byAE =(En+l-En)-(En+2-En+1 )=2H ux ( 3 which i s e x a t l y independent o f t h e quantum nus- b e r n, i n agreement w i t h t h e observed equal spa-
CN'
Stretching Absorption
CSCI
+
~ X I O - ~ C N -
tT=
2 3 ~
c i n g i n t h e CN- emission spectrum. By t h e way, (a)
\
Before Aggregationt h e d i s s o c i a t i o n energy i s(De-Eo) w i t h z e r o - p o i n t
2
energy E o = ~ w/2 -1( wx/4. The f i v e band s t r u c t u r e'$
remains r e s o l v e d a t h i g h e r temperatures(at l e a s t t o T = 120K) and the emission s t r e n g t h decreases&
o n l y g r a d u a l l y over t h i s temperature range, i n
-
c o n t r a s t t o t h e r a p i d tempera u r e decrease o f5
2
t h e normal F emission i n CsCl.
U ) cFor t h e h i g h CN- c o n c e n t r a t i o n case ( F i g .
2
5c and d ) , t h e T K emission spectrum l o o k s s i m i -
-
After Aggregation l a r , b u t c o n t a i n s an added e x t r a band a t t h es p e c t r a l p o s i t i o n o f t h e i s o l a t e d CN- d e f e c t
'z
emission. Modest temperature increase ( t o 36 K ? c!
i n Fig. 5d) produces s t r o n g decrease o f t h e F1, c e n t e r emission bands ( p a r t i c u l a r l y f o r t h e low n value t r a n s i t i o n s ) , combined w i t h t h e increase
o f t h e i s o l a t e d CN- d e f e c t n = l 4 emission 2 0 9 0 2 0 7 0 2 0 5 0 ban?. Note t h a t t h e i s o l a t e d CN- d e f e c t has 10 Wave Number (cm-I)
cm- h i g h e r n = 1 + 0 emission than t h e p e r t u r b e d
C7-290
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DE PHYSIQUECN- does, as impl i e d by the absorptjon ment i n F i g .4. Obviously f o r h i g h CN- d e f e c t d e n s i t i e s t h e e x c i t a t i o n o f t h e CN- i n t h e
FH(CN-) complex can e f f i c i e n t l y t r a n s f e r
--
w i t h t h e h e l p o f thermal a c t i v a t i o n energy--
i t s energy i n t o t h e abundant i s o l a t e d CN- defec system. i d i g r a t i n g by resonance energy t r a n s f e r through t h i s system, t h e CN- e x c i t a t i o n can he t r a n s p o r t e d t o some t r a p w i t h a r a d i a t i o n l e s s d e - e x c i t a t i o n channel (e.g. a CN- d e f e c t p a i r r e s u l t i n g i n a quick decay and disappearance o f t h e FH(CN-)emission w i t h i n c r e a s i n g temperature E v i d e n t l y , t h e d i l u t e case o f n o n - i n t e r a c t i n g FH(CN-) and i s o l a t e d CN- d e f e c t s has a much more ", simple, powerful and temperature independent behavior compared t o t h e concentrated CW- " i n t e r a c t i v e case".The symmetry o f t h e FH(CEI-) complex was determined by pol a r i zed em1 s s i o n measure~sents ( F i g . 6 ) , performed i n CsC1 :CN-crystal o r i e n t e d w i t h X-ray a n a l y s i s t o a <100> o r i e n t a t i o n . [ l o 0 1 p o l a r i z e d e x c i t a t i o n i n t h e F H ( l ) band produces n e a r l y completely para1 1 e l [ l O O ] p o l a- r i z a t i o n o f t h e CN- emission, preserved essen- t i a l l y under increase o f temperature t o 70K. The same [ l O O ] e x c i t a t i o n o f t h e F ( 2 ) a b s o r p t i o n
leads t o s t r o n g perpendicular
FOIOI
p o l a r i z a t i o n 1950 2000 2050 1950 2 0 W 2050 2100o f t h e CN- emission. These two p o l a r i z e d measure- Wave Number (cm-1)
ments c o n f i r m t h e model i l l u s t r a t e d i n Fig. 2:
( a ) < l o o > o r i e n t a t i o n o f t h e F/CN- complex, Fig .5 w i t h F H ( l ) and FH(2) absorptions p o l a r i z e d
s
andI t o t h e <loo> a x i s r e s p e c t i v e l y .
( b ) The CN- molecule ( a weakly hindered r o t o r i n t h e i s o l a t e d d e f e c t case) i s a l i g n e d along the <loo> p a i r a x i s by s t r o n g i n t e r a c t i o n w i t h t h e neighboring F center-even up t o h i g h temperatures.
I n Fig. 7a we summarize t h e measured time dependence o f f i v e FH(CN-) ernission bands a f t e r e x c i t a t i o n w i t h a 40 nsec excirner-laser pumped dye l a s e r pulse. The h i g h l e v e l emissions ( n = 5 4 and 4+3) s t a r t immediately and decay r a p i d l y . The
emissions from t h e l o v ~ e r l e v e l s occur w i t h small i n i t i a l value, h u t rnostly i n delayed b u i l d - u p (by t r a n s i t i o n s from the higher l e v e l s ) and subsequent slow emission decay. I n F i g . 7b we p l o t the r e l a t i v e s i z e o f t h e t = 0 i n i t i a l emission w i t h bars f o r a l l e x c i t e d s t a t e s , i l l u s t r a t i n g t h e i r d i r e c t pump p r o b a b i l i t y through coup1 i n g t o t h e e l e c t r o n i c e x c i t a t i o n . Obviously, pumping occurs w i t h a d i s t r i b u t i o n w i t h s t r o n g peak a t t h e 4 t h s t a t e ( 4 Evibr
-
1.0 eV) ,indi-. c a t i n g a very h i g h quantum e f f i c i e n c y o f A n = l v i b r a t i o n a l emission( ty4)and energy conver- s i o n e f f i c i e n c y ( q = q * Evibr/Eabs = 0.5).I n t h e presented emission measurements ( F i g s .5,6,7), t h e pump l i g h t i n t e n s i t y used was l o w enough t o e x c i t e FH(CN-) c e n t e r s o n l y w i t h t h e i r CN- i n t h e v i b r a t i o n a l ground s t a t e . Under considerably increased pump 1950 2000 2050 2100 ,95Q 2000 2050 21001 ig h t i n t e n s i t y (focused cw l a s e r ) , however,
complex. This leads t o p a r t i a l pump- i n g o f " v i b r a t i o n a l l y e x c i t e d FH c e n t e r s " i n t o h i g h e r CN- l e v e l s , as we c o u l d observe by a r e l a t i v e increase o f t h e 5 + 4 emission and an appearance o f a h i g h e r 6 + 5
-
.-
cemission band i n s p e c t r a l measure- ments o f t h e type i n Fig. 5.
4
-
21
-
Figure 7 i l l u s t r a t e s d i r e c t l y
'5
t h e p o p u l a t i o n i n v e r s i o n among t h e CN- 1 eve1 s achieved under p u l sed pumping ( s t r o n g e s t between n=4 andQ
31, i n d i c a t e d by t h e r a p i d i n i t i a l
.:
decay o f t h e 4 + 3 emission which wi s most 1 ik e l y by s t i m u l a t e d t r a n s i - t i o n s . Some questions w i l l be c e r - t a i n l y r a i s e d i f one r e c a l l s t h a t t h e t r a n s i t i o n p r o b a b i l i t y anlong t h e v a r i o u s l e v e l s o f CN- i s propor- t i o n a l t o t h e m a t r i x elements l i k e ,,
Wave Number (cm-') Time (msec)
Fig .7 w i t h p = t h e d i p o l e moment o f CN-. This t r a n s i t i o n r a t e i s c h a r a c t e r i z e d by t h e l i n e a r dependence o f t h e quantum number n. The discrepancy between t h i s general feature and what we observed i m p l i e s t h a t t h e a c t u a l dynarnical process occuring among t h e v a r i o u s l e v e l s o f
CN-
i s much more i n v o l v e d than i t looks l i k e .Experiments ~ i t h t h i s system f o r powerful 4.8 pn l a s e r a p p l i c a t i o n s a r e a c t i - v e l y under way. They p o s s i b l y a l l o w l a s e r l i g h t t l r n i n g by t h e above described s h i f t o f t h e i n v e r s i o n i n t o h i g h e r CN- l e v e l s under s t r o n g pump l i g h t i n t e n s i t y . E f f o r t s t o f u l l y understand t h i s h i g h l y e f f i c i e n t and novel energy t r a n s f e r process between F and CN- are a l s o under way--both t h e o r e t i c a l l y and by f u r t h e r experiments. Resonance Raman measurements on t h e FH(CN-) d e f e c t s N i l 1 c l e a r l y decide, i f t h e FH e l e c t r o n / CN- v i b r a t i o n a l c o u p l i n g occurs i n t h e a b s o r p t i o n process i t s e l f , o r d u r i n g and/or a f t e r e x c i t e d s t a t e r e l a x a t i o n . I f t h e l a t t e r i s the case, t h e e l e c t r o n i c energy o f t h e normal F emission (E = 1.25 eV) f i t s c l o s e l y t o t h e 4 t h
CN-
v i b r a t i o n a l l e v e l e x c i t a t i o n energy, suggesting a d i r e c t and h i g h l y e f f i c i e n t t r a n s f e r process between b o t h . Experiments on FIj c e n t e r s i n CsBr and CsI a r e a1 ready under way.Support by NSF Grant 81-05332 f o r t h i s work i s acknowledged. References
1. W.B. Fowler, i n "Physics o f Color Centers", e d i t e d by
W.B.
Fowler (Academic, New York, (1968) Chap. 2.2. Y . Yang and F. Luty, Phys. Rev. L e t t . 51, 419 (1983).
3. D.W. Lynch, A.D. Brothers, and D.A. RoFnson, Phys. Rev. 139, A285 (1965). 4. H. Dubost and R. Charneau, Chem. Phys. 12, 407 (1976).
