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HAL Id: jpa-00225082

https://hal.archives-ouvertes.fr/jpa-00225082

Submitted on 1 Jan 1985

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IR LASER-INDUCED UV-VISIBLE FLUORESCENCE

IN MATRIX-ISOLATED CO

J. Galaup, J. Harbec, R. Charneau, H. Dubost

To cite this version:

(2)

J O U R N A L

DE PHYSIQUE

Coiloque C7, supplgment au nOIO, Tome 46, octobre 1985 page C7-293

I R

LASER-INDUCED U V - V I S I B L E FLUORESCENCE I N MATRIX-ISOLATED CO

J.P. Galaup, J.Y. ~arbec', R. Charneau and H. Dubost

Laboratoire de Photophgsique MoZe'cuZaire du

CNRS"., B i i t

.

21 3,

UniversitS Paris-sud,

91405

Orsay Ceder, France

Resume

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Sous l ' e f f e t d ' u n e i r r a d i a t i o n p a r u n L a s e r i n f r a r o u g e a c e n t r e s c o l o r & s , u n f i l m d ' a r g o n s o l i d e c o n t e n a n t 1 % d e 1 3 ~ 1 8 0 emet u n e f l u o r e s c e n c e d a n s l a r e g i o n U V - v i s i b l e d u s p e c t r e . L a f r e q u e n c e d u l a s e r i n f r a r o u g e c o i n c i d e a v e c c e l l e d u c e n t r e d e l a r a i e d ' a b s o r p t i o n c o r r e s p o n d a n t a u p r e m i e r h a r m o n i q u e de La v i b r a - t i o n d e C O . L e s p e c t r e d ' e m i s s i o n e s t c o n s t i t u e d ' u n s y s t e m e com- p l e x e i n t e n s e e n t r e 6 5 0 e t 8 0 0 nm e t d ' u n e p r o g r e s s i o n s i m p l e p l u s f a i b l e q u i s ' e t e n d d e 3 2 0 a 6 4 0 nm. Ces d e u x b a n d e s d ' e m i s s i o n s o n t a t t r i b u e e s a d e s t r a n s i t i o n s e l e c t r o n i q u e s d e l ' o x y g e n e mo- l e c u l a i r e . Le s p e c t r e d ' e m i s s i o n i n f r a r o u g e m o n t r e q u e l e s mCca- n i s m e s de t r a n s f e r t V - V n o n r e s o n n a n t s p e u p l e n t d e s n i v e a u x v i - b r a t i o n n e l s t r e s e l e v e s d a n s l ' e t a t e l e c t r o n i q u e f o n d a m e n t a l de C O . L ' C m i s s i o n p r o v e n a n t d e s C t a t s e x c i t e s c3A, e t b1C; d e L ' o x y - g e n e e s t l ' e v i d e n c e d ' u n e c o n v e r s i o n d ' e n e r g i e v i b r a t i o n n e l l e e n e n e r g i e e l e c t r o n i q u e . A b s t r a c t

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U V - v i s i b l e f l u o r e s c e n c e f r o m a s a m p l e o f s o l i d a r g o n c o n t a i n i n g 1 % o f 1 3 c 1 8 0 i s o b s e r v e d u p o n i r r a d i a t i o n b y a c o l o r - c e n t e r l a s e r whose f r e q u e n c y i s c o i n c i d e n t w i t h t h e f i r s t o v e r t o - n e a b s o r p t i o n b a n d o f C O . T h i s e m i s s i o n c o n s i s t s o f a n i n t e n s e c o m p l e x s y s t e m b e t w e e n 6 5 0 a n d 8 0 0 nm a n d o f a w e a k e r s i n g l e p r o - g r e s s i o n e x t e n d i n g f r o m 3 2 0 t o 6 4 0 nm. B o t h s y s t e m s a r e a s s i g n e d t o e l e c t r o n i c t r a n s i t i o n s o f m o l e c u l a r o x y g e n . I n f r a r e d e m i s s i o n s p e c t r a i n d i c a t e t h a t h i g h L y i n g v i b r a t i o n a l l e v e l s o f t h e g r o u n d e l e c t r o n i c s t a t e o f C O ( u p t o v = 3 0 ) a r e p o p u l a t e d b y a n h a r m o n i c V - V p u m p i n g . The o b s e r v a t i o n o f s u b s e q u e n t e m i s s i o n f r o m t h e c 3 A U a n d b1C; e x c i t e d e l e c t r o n i c s t a t e s o f 0, d e m o n s t r a t e s t h a t v i b r a t i o n a l t o e l e c t r o n i c e n e r g y t r a n s f e r o c c u r s . I

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INTRODUCTION I n d i a t o m i c m o l e c u l e s , v i b r a t i o n a l e n e r g y t r a n s f e r t e n d s t o a c c u m u l a t e t h e v i b r a t i o n a l e n e r g y i n t h e h i g h - l y i n g L e v e l s . S u c h a n e x c i t a t i o n m e c h a n i s m i s known a s t h e a n h a r m o n i c V - V p u m p i n g a n d i t o c c u r s i n g a s p h a s e ( l t 2 ) as w e l l as i n t h e s o l i d p h a s e ( 3 ) . I n g a s e o u s C O ( ' 1 a n d NO ( * ) , i r r a d i a t i o n w i t h s e v e r a l w a t t s o f CW L a s e r r a d i a t i o n r e s u l t s i n UV o r v i s i b l e e m i s s i o n . I n t h e p r e s e n t p a p e r we r e p o r t a s i m i l a r e x p e - r i m e n t o n m a t r i x - i s o l a t e d C O w h i c h i s v i b r a t i o n a l l y , e x c i t e d t o t h e v

=

2 s t a t e b y a Low p o w e r c o l o r - c e n t e r l a s e r

(CCL).

I 1

-

EXPERIMENTAL RESULTS The C O m o l e c u l e s w e r e o p t i c a l l y pumped i n t o t h e v = 2 s t a t e u s i n g a m e c h a n i c a l l y c h o p p e d C W CCL d e l i v e r i n g 1 0 mW a t

X

= 2 . 4 7 pm t h e wa- v e l e n g t h o f t h e O + 2 o v e r t o n e a b s o r p t i o n b a n d c e n t e r o f 1 3 c 1 8 6 . The t u n i n g r a n g e o f o u r KC1:Li CCL e x t e n d s f r o m 2.45 t o 2 . 9 pm. T h e r e f o r e o n l y t h e h e a v i e s t i s o t o p i c s p e c i e s o f C O c o u l d b e o p t i c a l l y pumped w i t h t h i s s o u r c e . T h i n f i l m s o f a r g o n c o n t q i n i n g 1 % o f 1 3 c 1 8 0 w e r e +present address : DBpartement de Physique, U n i v e r s i t d de Montrdal, Case p o s t a l e 6128, succursale A Montrdal, QuBbec, Canada H3C 357

" ~ a b o r a t o i r e associB B l ' U n i v e r s i t 6 de Paris-Sud

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C7-294 JOURNAL

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PHYSIQUE o b t a i n e d b y d e p o s i t i n g a g a s e o u s m i x t u r e o f A r a n d C O a t t h e d e s i r e d r a t i o o n t o a g o l d - c o a t e d c o p p e r s u b s t r a t e h e l d a t l i q u i d h e l i u m t e m - p e r a t u r e s . I R e m i s s i o n s p e c t r a w e r e r e c o r d e d u s i n g a 1 m S o p r a mono- c h r o m a t o r w i t h a n I n S b o r HgCdTe p h o t o d i o d e . U V - v i s i b l e s p e c t r a w e r e o b t a i n e d w i t h t h e a i d o f a H R S J o b i n - Y v o n s p e c t r o m e t e r e q u i p p e d w i t h a R C A C31034 p h o t o m u l t i p l i e r t u b e .

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V i b r a t i o n a l e x c i t a t i o n o f C O i n s o l i d a r g o n O v e r t o n e e x c i t a t i o n o f C O i n s o l i d A r h a s r e v e a l e d t o b e a v e r y e f f i - c i e n t way t o p r o d u c e s t r o n g l y v i b r a t i o n a l l y e x c i t e d C O m o l e c u l e s . The e x c i t a t i o n m e c h a n i s m h a s b e e n s t u d i e d i n d e t a i l s u s i n g t i m e r e s o l v e d i n f r a r e d s p e c t r o s c o p y C 4 ) . Due t o t h e w e a k n e s s o f t h e O + 2 t r a n s i t i o n d i p o l e moment, t h e v

=

2 e x c i t a t i o n i s r a t h e r l o c a l i z e d a n d m i g r a t i o n a i d e d e n e r g y t r a n s f e r f r o m v = 2 i s i n h i b i t e d . H o w e v e r o v e r t o n e pum- p i n g p r o d u c e s a s t r o n g p o p u l a t i o n i n v e r s i o n b e t w e e n l e v e l 2 a n d l e v e l 1 r e s u l t i n g i n 2 + I s t i m u l a t e d e m i s s i o n . A c t u a l l y e v i d e n c e f o r s t i m u - l a t e d e m i s s i o n h a s b e e n o b t a i n e d f o r C O i n s o l i d N 2 (5). B r i e f l y , t h e e x c i t a t i o n o f h i g h - l y i n g v L e v e l s o f C O i n s o l i d a r g o n o c c u r s i n 4 s t e p s : 1 ) d i r e c t o p t i c a l p u m p i n g o f v

=

2 2 ) f a s t p o p u l a t i o n t r a n s f e r f r o m v = 2 t o v = 1 r e s u l t i n g f r o m 2 + l s t i m u l a t e d e m i s s i o n 3 ) m i g r a t i o n o f t h e v

=

1 e x c i t a t i o n 4 ) n o n - r e s o n a n t e n e r g y t r a n s f e r t h r o u g h t h e w e l l - k n o w n f u s i o n p r o c e s s : CO(v)

+

CO(1) + C O ( v + l ) + CO(0) + A E .

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( i n t e n s i t y , t i m e b e h a v i o u r , e x c i t a t i o n s p e c t r a ) . T h e r e f o r e e n e r g y t r a n s f e r p r o b a b l y o c c u r s v i a t w o d i s t i n c t c h a n n e l s .

The COCv)

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0 2 i n t e r a c t i o n i s t o o weak t o g i v e r i s e t o n o n r e s o n a n t e n e r g y t r a n s f e r i n v o l v i n g a c u r v e c r o s s i n g m e c h a n i s m . R e s o n a n t p r o c e s - s e s a r e m o r e L i k e l y e x p e c t e d t o b e e f f i c i e n t i n t h i s c a s e . A schema- t i c d i a g r a m o f t h e l o w e s t v i b r o n i c e n e r g y l e v e l s o f CO a n d 0 2 w h i c h may b e i n v o l v e d i n V-E t r a n s f e r a r e s h o w n i n F i g . 5 . O x y g e n i s u n i q u e i n h a v i n g t w o L o w - L y i n g e x c i t e d e l e c t r o n i c s t a t e s : alA a t 0 . 9 8 eV a n d b1C; a t 1.63 eV r e l a t i v e t o t h e g r o u n d s t a t e X ~ Z - . ~ S U C ~ e n e r g i e s c o r r e s p o n d y o t h a t o f a s m a l l n u m b e r o f C O v i b r a t i o - n a l q u a n t a . I t s h o u l d a l s o 32 3 b e p o i n t e d o u t t h a t .\Ivy C O ( X ' C + , v > 1 9 ) a n d 0 02 Cc3AU, V ) h a v e s i m i l a r e n e r i e s . The e n e r g y o f

9

C O C X C', v

> 2 9 )

a l s o m a t c h e s t h a t o f c0(a311, v ) . T h e r e - f o r e s e v e r a l n e a r l y r e s o n a n t V-E t r a n s f e r p r o c e s s e s may o c c u r . F i g u r e

5

S c h e m a t i c e n e r g y l e v e l d i a - g r a m s h o w i n g t h e v i b r o n i c L e v e l s o f t h e L o w e s t e l e c - t r o n i c s t a t e s o f CO a n d 02 m o 1 e c u l e . s . The f l u o r e s c e n t 02 l e v e l s a r e m a r k e d w i t h a s e m i c i r c l e . The wavy a r r o w s i n d i c a t e a f e w p o s s i b l e p a - t h w a y s f o r V-E t r a n s f e r .

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X1x+

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C7-298

JOURNAL

DE

PHYSIQUE Two p o s s i b l e e x p l a n a t i o n s may a c c o u n t f o r t h e p r e d o m i n a n c e o f t h e b-X e m i s s i o n o f "02 w i t h r e s p e c t t o t h a t o f 1 6 0 2 . E i t h e r ' ' 0 2 i s m o r e a b u n d a n t t h a n 1 6 0 2 o r a n i s o t o p i c a l l y s e l e c t i v e t r a n s f e r p r o c e s s o c - c u r s i n f a v o r o f t h e m o r e d i l u t e d 1 8 0 2 s p e c i e s . " 0 - e n r i c h e d m o l e c u l a r o x y g e n c o u l d b e c o n t a i n e d i n t h e i s o t o p i c a l l y e n r i c h e d C O g a s u s e d f o r p r e p a r i n g t h e s o l i d

A ~ : ~ ~ c ' ' o

s a m p l e s o r i t may r e s u l t f r o m t h e i n - s i t u d i s s o c i a t i o n o f 1 3 c 1 8 0 i n d u c e d b y t h e I R L a s e r r a d i a t i o n . L e t u s assume f o r i n s t a n c e t h a t 1 8 0 2 i s t h e m o s t a b u n d a n t s p e c i e s . The s e l e c t i v e e x c i t a t i o n o f 1 6 0 2 ( c ~ A ~ ) c o u l d t h e n r e s u l t f r o m m i g r a t i o n o f t h e t r i p l e t e x c i t a t i o n among t h e m o s t a b u n d a n t 1 8 0 2 m o l e c u l e s a n d t r a p p i n g b y t h e L e a s t a b u n d a n t 1 6 0 2 s p e c i e s . H o w e v e r s u c h a m e c h a n i s m w o u l d r e q u i r e a r a t h e r L a r g e "02 c o n c e n t r a t i o n a n d t h e e x i s t e n c e o f a n e n e r g y g a p b e t w e e n t h e

c 3 a U

s t a t e s o f "02 a n d 1 6 0 2 . I n s o l i d N2, t h e e n e r g y o f 1 6 0 2

( c 3 a U )

h a s b e e n f o u n d t o b e s m a l l e r t h a n t h a t o f 1 8 0 2

( c 3 a U )

b y 1 2 cm-' C 8 ) . E v e n s u c h a s m a l l d i f f e r e n c e i s L a r g e e n o u g h t o e n s u r e e n e r g y t r a p p i n g a t Low t e m p e r a t u r e s . On t h e c o n t r a r y i f 1 6 0 2 i s a s s u m e d t o b e m o r e a b u n d a n t t h a n "02, t h e s e l e c t i v e e x c i - t a t i o n o f ' ' 0 2 ( b l ~ + ) w o u l d b e d u e t o m i g r a t i o n o f t h e v i b r a t i o n a l e x c i t a t i o n among ' 6 8 2 a n d t r a p p i n g o n t h e h e a v i e s t i s o t o p i c s p e c i e s 1 8 02

-

I V

-

CONCLUSION The o b s e r v a t i o n o f I R - i n d u c e d U V - v i s i b l e f l u o r e s c e n c e i s t h e e v i d e n c e f o r V - E e n e r g y t r a n s f e r f r o m C O t o 0 2 . The e x p e r i m e n t a l o b s e r v a t i o n s s u g g e s t t h a t t h e 0 2 f l u o r e s c e n t e l e c t r o n i c s t a t e s a r e p o p u l a t e d v i a t w o i n d e p e n d e n t p r o c e s s e s . M o l e c u l a r o x y g e n i s p r e s e n t i n t h e s a m p l e s a s a n i m p u r i t y a n d b o t h t h e c o n c e n t r a t i o n a n d t h e i s o t o p i c r a t i o a r e u n k n o w n . A l t h o u g h i t i s d i f f i c u l t t o u n a m b i g u o u s l y i d e n t i f y t h e V-E t r a n s f e r mechanisms, t h e f o l l o w i n g 2 - c h a n n e l scheme r e a s o n a b l y a c - c o u n t s f o r t h e e x p e r i m e n t a l b e h a v i o u r : i) 0 2 ( c 3 ~ , ) i s f e e d e d b y i n t e r m o l e c u l a r E - E t r a n s f e r f r o m C O t o O 2 m e d i a t e d t h r o u g h i n t e r n a l c o n v e r s i o n X 1 z + + a 3 I I i n C O . ii) O ~ ( ~ ' X ; ) i s p o p u l a t e d e i t h e r b y d i r e c t V-E t r a n s f e r o r b y i n t e r - m o l e c u l a r V - V t r a n s f e r b e t w e e n C O a n d 02 f o l l o w e d b y i n t e r n a l c o n v e r - s i o n i n 0 2 . REFERENCES ( 1 J . W . R i c h a n d R . C . Bergman, Chem. P h y s . 4 4 ( 1 9 7 9 ) 5 3 . C 2 ) H. Dunnwald, E . S i e g e l , W . U r b a n , J . W . ~ z h , G . F . H o m i c z a n d M . J . W i l l i a m s , Chem. P h y s . 9 4 ( 1 9 8 5 ) 1 9 5 . C 3 ) H. D u b o s t , " S p e c t r o s c o p y o f T i b r a t i o n a 1 a n d r o t a t i o n a l l e v e l s o f d i a t o m i c m o l e c u l e s i n r a r e g a s c r y s t a l s " i n I n e r t Gases : P o t e n - t i a l , D y n a m i c s a n d e n e r g y t r a n s f e r i n d o p e d c r y s t a l s , S p r i n g e r s e r i e s i n C h e m i c a l P h y s i c s , v o l . 34, e d i t e d b y M.L. K l e i n ( S p r i n g e r V e r l a g , B e r l i n 1984), p. 1 4 5 - 2 5 7 . ( ) J . J . Zondy, J . Y . H a r b e c , J .P. G a l a u p , R . C h a r n e a u a n d H. D u b o s t , p r e c e d i n g p a p e r i n t h i s v o l u m e .

c 5 )

J . P . G a l a u p , J . Y . H a r b e c , J . J . Zondy, R . C h a r n e a u a n d H. D u b o s t , p r e c e d i n g p a p e r i n t h i s v o l u m e .

c 6 )

R . R o s s e t t i a n d L.E. B r u s , J . Chem. P h y s . 7 1 ( 1 9 7 9 ) 3 9 6 3 .

C 7 ) D . H . Katayama, T.A. M i l l e r , a n d V r E . B o n d y E y , J . Chem. Phys., 71 ( 1 9 7 9 ) 1 6 6 2 .

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