LASER INDUCED FLUORESCENCE AND HIGH RESOLUTION FOURIER TRANSFORM SPECTROSCOPY (L.I.F.F.T.S.)

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LASER INDUCED FLUORESCENCE AND HIGH RESOLUTION FOURIER TRANSFORM

SPECTROSCOPY (L.I.F.F.T.S.)

J. Vergès, C. Amiot

To cite this version:

J. Vergès, C. Amiot. LASER INDUCED FLUORESCENCE AND HIGH RESOLUTION FOURIER

TRANSFORM SPECTROSCOPY (L.I.F.F.T.S.). Journal de Physique Colloques, 1987, 48 (C7),

pp.C7-595-C7-600. �10.1051/jphyscol:19877145�. �jpa-00226966�

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JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n012, Tome 48, dgcembre 1987

LASER INDUCED FLUORESCENCE

AND

HIGH RESOLUTION FOURIER TRANSFORM SPECTROSCOPY (L.I.F.F.T.S.)

J . VERGES and

C.

AMIOT

Laboratoire Aim6 Cotton, Bdtiment 505, Campus d l O r s a y , F-91405 Orsay Cedex, France

I - INTRODUCTION

F o u r i e r t r a n s f o r m s p e c t r o s c o p y (F.T.S.) i s a p o w e r f u l s p e c t r o s c o p i c t e c h n i q u e . The a d v a n t a g e s of t h i s method a r e w e l l known :

i ) t h e m u l t i p l e x ( o r F e l l g e t t ) a d v a n t a g e which r e s u l t s i n t h e s i m u l t a n e o u s o b s e r v a - t i o n of a l l t h e s p e c t r a l e l e m e n t s d u r i n g t h e r e c o r d i n g t i m e . T h i s a d v a n t a g e i s f u l l y o b t a i n e d when t h e d ' e t e c t o r n o i s e i s much h i g h e r t h a n i t h e p h o t o n n o i s e .

i i ) t h e h i g h t h r o u g h p u t , ( o r J a c q u i n o t ) a d v a n t a g e which p e r m i t s , a t e q u a l r e s o l u t i o n , a b e t t e r p h o t o n g a t h e r i n g t h a n c l a s s i c a l g r a t i n g t e c h n i q u e s .

The a n a l y s i s of t h e s p e c t r a i s a l s o f a c i l i t a t e d by t h e w a v e l e n g t h a c c u r a c y : a whole s p e c t r u m c a n b e r e c o r d e d on a n i n t e r n a l l y c o n s i s t e n t w a v e l e n g t h s c a l e , t i e d d i r e c t l y t o a s i n g l e a c c u r a t e r e p r o d u c i b l e w a v e l e n g t h s t a n d a r d . Of e q u a l i m p o r t a n c e i s t h e f a c t , t h a t , by s i m p l e o p t i c a l and n u m e r i c a l f i l t e r i n g , t h e r e c o r d e d s p e c t r a l r e g i o n can b e t c h o s e n a t w i l l .

I1 - EXPERIMENT

L a s e r e x c i t a t i c n i s a c o n v e n i e n t a n d w i d e l y u s e d method f o r p r o d u c i n g mole- c u l e s i n a g i v e n e l e c t r o n i c e x c i t e d s t a t e . The f l u o r e s c e n c e l i g h t i s a n a l y z e d by means of F.T.S. u s i n g t h e e x p e r i m e n t a l s e t - u p shown i n F i g u r e 1 . I n t h i s p a r t i c u l a r c a s e , a l k a l i d i m e r s m o l e c u l e s a r e produced i n a h e a t p i p e oven. They a r e e x c i t e d by a t u n a b l e dye l a s e r beam. Backward f l u o r e s c e n c e l i g h t i s c o l l e c t e d by a n o p t i c a l a r r a n g e m e n t and imaged o n t o t h e e n t r a n c e h o l e o f t h e F o u r i e r s p e c t r o m e t e r . Forward f l u o r e s c e n c e i s c o l l e c t e d by a low r e s o l u t i o n g r a t i n g a p p a r a t u s and i s u s e d t o s e r v o e t h e l a s e r f r e q u e n c y , measured w i t h a lamda m e t e r .

F i g u r e 1 : E x p e r i m e n t a l s e t - u p . The m o l e c u l e s produced i n t h e h e a t - p i p e a r e e x c i t e d by a t u n a b l e l a s e r .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877145

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JOURNAL DE PHYSIQUE

I11

-

SPECTROSCOPIC RESULTS

Over t h e course of r e c e n t y e a r s t h e o r e t i c a l methods have been developped f o r t h e c a l c u l a t i o n of p o t e n t i a l energy curves of diatomic molecules. Consequently t h e r e appears a g r e a t need of e x t e n s i v e and a c c u r a t e experimental d a t a i n o r d e r t o t e s t t h e new t h e o r e t i c a l methods. For t h e f i l l f i l m e n t of t h i s p r o j e c t t h e s p e c t r a of a number of molecules have been i n v e s t i g a t e d using L.I.F.F.T.S., Table I shows a summary of t h e molecules and e l e c t r o n i c s t a t e s t h a t were c h a r a c t e r i z e d by t h i s method. I t g a i n s a c c e s s mostly t o those e x c i t e d s t a t e s having t h e same i n v e r s i o n symmetry a s t h a t of t h e ground e l e c t r o n i c s t a t e and which would be i n a c c e s s i b l e by d i r e c t a b s o r p t i o n from t h i s ground s t a t e .

From t h e s e s p e c t r o s c o p i c s t u d i e s l a r g e p o r t i o n s of p o t e n t i a l energy curves can be g e n e r a t e d . The example of t h e NaK molecule i s d e p i c t e d i n F i g u r e 2. (10).

TABLE I

ELECTRONIC STATES CHARACTERIZED BY L

.

^I.^F

.

^F

.

^T.^S^{. a}

Molecules

t

12

Bi, L i z

*

KH CsH BaH

*

BaF

E l e c t r o n i c S t a t e s

X '2; alg a ' 0;

Ref 1 2

3 4

5

6

7

a For m o l e c u l e s marked w i t h a s t a r i s o t o p i c s p e c i e s h a s been s t u d i e d . D i s s o c i a t i o n l i m i t s determined and l o n g range study w a s performed f o r u n d e r l i n e d s t a t e s .

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F i g u r e 2 : An example of p o t e n t i a l energy

P.

c u r v e s f o r NaK d e r i v e d from L.I.F.F.T.S. ccw's"O s t u d i e s ( 1 0 ) .

I V

-

CONSEQUENCES OF THE SPECTROSCOPIC STUDIES

S e v e r a l remarkable p h y s i c a l e f f e c t s were p u t i n e v i d e n c e a f t e r t h e p r e v i o u s l y r e p o r t e d s p e c t r o s c o p i c s t u d i e s .

.

Long r a n g e s t u d i e s

For t h e e l e c t r o n i c s t a t e s u n d e r l i n e d i n Table I , a l o n g r a n g e s t u d y of t h e p o t e n t i a l energy c u r v e was performed. Accurate v a l u e s of t h e d i s s o c i a t i o n e n e r g i e s and of t h e m u l t i p o l a r development c o e f f i c i e n t s were d e r i v e d . T r a n s i t i o n s i n v o l v i n g t h e l a s t bound v i b r a t i o n a l l e v e l s have been observed i n L i , , KH and CsH. F i g u r e 3 shows t h e p r e d i s s o c i a t e d P I 1 l i n e i n t h e X

'c+

s t a t e of CsH. ( 1 2 ) .

F i g u r e 3 : The l a s t observed d o u b l e t n e a r t h e CsH d i s s o c i a t i o n l i m i t . The l i n e PI1 i s p r e d i s s o c i a t e d ( 1 2 ) .

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C7-598 JOURNAL

DE

PHYSIQUE

.

Double minimum p o t e n t i a l energy curve i n t h e 2 'E: Na, e l e c t r o n i c s t a t e

(8,

Can. J. Phys. 1984)

Accurate informations on t h i s s t a t e have been obtained both by one and two s t e p e x c i t a t i o n p r o c e s s e s . The p o t e n t i a l curve, shown i n f i g u r e 4 i s due t o an avoi- ded c r o s s i n g of two d i a b a t i c s t a t e s : t h e i n t e r n a l well comes from a Rydberg s t a t e and t h e second d i a b a t i c s t a t e has a s t r o n g i o n i c c h a r a c t e r a t l a r g e i n t e r n u c l e a r d i s t a n c e s . However a simple i o n i c curve, shown a s a dashed l i n e , does not adequa- t e l y reproduce t h e o u t e r limb.

p o t e n t i a l f o r

.

E l e c t r o n i c energy t r a n s f e r s B 'n

-

2 Iz+ i n Na, and L i ,

(3,

^{J .}Mol. S p e c t r . 1985,

5,

Phys. Rev. L e t t 2 1 9 8 6 ) . ~ g

The pumping scheme i s r e p r e s e n t e d i n F i g u r e 5. An argon l a s e r l i n e e x c i t e s a s i n g l e r o v i b r a t i o m a l l e v e l of t h e B 'n s t a t e . Extensive i n f r a r e d f l u o r e s c e n c e appears which i s a s s i g n e d without ambiguity t o t h e 2 'c: + A

'z:

t r a n s i t i o n . The 2

'c;

s t a t e being of t h e same symmetry a s t h e ground s t a t e cannot be e x c i t e d d i r e c t l y from-this s t a t e : energy i s t r a n s f e r e d from B 'IT t o 2

'xi.

Due t o high r e s o l u t i o n of L . I.F.F.T. S. a s s o c i a t e d t o t h e p o s s i b i l i t y of me2suring r e l a t i v e i n t e n s i t i e s on l a r g e

s p e c t r a l ranges, t h e p o p u l a t i o n s of a l l t h e v i b r o r o t a t i o n a l l e v e l s populated i n t h e t r a n s f e r h a s been d e r i v e d . P r o p e n s i t y r u l e s have been put i n evidence : c o n s e r v a t i o n of t h e n u c l e a r symmetry and of t h e r o t a t i o n a l number J f o r quasi-resonant t r a n s f e r s . T h e o a e t i c a l model h a s been proposed t o a t t e m p t t o g i v e adequate e x p l a n a t i o n f o r a l l t h e experimental r e s u l t s . Cross s e c t i o n s have been c a l c u l a t e d f o r each v i b r o r o - t a t i o n a l channel. F a i r l y good agreement i s o b t a i n e d w i t h experimental r e s u l t s .

F i g u r e 5 : P o s i t i o n of t h e lowest l y i n g s i n g l e t e l e c t r o n i c s t a t e s of Na,

.

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