CHEMILUMINESCENT CHANNELS OPENED BY PHOTON ABSORPTION DURING REACTIVE COLLISIONS

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CHEMILUMINESCENT CHANNELS OPENED BY PHOTON ABSORPTION DURING REACTIVE

COLLISIONS

Ph. Brooks, R. Curl, T. Maguire

To cite this version:

Ph. Brooks, R. Curl, T. Maguire. CHEMILUMINESCENT CHANNELS OPENED BY PHOTON

ABSORPTION DURING REACTIVE COLLISIONS. Journal de Physique Colloques, 1985, 46 (C1),

pp.C1-303-C1-312. �10.1051/jphyscol:1985130�. �jpa-00224507�

JOURNAL DE PHYSIQUE

Colloque C I , suppl6ment a u nO1, Tome 46, janvier 1985 page CI-303

C H E M I L U M I N E S C E N T CHANNELS OPENED BY PHOTON A B S O R P T I O N D U R I N G R E A C T I V E C O L L I S I O N S

Ph. R. Brooks, R.F. Curl and T.C. Maguire

Department of Chemistry and Rice Quantum I n s t i t u t e , Rice University, Houston, TX 7 7 2 5 1 , U.S.A.

Resum6 - Des lasers sont utilises pour sonder les especes transitoires qui existent pendant que les partenaires de r6action se transforment en produits de reaction dans une reaction chimique. Des faisceaux mol6culaires de K et NaCl se croisent B l1int6rieur de la cavite d'un laser h colorant continu et les photons absorb6s par les esp&ces transitoires permettent d'ouvrir la voie

"chimi-luminescente" conduisant 2 la formation de KC1 et ~ a * .

A b s t r a c r

-

I. I n t r o d u c t i o n

C o l l i s i o n s between atoms a r e u s u a l l y d e s c r i b e d i n terms of t h e p o t e n t i a l energy between t h e atoms, and one can f r e q u e n t l y c a l c u l a t e o r e x p e r i m e n t a l l y determinc t h e p o t e n t i a l energy curve a s a f u n c t i o n of i n t e r n u c l e a r s e p a r a t i o n . Numerous examples of such p o t e n t i a l curves can be found i n o t h e r a r t i c l e s i n t h i s volume, and t h e i r u t i l i t y i n i n t e r p r e t i n g d i v e r s e phenomena can be seen from a p e r u s a l of t h e s e a r t i - c l e s .

Chemically r e a c t i v e c o l l i s i o n s , on t h e o t h e r hand, a r e much more complicated.

F i r s t , the s i m p l e s t r e a c t i o n i n v o l v e s an atom r e a c t i n g with a diatomic molecule,

which i s a t h r e e ( o r more) body problem. The choice of c o o r d i n a t e system i s complicated by t h e f a c t t h a t t h e f i n a l system d i f f e r s from t h e i n i t i a l system. For t h e "simple" system of r e a c t i o n I, t h e p o t e n t i a l energy w i l l depend upon t h e d i s t a n c e between each p a i r of atoms, r m , . r B c , and ZAC. The p o t e n t i a l energy i s t h u s a s u r f a c e i n f o u r dimensional space whxch i s d i f f i c u l t t o imagine. I n o r d e r t o s i m p l i f y c a l c u l a t i o n s and p r o v i d e a conceptual b a s i s f o r d i s c u s s i o n , one f r e q u e n t l y d i s c u s s e s c o l l i n e a r r e a c t i o n s , where, f o r example, t h e atoms of r e a c t i o n 1 a r e confined t o a l i n e . The p o t e n t i a l energy f o r t h e system t h e n depends on only two d i s t a n c e s , rm and rgc and contour p l o t s can be used t o d e s c r i b e t h e system.

The g e n e r a l form such a contour p l o t t a k e s i s shown i n Fig. 1 which i s q u a l i t a t i v e l y s i m i l a r t o t h e p o t e n t i a l energy s u r f a c e s (PES) c a l c u l a t e d by Eyring and h i s co-workers i n t h e 1930's. I n t h e asymptotic e n t r a n c e o r e x i t channels, t h e canyon i n c r o s s s e c t i o n i s j u s t t h e p o t e n t i a l curve f o r a diatomic molecule. As t h e r e a g e n t s approach one a n o t h e r , c o n s i d e r a b l e i n t e r a c t i o n d i s t o r t s t h e simple diatomic molecules and t h e p o t e n t i a l energy may r i s e t o a c o l b e f o r e descending i n t o t h e product canyon. The w e l l shown i n Fig. 1 o r i g i n a l l y appeared a s an a r t i f a c t of t h e c a l c u l a t i o n f o r 8 + 8 , d 82 + 8. and w h i l e i t i s known t h a t no w e l l e x i s t s f o r B + 8 % . r g a c t i o n s such a s 8 + 08

+

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985130

C1-304 JOURNAL DE PHYSIQUE

energy, shown a s a dashed carve i n Fig. 1, i s presumed t o d e s c r i b e t h e g r o s s behavior of t h e system and t h e p o t e n t i a l energy p l o t t e d v e r s u s d i s t a n c e along t h i s

" r e a c t i o n c o o r d i n a t e " i s shown s c h e m a t i c a l l y i n Fig. 2 . P l o t s of t h i s type a r e used i n q u a l i t a t i v e d i s c u s s i o n of chemical r e a c t i o n s , b u t one must b e a r i n mind t h a t p l o t s such as Fig. 2 r e p r e s e n t a s p e c i a l two-dimensional c u t through a s u r f a c e of a t l e a s t f o u r dimensions.

Fig. 1 Fig. 2

Fig. 1. P o t e n t i a l energy contour p l o t (schematic) f o r c o l l i n e a r r e a c t i o n of A w i t h BC. Dashed l i n e corresponds t o minimum energy path.

Fig. 2. P o t e n t i a l energy

E.

The time e v o l u t i o n of a chemical system a c r o s s t h e PES i s a problem of fundamental i n t e r e s t t o chemistry. Experimentally, of c o a r s e , r e a c t i o n s do t a k e p l a c e , and a g r e a t d e a l has been l e a r n e d about t h e s e p r o c e s s e s by measuring t h e r a t e of e v o l u t i o n from r e a g e n t s t o products. Molecular beam and l a s e r techniques has made p o s s i b l e s t a t e - t e s t a t e chemistry, t h e s t u d y of how i n d i v i d u a l quantum s t a t e s of t h e r e a g e n t s a f f e c t r e a c t i v i t y and of how t h e p r o d u c t s a r e d i s t r i b u t e d among d i f f e r e n t p o s s i b l e f i n a l s t a t e s . But these techniques examine the asymptotic s t a t e s of the system.

The r e g i o n of t h e PES of g r e a t e s t i n t e r e s t n e a r the c o l i s only i n d i r e c t l y sampled.

T h e o r e t i c a l treatment of t h e time e v o l u t i o n i s v e r y d i f f i c u l t because t h e dynamics ( e i t h e r c l a s s i c a l o r quanta11 of t h e system a r e v e r y s e n s i t i v e t o t h e p r e c i s e d e t a i l s of t h e s u r f a c e , e s p e c i a l l y n e a r t h e c o r n e r .

I n o r d e r t o provide d i r e c t i n f o r m a t i o n about c r n c i a l r e g i o n s of t h e PES, we have t r i e d t o s p e c t r o s c o p i c a l l y i n t e r r o g a t e chemically r e a c t i v e systems i n t h e few picoseconds d u r i n g t h e course of t h e r e a c t i o n where t h e system i s between r e a g e n t s

and products. I n t h i s r e g i o n one h a s n e i t h e r r e a g e n t s nor p r o d u c t s but r a t h e r a system i n which old bonds a r e being broken and new bonds a r e being formed. We use t h e term " r e a c t i o n complex" o r " t r a n s i t i o n s t a t e " t o denote these molecular c o n f i g u r a t i o n s . As suggested i n P i g . 2 , t h i s r e g i o n i s expected t o encompass many n u c l e a r c o n f i g u r a t i o n s , and i s expected t o be r a t h e r s h o r t l i v e d .

11. E x ~ e r i m e n t a l C o n s i d e r a t i o n s

There a r e b a s i c a l l y t h r e e problems a s s o c i a t e d w i t h studying t h e s e r e a c t i o n complexes: f o r m a t i q g of t h e r e a c t i o n complex; d e t e c t i o n of t h e r e a c t i o n complex of i t s l i g h t a b s o r p t i o n ; and i n t e r ~ r e t u of t h e r e s u l t s . We s h a l l d i s c u s s t h e f i r s t two problems h e r e and t h e l a s t i n t h e d i s c u s s i o n s e c t i o n .

Fotmation of t h e r e a c t i o n complex i s expected on a t l e a s t each r e a c t i v e e v e n t , but t h e c o n c e n t r a t i o n of complexes which can be o b t a i n e d i s expected t o be v e r y low a s experimental e s t i m a t e s f o r t h e " l i f e t i m e s " of such r e a c t i o n complexes a r e t y p i c a l l y

<

Thus r e a c t i o n s which occur on every c o l l i s i o n a r e most d e s i r a b l e i n terms of produc- ing a high c o n c e n t r a t i o n of r e a c t i o n complexes. This p r e c l u d e s c a r r y i n g o u t t h e r e a c t i o n i n some s o r t of c e l l as, even i f t h e formidable problem of mixing t h e r e a g e n t s were solved, t h e r e a c t i o n would be complete on a v e r y s h o r t time s c a l e . To s o l v e t h e problem of mixing t h e r e a g e n t s , t h e method of crossed molecular beams was adopted. This a l s o has t h e advantage of e l i m i n a t i n g some p o s s i b l e a r t i f a c t s , such a s w a l l r e a c t i o n s .

The number of r e a c t i o n complexes e x c i t e d i s expected t o be low. /I/ D e t e c t i n g exci- t a t i o n of t h e r e a c t i o n complex i s t h u s q u i t e a n o n - t r i v i a l assignment even w i t h t h e h i g h photon f l u x e s o b t a i n a b l e w i t h Z r s e r s , and even i f t h e t r a n s i t i o n p r o b a b i l i t y f o r e x c i t i n g t h e complex i s high. As t h e b e s t chance f o r accomplishing t h i s . we t h e r e f o r e decided t o s e a r c h f o r a chemical r e a c t i o n i n which a luminescing product could be formed i f t h e r e a c t i o n complex absorbed l i g h t ,

where A _and B a r e g e n e r a l i z e d r e a g e n t s (atoms o r molecules); [ABJ' i s t h e r e a c t i o n complex, o r t r a n s i t i o n s t a t e ; C and D a r e g e n e r a l i z e d products; and t h e a s t e r i s k d e n o t e s e l e c t r o n i c e x c i t a t i o n . g x c i t a t i o n of t h e r e a c t i o n complex would t h u s be d e t e c t e d by observing emission of C

.

I n o r d e r t o c a r r y out t h e experiment most e f f i c i e n t l y one wishes t o maximize t h e number of C* s p e c i e s formed i n t h e time a v a i l a b l e f o r observation. The l i f e t i m e of t h e r e a c t i o n complexes i s so s h o r t t h a t power s a t u r a t i o n i s not expected a t moderate photon i n t e n s i t i e s , and t h e r a t e of r e a c t i o n ( 2 ) i s expected t o be l i n e a r i n l a s e r i n t e n s i t y . Therefore, t h e h i g h e s t o b t a i n a b l e average l a s e r power should be used. It i s a l s o important n o t t o use l a s e r s with high peak powers i n o r d e r t o avoid, o r a t l e a s t minimize, multiphoton processes. C l e a r l y , cw o p e r a t i o n provides

C1-306 JOURNAL DE PHYSIQUE

t h e lowest peak power i n combination w i t h t h e h i g h e s t average power. A t p r e s e n t t h e h i g h e s t t u n a b l e average power i s o b t a i n e d i n t r a c a v i t y i n a cw dye l a s e r .

The system s e l e c t e d f o r t h e experiments d e s c r i b e d h e r e i s K + NaCl + lrP

+

.

An energy l e v e l diagram i s shown i n Fig. 3 f o r t h i s system.

EMISSION 5 8 9 nrn

Fig. 3

Fig. 3. Energy l e v e l diagram f o r K r e a c t i n g w i t h NaC1. I f a r e a c t i o n complex were t o absorb l i g h t with k ( $57 urn t h e e x c i t e d ~ o m p l e x would have enough energy t o decompose t o KC1 and Na

.

This r e a c t i o n meets t h e experimental c r i t e r i a j u s t s e t f o r t h :

( a ) The a c t i v a t i o n energy i s low and r e a c t i o n o c c u r s on each hard sphere c o l l i - sion. I n a d d i t i o n a complex i s formed which seems t o p e r s i s t f o r a few ps.

( b ) The r e a c t i o n i s e x o e r g i c so b l u e - s h i f t e d chemiluminescence can be observed f o r wavelengths n e a r t h r e s h o l d .

( c ) No single-photon a b s o r p t i o n s a r e known f o r t h e r e a g e n t s o r p r o d u c t s i n t h e r e g i o n n e a r t h e t h r e s h o l d .

( d ) Beams of a l l r e a g e n t s

--

--

generated.

( e ) The t h r e s h o l d photon energy f o r t h e new channel occurs i n wavelength re- g i o n s e a s i l y a c c e s s i b l e t o cw dye l a s e r o p e r a t i o n .

The a p p a r a t u s i s more completely d e s c r i b e d elsewhere 131 and o n l y a b r i e f summary w i l l be given h e r e : c o l l i m a t e d molecular beams c r o s s i n s i d e t h e c a v i t y of a cr dye l a s e r ( s e e below). Fluorescence from t h e r e a c t i o n zone (RZ) i s collimated. passed through a n i n t e r f e r e n c e f i l t e r t o r e j e c t wavelengths not r e s o n a n t with t h e

f l u o r e s c i n g s p e c i e s (and t h e r e b y e x c l u d i n g c o p i o u s amounts o f s c a t t e r e d l a s e r l i g h t ) and d e t e c t e d w i t h a c o o l e d p h o t o m u l t i p l i e r o p e r a t e d i n t h e p u l s e - c o u n t i n g mode. The c a v i t y of a S p e c t r a P h y s i c s Model 375 d y e l a s e r i s e x t e n d e d t o i n c l u d e t h e m o l e c u l a r beam a p p a r a t u s by r e p l a c i n g t h e o u t p u t c o u p l e r w i t h a m i r r o r 1 . 5 m downstream from t h e l a s e r . Cooled s o l u t i o n s of Rhodamine 66 and DCM i n soap s o l u t i o n a r e pumped w i t h t h e 514.7 nm l i n e of a CR-18 ~ r + l a s e r . The dye l a s e r is not f o c u s s e d a t t h e RZ, and h a s a d i a m e t e r of about 2 mm t h e r e .

The m o l e c u l a r beams c o n t a i n t r a c e amounts o f Na which a r e e x c i t e d by t h e (weak) s p o n t a n e o u s f l u o r e s c e n c e of t h e dye s t r e a m a t t h e D l i n e , even i f t h e l a s e r i s l a s - i n g a t f r e q u e n c i e s f a r from t h e Na D l i n e . T h e r e f o r e , f l u o r e s c e n c e from t h e dye s t r e a m a t t h e Na D l i n e s was removed by a s m a l l i n t r a c a v i t y h e a t p i p e oven c o n t a i n - i n g Na and a few t o r r o f Argon. The l a s e r i s tuned by a 3 - p l a t e b i r e f r i n g e n t f i l t e r w i t h a bandwidth

-

.

The f l u o r e s c e u c e from t h e RZ was p a s s e d t h r o u g h an i n t e r f e r e n c e f i l t e r c e n t e r e d a t 589.0 nm w i t h a band p a s s o f 0.47 nm which p a s s e d o n l y t h e 589.0 nm D l i n e . Some l i g h t i s e m i t t e d from t h e RZ ( s c a t t e r e d l i g h t , photoluminescence, e t c . ) and may s t i l l p a s s t h r o u g h t h e f i l t e r f o r v a r i o u s beam combinations. I n o r d e r t o a c c o u n t f o r t h e s e v a r i o u s c o n t r i b u t i o n s , t h e K and l i g h t beams were chopped a t 20 and 90 Hz r e s p e c t i v e l y , and t h e NaCl beam was i n t e r r u p t e d by a beam f l a g . A LSI-11 mini-computer w i t h Camac i n t e r f a c e was used t o a c q u i r e d a t a i n a l l 8 beam on-off c o n f i g u r a t i o n s . The s i g n a l o f i n t e r e s t , t h e three-beam s i g n a l (3BS). i s t h a t s i g n a l which a p p e a r s o n l y when a l l t h r e e beams a r e on.

IV. R e s u l t s

T y p i c a l r e s u l t s f o r a c o u n t i n g p e r i o d of about 5 minutes a r e shown i n Table I.

Count r a t e s a r e shown f o r v a r i o u s e l e m e n t a r y s i g n a l p r o c e s s e s R. j k where i, j , k a r e 0 o r 1 t o i n d i c a t e K, NaCl o r l a s e r , o f f o r on, r e s p e c t i v e i y . These e l e m e n t a r y r a t e s a r e a s s m e d t o be a d d i t i v e s o t h a t t h e s i g n a l observed w i t h a l l beams on, Slll i s g i v e n by

T a b l e I

Count r a t e s ( 5 1 o) f o r o b s e m a t i o n of NaD r a d i a t i o n w i t h v a r i o u s c o m b i n a t i o n s o f K, NaCl and l i g h t beams f o r i r r a d i a t i o n a t X = 630 ^nm.

- - - - --- --- - - -

S i g n a l O r i g i n Count R a t e (s-')

Roo o Dark C u r r e n t 4.2 f 1

Roo 1 S c a t t e r e d L i g h t 23.3 2 2

Rlo o K Background 0.0

-+

Ro zo NaCl Background 24.2 f 2

Rlo 1 K Photoluminescence 146.4 2 5

Ro 11 NaCl Photoluminescence 29.4 ? 4

Rlzo Chemiluminescence 0.0 2 3

R111 3 Beam S i g n a l (3BS) 559.3 2 1 3

The 3BS, R ~ z z , i s p o s i t i v e and h i g h l y s i g n i f i c a n t s t a t i s t i c a l l y . It should be emphasized t h a t t h i s s i g n a l i s p r e s e n t & when a l l t h r e e beams a r e on. I f any one beam i s missing. R ~ l l i s z e r o . T h i s s i g n a l i s r e l a t i v e l y r e p r o d u c i b l e / 4 / and i s p r e s e n t o v e r a v e r y wide range of e x c i t a t i o n wavelengths. Fig. 4 shows t h e 3-beam

CI-308 JOURNAL DE PHYSIQUE

s i g n a l ( c o r r e c t e d f o r d r i f t s and normalized f o r comparison between d i f f e r e n t days) v e r s u s l a s e r wavelength. The s c a t t e r i n p o i n t s does n o t seem t o r e f l e c t s p e c t r a l f e a t u r e s , but r a t h e r t o be due t o f l u c t u a t i o n s i n t h e i n t e n s i t i e s of one o r more of t h e beams. As suggested i n Fig. 4 , a t h r e s h o l d f o r R,,, i s observed n e a r 720 nm, and p r e l i m i n a r y r e s u l t s f o r t h e t h r e s h o l d a r e shown i n Fig. 5.

9

-

+

-, +

k v,

*

z ^{' t}

W

g

+

'?.

+, ^.

I I I I

600 620 640 660 680 6 8 0 7 0 0 7 2 0 7 4 0

W A V E L E N G T H (nrn) E X C I T A T I O N W A V E L E N G T H ( n m )

Fig. 4 Fig. 5

Fig. 4. Three beam s i g n a l , R,,,, v e r s u s e x c i t a t i o n wavelength. Various symbols r e p r e s e n t d i f f e r e n t r u n s which a r e normalized t o one another. E r r o r b a r s a r e 5 1 a and a r e shown f o r v a r i o u s runs. The shaded a r e a r e p r e s e n t s t h e average of d i f f e r e n t r u n s

+

Fig. 5. Three beam s i g n a l , R,,,, v e r s u s e x c i t a t i o n wavelength i n t h e r e g i o n n e a r t h e t h r e s h o l d . Dashed l i n e i s f o r i l l u s t r a t i o n .

The dependence of R,,, on l a s e r power i s shown i n Fig. 6. The power was monitored by d e t e c t i n g t h e v e r t i c a l l y p o l a r i z e d l i g h t t r a n s m i t t e d by t h e "l00k r e f l e c t i n g " end l a s e r m i r r o r ; t h i s p o l a r i z a t i o n was

not

'X photoluminescence." R,,,, i s presumed t o a r i s e from K dimers, which a r e well-known i n a l k a l i metal beams, and t h i s photolnminescence i s q u i t e l a r g e i f viewed through a r e d - p a s s f i l t e r . Excited Ka a r e t h u s c l e a r l y formed by t h e l a s e r , and s e q u e n t i a l p r o c e s s e s i n v o l v i n g K,, such a s

K,* + NaCl 4 ~ a * + KaCl ( 5 )

must be considered (although i t should be noted t h a t ( 6 ) i s n o t e n e r g e t i c a l l y open u n l e s s two photons a r e absorbed which i s hard t o c o r r e l a t e w i t h t h e observed l i n e a r dependence of 3BS on power. Fig. 6. Because of l a c k of information r e g a r d i n g t h e

0.2 0 4 0.6 0.8 1.0 R E L A T I V E P O W E R

Fig. 6. Three beam s i g n a l , ^Rl,,, v e r s u s l a s e r power. The l i n e i s drawn through t h e d a t a .

bond e n e r g i e s of KaCl, i t i s impossible t o know i f (5) i s e n e r g e t i c a l l y open). Sig- n a l s were compared f o r l a s e r e x c i t a t i o n on a Ka bandhead and e x c i t a t i o n a t an a d j a c e n t minimum. Even though t h e photoluminescence v a r i e d by a f a c t o r of 1 5 , R,,, v a r i e d by l e s s than 10%. We conclude t h a t r e a c t i o n ( 5 ) cannot account f o r t h e s i g - n a l we r e p o r t .

The 3 beam s i g n a l i s from t h e f l u o r e s c e n c e of Na atoms, presumably those formed i n a chemical r e a c t i o n . But both beams c o n t a i n t r a c e amounts of Na i m p u r i t i e s and exci- t a t i o n of t h e impurity Na v i a s e q u e n t i a l energy t r a n s f e r from an a r t i f a c t A

would appear a s a 3 beam s i g n a l . Species A cannot be e i t h e r K o r NaCl s i n c e no sin- g l e photon a b s o r p t i o n s a r e known i n t h i s wavelength region, nor can it be t h e K dimer s i n c e t h e 3BS i s independent of d i r e c t e x c i t a t i o n of K,. I n o r d e r t o r u l e o u t o t h e r , unknown. a r t i f a c t s p e c i e s , we performed a u x i l i a r y experiments on Na + NaCl and K + Na i n which t h e Na beam was a d j u s t e d t o be comparable t o Na i n t e n s i t i e s i n t h e - m a i n experiments. I n both c a s e s t h e chemical r e a c t i o n K + NaCl i s removed, b a t t h e p o s s i b i l i t y of energy t r a n s f e r from some impurity i s s t i l l p r e s e n t . No 3BS i s observed under t h e s e o t h e r c o n d i t i o n s , and we conclude t h a t such an a r t i f a c t source can be r u l e d o u t .

Fluorescence i s observed from t h e r e g i o n of t h e o v e r l a p of molecular beams of K and NaCl which c r o s s i n s i d e t h e c a v i t y of a cw dye l a s e r . This f l u o r e s c e n c e i s h i g h l y s t a t i s t i c a l l y s i g n i f i c a n t and a r i s e s from t h e emission of an e x c i t e d sodium atom.

~ a * . Because of t h e low d e n s i t i e s i n t h e molecular beams, t h e p r o c e s s r e s p o n s i b l e f o r these s i g n a l s most l i k e l y involves b i n a r y molecular c o l l i s i o n s between s p e c i e s from each molecular beam. Wall e f f e c t s and secondary c o l l i s i o n s may be r u l e d o u t , and we i n t e r p r e t t h e p r o c e s s r e s p o n s i b l e f o r t h e s e s i g n a l s a s t h e l a s e r a s s i s t e d r e a c t i o n

K + NaC1 + U

-+

CI-310 JOURNAL DE PHYSIQUE

Our i n t e r p r e t a t i o n i s based on t h e following experimental o b s e r v a t i o n s :

( a ) The s i g n a l occurs o n l y when a l l t h r e e r e a g e n t s a r e p r e s e n t . A l l one- and two-beam c o n t r i b u t i o n s t o t h e s i g n a l (e.g., d a r k c u r r e n t , s c a t t e r e d l i g h t o r photoluminescence, e t c . ) have been s u b t r a c t e d from t h e raw s i g n a l . ( b ) The s i g n a l i s l i n e a r i n l a s e r i n t e n s i t y which s u g g e s t s t h a t only one photon

i s r e s p o n s i b l e f o r t h i s process.

( c ) No s i n g l e photon a b s o r p t i o n s a r e known i n t h i s wavelength range f o r e i t h e r t h e r e a g e n t s (K, NaCl) o r t h e p r o d u c t s (Na, KC1).

( d l The s i g n a l i s due t o f l u o r e s c e n c e a t t h e Na D l i n e and i s assumed t o be due t o Na

.

( e ) The s i g n a l (emission of ~ a * ) i s observed o v e r a wide range of e x c i t i n g wavelengths. This emission i s blae s h i f t e d from t h e e x c i t i n g l i g h t .

( f ) The s i g n a l i s l i n e a r i n NaCl i n t e n s i t y and appears t o be l i n e a r i n K i n t e n - s i t y .

Observation of r e a c t i o n (3) i s c o n s i s t e n t with a l l of t h e s e o b s e r v a t i o n s . We have not been a b l e t o t h i n k of any o t h e r p r o c e s s which i s c o n s i s t e n t with a l l of t h e experimental f a c t s , and we conclude t h a t we have observed t h e l a s e r - a s s i s t e d reac- t i o n

K + NaCl + Id

-+

The r e a c t i o n e x o e r g i c i t y can provide t h e energy r e q u i r e d f o r t h e b l u e - s h i f t e d emis- s i o n , and energy balance r e q u i r e s

where ei and et denote i n t e r n a l energy and t r a n s l a t i o n a l energy r e s p e c t i v e l y , primes denote r e a c t i o n p r o d u c t s , *L i s t h e frequency of t h e l a s e r , 690' is t h e d i f f e r e n c e i n d i s s o c i a t i o n e n e r g i e s of p r o d u c t s and r e a g e n t s , (1750 2 1000 cm-') and eb i s t h e energy of t h e Na D l i n e (16,970 cm-'1. The t h r e s h o t d l a s e r energy w i l l occur when (8: + e i )

-

-

.

f l f f h

It = 840 -70 +80 nm. The experimental t h r e s h o l d a t 740 nm i s c o n s i s t e n t w i t h t h i s v a l u e because i t i s u n l i k e l y t h a t a l l of t h e thermal energy can be u t i l i z e d i n t h e blue s h i f t . ( R o t a t i o n a l energy of t h e diatomic i s l i k e l y t o be roughly c o n s t a n t t o conserve t o t a l a n g u l a r momentum. f o r example. )

I n t h e absence of l a s e r l i g h t , crossed molecular beam experiments 151 have been shown t h a t t h e ground s t a t e r e a c t i o n

K + NaCl

-+

proceeds on n e a r l y every hard-spherg c o l l i s i o n . The a n g u l a r d i s t r i b u t i o n s of t h e p r o d u c t s shows t h a t a complex, [KNaCll

.

-

We t h u s view t h i s p r o c e s s a s a " s t i c k y c o l l i s i o n " , f o l l o w e d by a c o n v e n t i o n a l l i g h t a b s o r p t i o n p r o c e s s . ( A t t h e power l e v e l s used,

-

The spectrum r e f l e c t s t h e energy d i f f e r e n c e between t h e upper and lower PES f o r t h e system. S e v e r a l g e n e r a l i t i e s I 8 1 c a n be expressed. I f t h e system spends more time i n a small r e g i o n of t h e PES, t h e spectrum would be e x p e c t e d t o d i s p l a y a maximum a t w a v e l e n g t h s c o r r e s p o n d i n g t o t h e s e p a r a t i o n between t h e s u r f a c e s f o r t h e confinement r e g i o n . Likewise, i f t h e s u r f a c e s a r e ' b a r a l l e l " o v e r a range of c o n f i g u r a t i o n s , a d i f f e r e n t type o f maximum would be o b t a i n e d a s a r e s u l t o f many c o n f i g u r a t i o n s b e i n g c a p a b l e of a b s o r b i n g p h o t o n s o f e s s e n t i a l l y t h e same wavelength.

Although t h e signal-to-noise s o f a r i s i n s u f f i c i e n t t o be c e r t a i n , t h e e x p e r i m e n t a l spectrum d o e s n o t seem t o e x h i b i t any f e a t u r e s . I t i s p o s s i b l e t h a t t h e v e r y h i g h e f f e c t i v e " t e m p e r a t u r e s " of t h e complexes which would a v e r a g e out any s u c h f e a t u r e s . It i s a l s o p o s s i b l e t h a t t h e PES a r e of t h e wrong s e l e c t i v e s l o p e s f o r t h e f o r m a t i o n o f s t r u c t u r e . F i g . 7 shows a s e c t i o n of t h e f i r s t t h r e e c o l l i n e a r PES o b t a i n e d from p s e u d o p o t e n t i a l c a l c u l a t i o n s by Roach and Child. 161 F o r t h e 180° approach ( c o l - l i n e a r ) t h e r e i s no w e l l i n t h e ground s u r f a c e , b u t f o r b e n t g e o m e t r i e s a w e l l a p p e a r s ; changes a r e a l s o e x p e c t e d f o r t h e e x c i t e d s t a t e s i n b e n t g e o m e t r i e s . The ground and e x c i t e d s t a t e c u r v e s a r e r a t h e r d i s s i m i l a r and i t seems l i k e l y t h a t a n o p t i c a l t r a n s i t i o n would r e s u l t i n a f a i r l y wide a b s o r p t i o n band because t h e Franck-Condon t r a n s i t i o n p r o b a b i l i t i e s a r e e x p e c t e d t o be r e l a t i v e l y c o n s t a n t (and r e l a t i v e l y s m a l l ) o v e r a wide range of wavelength.

These e x p e r i m e n t s d e m o n s t r a t e t h a t d i r e c t s p e c t r o s c o p i c o b s e r v a t i o n of a system i n t h e p r o c e s s o f chemical r e a c t i o n i s p o s s i b l e . What have we l e a r n e d and what d i r e c - t i o n should f u t u r e work t a k e ? F i r s t , i n t e r m s o f t h e p a r t i c u l a r r e a c t i o n s t u d i e d h e r e , t h e e n t i r e p i c t u r e i s c o n s i s t e n t q u a l i t a t i v e l y w i t h t h e n a t u r e of ground and l o w e s t e x c i t e d p o t e n t i a l c u r v e s o f Roach and C h i l d which have v e r y d i f f e r e n t shapes and would t h e r e f o r e be e x p e c t e d t o g i v e r i s e t o a b r o a d spectrum. Also because t h e e x c i t e d s t a t e and ground s t a t e c u r v e s a r e r e l a t e d by a n avoided c r o s s i n g between two d i f f e r e n t k i n d s o f i o n i c c u r v e s , i t i s r e a s o n a b l e t o e x p e c t a v e r y l a r g e e l e c t r o n i c d i p o l e t r a n s i t i o n moment. T h i s i s a t l e a s t r e a s s u r i n g . E x p e r i m e n t a l l y , i t i s l o g i - c a l t o i n v e s t i g a t e some s i m i l a r systems such a s Rb + NaCl i n o r d e r t o l o o k f o r s y s t e m a t i c changes. The p u r s u i t of s t r u c t u r e i n t h e spectrum c o u l d be advanced by u s i n g r e a c t a n t beams c o o l e d by s u p e r s o n i c e x p a n s i o n i n o r d e r t o remove thermal e n e r g y from t h e r e a g e n t s . A i n t e r e s t i n g p o s s i b i l i t y i s introduced by t u n i n g t h e l a s e r t o t h e r e d o f t h e K D l i n e s where t h e e x c i t e d s t a t e of t h e complex i s bound.

t h e n l o o k i n g f o r b l u e - s h i f t e d bound-bound f l u o r e s c e n c e i n t o t h e lower s t a t e poten- t i a l w e l l .

JOURNAL DE _PHYSIQUE

- 4 -2 0 2 4

R E A C T I O N P A T H

( % I

Fig. 7. P o t e n t i a l energy

s.

we g r a t e f u l l y acknowled@ support of t h i s r e s e a r c h by t h e National Science Founda- t i o n under g r a n t s CBE 8007651 and CBE 8317764. and by t h e Robert A. Welch Founda- t i o n .

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BROOKS P. R., CURL R. F. and MAGUIRE T. C.,

Ber.

(1982) 401.

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A

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5. MILLER W. B., SAFRON S. A. and BERSCBBACE D. R., Discuss. F a r a d a ~

&. ^44.

(1967) 108; MILLER W. B. and SAFRON S. A.. W.D. T h e s i s , Barvard U n i v e r s i t y , (1969).

6. ROACH A. C. and CHILD M. J., Mol. Phvs. 14, (1968) 1.

7. KWEI G. 8.. BOFFARDI B. P. and SUN S. F.. 1. Chem. PhJrs.

58,

8. POLANYI J. C. and WOLF R. J., Chem, WJrs.

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