HIGH RESOLUTION SPECTROSCOPIC STUDIES OF SMALL MOLECULES

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HIGH RESOLUTION SPECTROSCOPIC STUDIES OF SMALL MOLECULES

R. Field

To cite this version:

R. Field. HIGH RESOLUTION SPECTROSCOPIC STUDIES OF SMALL MOLECULES. Journal

de Physique Colloques, 1987, 48 (C7), pp.C7-17-C7-28. �10.1051/jphyscol:1987703�. �jpa-00226908�

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JOURNAL DE PHYSIQUE

Colloque C7, supplbment au n012, Tome 48, dbcembre 1987

HIGH RESOLUTION SPECTROSCOPIC STUDIES OF SMALL MOLECULES

R.W. FIELD

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, M A 02139, U.S.A.

A b s t r a c t

The s p e c t r a o f small molecules i n t h e gas phase a r e o f t e n e x t r e m e l y complicated. Much o f t h i s c o m p l e x i t y i s g r a t u i t o u s , and can be made t o d i s a p p e a r when a s u i t a b l e d o u b l e resonance scheme i s employed. Sometimes i t i s necessary t o d e v i s e devious schemes f o r c i rcumventing o b s t a c l e s ( s e l e c t i o n r u l e s , Franck-Condon f a c t o r s , p o t e n t i a l energy b a r r i e r s ) t o i m p o r t a n t t y p e s of m o l e c u l a r s t r u c t u r a l i n f o r m a t i o n

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d i s s o c i a t i o n e n e r g i e s ) o r c l a s s e s of m o l e c u l a r energy l e v e l s (9. t r i p l e t s , l o c a l i z e d v i b r a t i o n a l e x c i t a t i o n s s u i t e d f o r bond s p e c i f i c photochemistry). The t r a d i t i o n a l s p e c t r o s c o p i c concepts and we1 1 - t r o d d e n p a t h f r o m spectrum t o p o t e n t i a l energy s u r f a c e m o l e c u l a r c o n s t a n t s i s ill s u i t e d f o r c h a r a c t e r i z i n g t h e l a r g e a m p l i t u d e d i s p l a c e m e n t s o f atoms from t h e i r e q u i l i b r i u m p o s i t i o n s which o c c u r i n s i m p l e unimol e c u l a r dynamical processes. C r o s s - c o r r e l a t i o n o f double resonance s p e c t r a may p r o v i d e a d i r e c t view o f s p e c i f i a b l e i n t r a m o l e c u l a r processes.

F o u r s p e c t r o s c o p i c examples w i l l b e discussed: ( i ) S t i m u l a t e d Emission Pumping ( s ~ P ) spectroscopy o f formaldehyde, an example o f a we1 1 -behaved m o l e c u l e i n t h e small a m p l i t u d e l i m i t , f o r which a complete s e t o f harmonic v i b r a t i o n a l f r e q u e n c i e s (wi ) and a n h a r m o n i c i t i e s (xi j) i s determined; ( i i ) P e r t u r b a t i o n F a c i 1 i t a t e d O p t i c a l - O p t i c a l Double Resonance (PFOOOR)

s p e c t r o s c o p y of L i z , i l l u s t r a t i n g access t o t r i p l e t s t a t e s and t h e phenomenon o f " a c c i d e n t a l p r e d i s s o c i a t i o n ; ( i i i ) d e t e r m i n a t i o n o f an upper bound t o t h e HCC-H d i s s o c i a t i o n energy o f a c e t y l e n e by Zeeman A n t i - C r o s s i n g (ZAC)

spectroscopy, a c o o p e r a t i v e p r e d i s s o c i a t i o n scheme s i m i l a r t o t h e a c c i d e n t a l p r e d i s s o c i a t i o n i n L i p whereby, d e s p i t e p o t e n t i a l energy b a r r i e r s , one can e n s u r e t h a t t h e onset o f d i s s o c i a t i o n w i l l be d e t e c t e d n o t f a r above t h e thermochemical l i m i t ; ( i v ) d e t e c t i o n and c h a r a c t e r i z a t i o n o f a v i n y l i d e n e v i b r a t i o n a l l e v e l t h r o u g h t h e e f f e c t o f a c e t y l e n e - v i n y l i d e n e i s o m e r i z a t i o n on t h e c r o s s - c o r r e l a t i o n o f a c e t y l e n e SEP s p e c t r a , i l l u s t r a t i n g a new, d i r e c t way o f sampl i ng i n t r a m o l e c u l a r dynamics a t such h i g h i n t e r n a l e n e r g i e s t h a t v i b r a t i o n a l s p e c t r a a r e i n t r i n s i c a l l y unassignable.

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

High r e s o l u t i o n s p e c t r a o f s m a l l , gas phase molecules can c o n t a i n an enormous q u a n t i t y o f i n f o r m a t i o n . F o r example, a s i n g l e v ' - v " v i b r a t i o n a l band o f a 7 ~ - 7 c + system c o n s i s t s o f 147 r o t a t i o n a l branches, each comprised o f t e n s o f r o t a t i o n a l l i n e s . F o r MnH where t h e n u c l e a r s p i n s a r e IMn = 512 and

IH = 112, each r o t a t i o n a l l i n e s p l i t s i n t o (21Mn+1)(?IH+1) = 12 s t r o n g

h y p e r f i n e components. Thus a s i n g l e v i b r a t i o n a l band m i g h t c o n s i s t o f as many as 104 f e a t u r e s , each measured t o -1 p a r t i n 108. Yet t h e i n f o r m a t i o n c o n t e n t o f such a band, when reduced t o t h e -20 m o l e c u l a r c o n s t a n t s capable o f

r e p r o d u c i n g a l l measurable f r e q u e n c i e s and i n t e n s i t i e s , i s q u i t e modest. T h i s i s g r a t u i t o u s c o m p l e x i t y i n i t s s i m p l e s t and l e a s t i n s i d i o u s form.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987703

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C7-18 JOURNAL DE PHYSIQUE

I t can be v e r y easy t o r e c o r d an u n s p e c i f i e d spectrum, b u t o f t e n much more d i f f i c u l t t o r e c o r d

the

spectrum t h a t r e v e a l s t h e s o u g h t - f o r b i t o f i n f o r m a t i o n about a s p e c i f i c molecule. Some s p e c t r a a r e r e l a t i v e l y easy t o a s s i g n and f i t t o m o l e c u l a r constants. However, several innocent appearing, c h e m i c a l l y s i g n i f i c a n t , and f r e q u e n t l y asked q u e s t i o n s such as "what i s t h e A b A + B d i s s o c i a t i o n energy" o r "where i s t h e l o w e s t 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 " can be e x c e e d i n g l y d i f f i c u l t t o answer because o f s p e c t r o s c o p i c

o b s t a c l e s . Franck-Condon and e l e c t r o n i c s e l e c t i o n r u l e r e s t r i c t i o n s a r e o f t e n f o r m i d a b l e o b s t a c l e s t o t h e e x t r a c t i o n o f c r u c i a1 i n f o r m a t i o n from t h e s p e c t r a of even t h e s i m p l e s t o f molecules.

Polyatomic molecules a r e n o t r i g i d bodies i n which t h e c o n s t i t u e n t atoms undergo o n l y i n f i n i t e s i m a l displacements from t h e i r e q u i l i b r i u m p o s i t i o n s . Textbooks t e l l us about v i b r a t i o n a l normal modes and l e a d us t o b e l i e v e t h a t each v i b r a t i o n a l l e v e l may be assigned a s e t o f normal mode v i b r a t i o n a l quantum numbers, t h a t t h e v i b r a t i o n a l energy l e v e l s may be represented by a s e t o f fundamental v i b r a t i o n a l f r e q u e n c i e s (wi) and anharmonici t i e s (xi j), and t h a t t h e s e v i b r a t i o n a l constants determine t h e p o t e n t i a l energy surface, V(1).

U n f o r t u n a t e l y , t h e (wi,xij) determine o n l y a r e s t r i c t e d r e g i o n o f V(2) near e q u i l i b r i u m

(4,);

moreover, f o r four-atom and l a r g e r molecules, f a r more f o r c e c o n s t a n t s t h a n may be i n f e r r e d from s p e c t r a l d a t a a r e r e q u i r e d t o o b t a i n even t h e most p r i m i t i v e r e p r e s e n t a t i o n o f t h e i n t r a - and inter-mode

anharmonici t i e s .

The t r a d i t i o n a l approach t o t r a d i t i o n a l l y recorded v i b r a t i o n - r o t a t i o n s p e c t r a can p r o v i d e l i t t l e i n s i g h t i n t o l a r g e a m p l i t u d e processes. For example, a s i d e from l o w b a r r i e r ((10 kcal/mole) i n t e r n a l r o t a t i o n processes, h i g h r e s o l u t i o n spectroscopy has been a b l e t o t e l l us v e r y l i t t l e about t h e mechanism o r b a r r i e r shape f o r u n i m o l e c u l a r i s o m e r i z a t i o n processes o r t h e s t r u c t u r e o f u n s t a b l e isomers. D e s p i t e t h e t r u i s m t h a t m o l e c u l a r e i g e n s t a t e s a r e s t a t i o n a r y , p o l y a t o m i c rnolecwles are much Tore l i k e t h e dynamical o b j e c t s suggested by our experience w i t h b a l l -and-spring models t h a n t h e s t a t i c o b j e c t s suggested by quantum mechanical s t a t i o n a r y s t a t e s . The spectrum does c o n t a i n dynamical i n f o r m a t i o n , b u t we must d i s c o v e r new ways o f r e a d i n g t h i s i n f o r m a t i o n i n t h e spectrum.

T h i s paper i l l u s t r a t e s some h i g h r e s o l u t i o n s p e c t r o s c o p i c s o l u t i o n s t o t h e problems o f g r a t u i t o u s c o m p l e x i t y , s p e c t r o s c o p i c o b s t a c l e s , s y s t e m a t i c c h a r a c t e r i z a t i o n o f t h e small a m p l i t u d e r e g i o n o f V( ) , and l a r g e a m p l i t u d e i n t r a m o l e c u l a r dynamics. Double resonance schemes p

P

ay a c r u c i a l r o l e by r e d u c i n g t h e c o m p l e x i t y o f #a spectrum, by f o r c i n g a c r u c i a l b u t weak s p e c t r a l f e a t u r e t o emerge from concealment under more numerous and s t r o n g e r f e a t u r e s , by t a k i n g advantage o f a c c i d e n t s o f n a t u r e ( s p e c t r o s c o p i c p e r t u r b a t i o n s ) which a l l o w s e l e c t i o n r u l e s t o be bent, and when complemented by s t a t i s t i c a l methods f o r p r o c e s s i n g m a x i m a l l y s i m p l i f i e d s p e c t r a t o r e v e a l dynamics.

11. E f f e c t s and Techniques

It w i l l be u s e f u l t o d e f i n e o r d e s c r i b e s e v e r a l s p e c t r o s c o p i c e f f e c t s and techniques t h a t p l a y a c e n t r a l r o l e i n t h e f o u r examples t o be discussed i n t h e n e x t section.

A. S p e c t r o s c o p i c p e r t u r b a t i o n s a r e e x h a u s t i v e l y discussed i n Ref. [I].

F i g u r e s 1.1 and 1.2 of Ref. L l ] i l l u s t r a t e how a p e r t u r b a t i o n d i s r u p t s t h e r e g u l a r p a t t e r n o f r o t a t i o n a l 1 in e s i n a spectrum. P e r t u r b a t i o n s occur because c e r t a i n terms i n t h e exact m o l e c u l a r Hamiltonian, which we choose t o n e g l e c t i n o r d e r t o d e f i n e separate e l e c t r o n i c s t a t e s and t o f a c t o r t h e w a v e f u n c t i o n i n t o e l e c t r o n i c , v i b r a t i o n a l

,

and r o t a t i o n a l p a r t s , a r e n o t always n e g l i g i b l e . The usual Born-Oppenheimer way o f t h i n k i n g about m o l e c u l a r s t a t e s serves us v e r y w e l l f o r -99% o f t h e observable m o l e c u l a r e i g e n s t a t e s . However, u s u a l l y because of an a c c i d e n t a l near degeneracy, -1% o f t h e e i g e n s t a t e s a r e o f mixed e l e c t r o n i c c h a r a c t e r

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where

l i

,J> i s t h e i - t h e i g e n s t a t e w i t h t h e r i g o r o u s l y good J r o t a t i o n a l quantum number, lelvlJ> and le2v2J> a r e Born-Oppenheimer r o t a t i o n - v i b r a t i o n - e l e c t r o n i c b a s i s f u n c t i o n s a s s o c i a t e d w i t h p o t e n t i a l energy curves 1 and 2 and z e r o - o r d e r e n e r g i e s EO

,

and a i J i s a m i x i n g c o e f f i c i e n t

P e r t u r b a t i o n s can be a nuisance because t h e y can make a spectrum d i f f i c u l t t o a s s i g n and d e s c r i b e i n terms o f s t a n d a r d m o l e c u l a r constants, b u t t h e y a r e a g r e a t boon t o w r i t e r s o f spectroscopy t e x t b o o k s ( o t h e r w i s e t h e r e m i g h t be l i t t l e t o d i s c u s s a f t e r t h e f i r s t c h a p t e r ) and t o s p e c t r o s c o p i s t s i n need o f a window onto o t h e r w i s e unobservable s t a t e s and a r e s t r i c t e d 1 ic e n s e t o v i o l a t e s e l e c t i o n r u l e s .

B. P r e d i s s o c i a t i o n i s a k i n d o f p e r t u r b a t i o n where t h e p e r t u r b e d l e v e l lelvlJ> i s bound and t h e p e r t u r b i n g s t a t e le2a2J> i s unbound and a s s o c i a t e d w i t h t h e v i b r a t i o n a l continuum o f t h e e2 e l e c t r o n i c s t a t e . See c h a p t e r 6 o f Ref. [l]. The e f f e c t o f a p r e d i s s o c i a t i o n i s t o cause t h e p r e d i s s o c i a t e d 1 eve1 t o be broadened ( s h o r t e r l i f e t i m e , l o w e r f l u o r e s c e n c e quantum y i e l d ) and s h i f t e d . P r e d i s s o c i a t i o n can o n l y o c c u r a t an energy above t h e l o w e s t energy A&A+B d i s s o c i a t i o n l i m i t and t h e r e f o r e t h e o b s e r v a t i o n o f a p r e d i s s o c i a t i o n p r o v i d e s a r i g o r o u s upper bound t o t h e m o l e c u l a r d i s s o c i a t i o n energy, DO0 (AB).

P o t e n t i a l energy b a r r i e r s , e l e c t r o n i c s e l e c t i o n r u l e s , and Franck-Condon 1 im i t a t i o n s on v i b r a t i o n a l o v e r l a p t y p i c a l l y cause p r e d i s s o c i a t i o n e f f e c t s t o b e unobservably small up t o an energy w e l l above t h r e s h o l d . To o b t a i n a u s e f u l l y t i g h t upper bound on DoO f r o m p r e d i s s o c i a t i o n e f f e c t s o f t e n r e q u i r e s s p e c t r o s c o p i c t r i c k e r y .

C. A c c i d e n t a l P r e d i s s o c i a t i o n i s a t h r e e s t a t e e f f e c t whereby s t a t e 1 i s p e r t u r b e d by s t a t e 2 which, i n t u r n , i s p r e d i s s o c i a t e d by s t a t e 3. I n t h e absence o f s t a t e 2, t h e e f f e c t o f t h e s t a t e 3 continuum on s t a t e 1 i s

u n d e t e c t a b l y small. A c c i d e n t a l p r e d i s s o c i a t i o n i s an example o f a molecule o u t s m a r t i n g i t s e l f ; a 1+3 d i s s o c i a t i o n channel which i s d e v i o u s l y concealed because o f e l e c t r o n i c s e l e c t i o n r u l e s o r n e g l i g i b l e v i b r a t i o n a l o v e r l a p i s i n a d v e r t e n t l y r e v e a l e d by an a c c i d e n t a l 1-2 degeneracy. Sometimes, when n a t u r e does n o t p r o v i d e a s u i t a b l e p r o m o t i n g l e v e l - 2 , a d d i t i o n o f a s u i t a b l e DC o r o s c i l l a t i n g e l e c t r i c o r magnetic f i e l d can t u n e a nearby l e v e l i n t o resonance w i t h l e v e l - 1 t h e r e b y v a s t l y enhancing t h e o b s e r v a b i l i t y o f t h e 1+3 p r e d i s s o c i a t i on.

D. An A n t i c r o s s i n g occurs when two z e r o - o r d e r b a s i s s t a t e s (Elo,

lel,v1,J> and E20, le2,v2,J>) b e l o n g i n g t o t h e same v a l u e s o f r i g o r o u s quantum numbers

(x.

J, p a r i t y , r o v i b r o n i c i r r e d u c i b l e r e p r e s e n t a t i o n l a b e l ) a r e t u n e d by a p p l i c a t i o n o f an e x t e r n a l magnetic o r e l e c t r i c f i e l d , t h r o u g h e x a c t degeneracy. Two s t a t e s o f t h e same symmetry cannot cross. An a n t i c r o s s i n g i s a p e r t u r b a t i o n induced by an e x t e r n a l f i e l d . The e x t e r n a l f i e l d can induce t h e a n t i c r o s s i n g by c a u s i n g Elo-E20 t o t u n e t h r o u g h z e r o and by causing t h e i n t e r a c t i o n m a t r i x element <e vlJ1 I t i ( e 2 v 2 J 2 > t o become nonzero by d e s t r o y i n g a r i g o r o u s z e r o - f i e l d symmetry

t3.

^{J l}⁼^{J Z}ⁱ^1,

^+--

^{and g-u} i n e l e c t r i c f i e l d ) . An a n t i c r o s s i n g i s d e t e c t e d by a resonant change i n t h e r a d i a t i v e p r o p e r t i e s ( l i f e t i m e , quantum y i e l d , p o l a r i z a t i o n , r o t a t i o n a l branch p a t t e r n , f l u o r e s c e n c e b r a n c h i n g r a t i o ) . See S e c t i o n s 5.5.2 and 5.5.3 o f Ref. [I].

E. O p t i c a l O p t i c a l Double Resonance (OODR) i s a two-step e x c i t a t i o n scheme whereby a PUMP l a s e r a t w21 e x c i t e s t h e e 2 v 2 J y e l v l J l t r a n s i t i o n and t h e n a PROBE l a s e r a t u 2 e x c i t e s t h e e3v3J3+ezv2J t r a n s i t i o n [2,3]. F i g u r e

1 i l l u s t r a t e s t h e OODR Qevel scheme f o r BaO C l n + + ~ f c + + x l z + [4] and t h a t t h e OODR e f f e c t may be d e t e c t e d a g a i n s t a d a r k background by t h e appearance of wf

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C7-20 JOURNAL DE PHYSIQUE

f l u o r e s c e n c e f r o m l e v e l - 3 a t w32 + ~ 2 1 2 W? >> ~ 2 1 and LO32 o r as a decrease ( a 40% change i s shown) i n f l u o r e s c e n c e i n t e n s i t y f r o m l e v e l - 2 r Both d e t e c t i o n schemes g i v e sub-Doppler l i n e w i d t h s ( D o p p l e r w i d t h

-

1000 MHz FWHM),

r e g a r d l e s s o f whether PUMP and PROBE a r e co- o r counter-propagating.

Second Excita+ion R I O I /

Resolved P ( I ) First E x c i l a t ~ o n P ( 1 )

x 'z*

10 ^- - 10

' 4

-

OOOR excttotlon spectrum of B o F

-

Y) C

Loser wavelength

t

+200 +loo ^UO ^-100 -200 F i g u r e 2

Loser Frequency ( M H z )

F i g u r e 1

F i g u r e 1. OODR spectroscopy. A l t e r n a t i v e d e t e c t i o n schemes f o r sub-Doppler OODR. PUMP e x c i t e s BaO A-X 1-0 P ( 1 ) , PROBE e x c i t e s C-A 3-1 R(0). The upper t r a c e was d e t e c t e d t h r o u g h a UV passing, v i s i b l e a b s o r b i n g c o l o r e d - g l a s s f i l t e r . The upper t r a c e was o b t a i n e d by m o n i t o r i n g a decrease i n t h e r e s o l v e d f l u o r e s c e n c e i n t e n s i t y of t h e A-X 1-2 P ( l ) l i n e .

F i g u r e 2. OODR E x c i t a t i o n Spectrum o f BaF. PUMP i s a single-mode, cw dye l a s e r e x c i t i n g BaF B-X 0-0. PROBE i s a broadband, cw dye l a s e r .

F i g u r e 2 [5] i l l u s t r a t e s how OODR s i m p l i f i e s a spectrum t o t h e minimum i n f o r m a t i o n r e q u i r e d t o make d e f i n i t i v e e l e c t r o n i c assignments and t o d e t e r m i n e r o t a t i o n a l and v i b r a t i o n a l constants. The PUMP s e l e c t s one o f -100 t h e r m a l l y p o p u l a t e d r o t a t i o n a l l e v e l s i n t h e BaF X2z+ s t a t e and p o p u l a t e s a s i n g l e l e v e l o f known e l e c t r o n i c , v i b r a t i o n a l

,

r o t a t i o n a l , and p a r i t y quantum numbers i n t h e B2z+ s t a t e ( t h e BaF B-X t r a n s i t i o n had been analyzed p r e v i o u s l y [ 6 ] ) . The PROBE t h e n e x c i t e s from t h e

only

a p p r e c i a b l y p o p u l a t e d l e v e l o f t h e B2z+ s t a t e i n t o e i t h e r E2z+ o r F2n. The e n t i r e EtB OODR e x c i t a t i o n spectrum c o n s i s t s of two l i n e s , R(N) and P(N), and t h e E - s t a t e r o t a t i o n a l c o n s t a n t (BE3) i s g i v e n by

The FcB ODOR e x c i t a t i o n spectrum c o n s i s t s o f two groups o f t h r e e l i n e s , from which t h e r o t a t i o n a l , s p i n - o r b i t , and A - d o u b l i n g c o n s t a n t s o f t h e F*n s t a t e , may be d e r i v e d . The t w o - l i n e

s.

6 - l i n e p a t t e r n f o r 2 z + t 2 ~ + ^VS. 2 n t 2 z + t r a n s i t i o n s t y p i f i e s t h e e l e c t r o n i c s t a t e symmetry d i a g n o s t i c p o w e r of OODR.

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The wide expanses o f f l a t b a s e l i n e suggest t h a t OODR e x c i t a t i o n spectroscopy w i l l be e x t r a o r d i n a r i l y s e n s i t i v e t o weak, n o m i n a l l y f o r b i d d e n t r a n s i t i o n s t h a t would n o r m a l l y be obscured under s t r o n g e r f e a t u r e s i n t h e t y p i c a l , r o t a t i o n a l l y congested, s i n g l e resonance spectrum.

F. S t i m u l a t e d Emission Pumping (SEP) i s a f o l d e d OODR scheme [7-91 i l l u s t r a t e d by F i g u r e 3. The second l a s e r , c a l l e d t h e DUMP, s t i m u l a t e s e m i s s i o n f r o m t h e i n t e r m e d i a t e l e v e l - 2 i n t o t h e t a r g e t l e v e l - 3 which i s a h i g h l y e x c i t e d v i b r a t i o n a l l e v e l o f t h e e l e c t r o n i c ground s t a t e . The SEP e f f e c t i s d e t e c t e d by t h e decrease i n s i d e f l u o r e s c e n c e ( " f l u o r e s c e n c e d i p " ) scheme f i r s t i l l u s t r a t e d f o r OODR i n F i g u r e 1. Many o t h e r schemes f o r d e t e c t i n g SEP have been demonstrated [8, 9 and r e f e r e n c e s t h e r e i n ] .

J u s t as OODR e x c i t a t i o n s p e c t r a a r e so s i m p l e t h a t e l e c t r o n i c s t a t e assignments may be deduced t r i v i a l l y , so t o o do SEP s p e c t r a c o n t a i n t h e minimum i n f o r m a t i o n needed t o deduce v i b r a t i o n a l l e v e l symmetry and B and C r o t a t i o n a l c o n s t a n t s f o r asymmetric t o p p o l y a t o m i c molecules

[lo].

T h i s i s i l l u s t r a t e d f o r H2CO by F i g u r e s 4 and 5. F i g u r e 4 shows p r e d i c t e d r o t a t i o n a l branch frequency and i n t e n s i t y p a t t e r n s f o r t h r e e ( o f f o u r p o s s i b l e ) symmetry c l a s s e s o f v i b r a t i o n a l l e v e l s reached by DUMP t r a n s i t i o n s p o l a r i z e d a l o n g t h e a-, b-, o r c-axes o f t h e b o d y - f i x e d i n e r t i a l a x i s system. F i g u r e 5 shows how, a t 8530 cm-1 o f v i b r a t i o n a l e x c i t a t i o n . i n t e r l e a v e d r o t a t i o n a l t r a n s i t i o n s o f

0 1 0 0 . 1 0 1 0 0 1 0

f o u r v i b r a t i o n a l bands

( k i

2540, 11224~. 31446~, and 224251) may be assigned

[lo].

Two o f t h e v i r t u e s o f SEP are: a s p e c t r a l r e g i o n which would be so congested i n a s i n g l e resonance as t o p r o h i b i t r o t a t i o n - v i b r a t i o n assignment becomes t r i v i a1 l y assignable; v i b r a t i o n a l l e v e l s t h a t c o u l d n o t n o r m a l l y be accessed d i r e c t l y from t h e z e r o - p o i n t v i b r a t i o n a l l e v e l o f t h e

k

s t a t e because o f v a n i s h i n g l y small 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 a s i l y reached because o f t h e i r good Franck-Condon o v e r l a p w i t h a we1 1 c h a r a c t e r i z e d v i b r a t i o n a l l e v e l o f t h e

a

s t a t e .

a

-

t y p e

c - t y p e 'RI.~ (11

F i g u r e 3

Pa,.l (21 P~l.,c3~

F i g u r e 4

b - t y p e

F i u r e 3. SEP PUMPIDUMP Level Diagram.

h.

P r e d i c t e d SEP P a t t e r n s f o r 3 Types o f Y i b r o n i c Bands. C a l c u l a t e d r e l a t i v e i n t e n s i t i e s f o r r o t a t i o n a l t r a n s i t i o n s from J K ,K = Z0,2 of H~~~ A

a c

4 l . H2CO i s a near p r o l a t e t o p (K = -0.9614) b u t i s t r e a t e d as a symmetric t o p w i t h A:B:C = 9.4:l.Z:l.Z.

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JOURNAL DE PHYSIQUE

F i g u r e 5

F i u r e 5 SEP Spectrum o f HzCO a t EVIB = 8530 cm-1. Three b-type bands

h i ,

^{bKC =}^il)and one a-type band (nKa = 0, &Kc = t l ) a r e i n t e r l e a v e d . The upper l e v e l i s

A

4 l JKa,,, = l 0 , l '

6 . P e r t u r b a t i o n F a c i l i t a t e d O p t i c a l O p t i c a l Double Resonance (PFOODR) [ll] i s a double resonance scheme t h a t t a k e s advantage o f a p e r t u r b a t i o n i n t h e i n t e r m e d i a t e s t a t e i n o r d e r t o g a i n access t o a c l a s s of l e v e l s t h a t would o t h e r w i s e be unobservable because o f r e s t r i c t i v e s e l e c t i o n r u l e s o r

Franck-Condon f a c t o r s [I?]. The ground s t a t e o f a l l a l k a l i dimers i s X1c+ and t h e s p i n s e l e c t i o n r u l e AS = 0 p r o h i b i t s e x c i t a t i o n o f t r a n s i t i o n s f r o m X ~ C + i n t o any t r i p l e t s t a t e . The s p i n - o r b i t t e r m i n ,tJ i s r e s p o n s i b l e f o r s e v e r a l Alz+-b3n p e r t u r b a t i o n s where t h e malecul a r e i g e n s t a t e s acqui r e a p p r e c i a b l e simultaneous A - s t a t e and b - s t a t e c h a r a c t e r [ c f . Eq. ( I ) ] , The f r a c t i o n a l A - s t a t e c h a r a c t e r ( 1

-

abJ2) makes i t p o s s i b l e f o r t h e PUMP t o e x c i t e i n t o t h e A-b mixed e i g e n s t a t e f r o m X ~ E + w h i l e t h e b - s t a t e c h a r a c t e r (abJ2) a l l o w s t h e PROBE t o e x c i t e f r o m t h e mixed l e v e l i n t o a p u r e t r i p l e t e i g e n s t a t e .

LASER -I

3=;

F i g u r e 6. PFOODR Level Diagram f o r L i z and Na2.

F i g u r e 6 i l l u s t r a t e s t h e two k i n d s o f i n f o r m a t i o n about t h e t r i p l e t s t a t e s of Na2 [13] and L i z E l 4 1 o b t a i n a b l e by PFOODR spectroscopy. PFOODR e x c i t a t i o n spectroscopy (PUMP frequency f i x e d , PROBE frequency scanned,

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f i 1 te r e d f l u o r e s c e n c e d e t e c t e d ) samples t h e 3~ Rydberg s t a t e s [13a ,14a].

PFOODR r e s o l v e d f l u o r e s c e n c e spectroscopy (PUM? and PROBE f r e q u e n c i e s f i x e d , emission spectrum recorded by a scanning monochromator) samples t h e l o w - l y i n g 3 ~ u (b3nu and a3z +) v a l e n c e s t a t e s [13b,14b], i n c l u d i n g bound+bound,

bound-quasi bound

Y L ~

p A16U+-b3nU-a3zU+ a c c i d e n t a l p r e d i s s o c i a t i o n , L i p and Na2 a3zu+ l e v e l s quasi bound b e h i n d a c e n t r i f u g a l b a r r i e r ) , and bound+f r e e

(3hg+a3zu+ o s c i l l a t o r y continuum) t r a n s i t i o n s .

H. Zeeman A n t i c r o s s i n S e c t r o s c o (ZAC) was f i r s t a p p l i e d t o a m o l e c u l e (CN), i n a zero-1as;r Porm, by : i d f o r d and B r o i d a [15]. The example

r e l e v a n t t o t h e p r e s e n t d i s c u s s i o n i n v o l v e s t h e

1 l ~ ~ + X l ~ ~ +

t r a n s i t i o n o f acetylene. A s i n g l e v,J,K,,MJ l e v e l o f t h e HCCH A - s t a t e i s e x c i t e d by l i g h t from a p u l s e d frequency doubled dye l a s e r and t h e i n t e g r a t e d , undispersed f l u o r e s c e n c e i s recorded a t f i x e d l a s e r frequency w h i l e t h e magnetic f i e l d s t r e n g t h i s scanned. Numerous sharp d i p s i n t h e d e t e c t e d f l u o r e s c e n c e a r e observed as l e v e l s , which a r e n o t d e t e c t a b l e i n t h e f l u o r e s c e n c e e x c i t a t i o n spectrum ( " d a r k " l e v e l s ) , a r e Zeeman-tuned t h r o u g h a n t i c r o s s i n g s w i t h t h e r e l a t i v e l y Zeeman-inactive l e v e l o f t h e s p e c t r o s c o p i c a l l y a c c e s s i b l e

( " b r i g h t " l e v e l ) 1 - s t a t e . When a b r i g h t s t a t e i s a n t i c r o s s e d by a dark s t a t e , t h e r a d i a t i v e c h a r a c t e r o f t h e b r i g h t s t a t e i s d i v i d e d among two mixed e i g e n s t a t e s , b o t h o f which r a d i a t e more s l o w l y hence a r e more e f f i c i e n t l y quenched t h a n t h e o r i g i n a l , u n p e r t u r b e d b r i g h t s t a t e . T h i s quenching-induced decrease i n f l u o r e s c e n c e quantum y i e l d a t a n t i c r o s s i n g s between b r i g h t and dark s t a t e s i s u s u a l l y t h e most i m p o r t a n t cause o f t h e f l u o r e s c e n c e d i p a t an a n t i c r o s s i n g (see S e c t i o n 5.5.3 o f Ref. [ I ] ) . Other e f f e c t s , such as a decrease i n o p t i c a l e x c i t a t i o n e f f i c i e n c y o r a c c i d e n t a l p r e d i s s o c i a t i o n by a p r e d i s s o c i a t e d dark s t a t e , a r e o f t e n i m p o r t a n t as w e l l .

I n t h e case o f t h e a c e t y l e n e ZAC spectrum, i t i s p o s s i b l e t o e s t i m a t e t h e number of d a r k s t a t e v i b r a t i o n a l l e v e l s p e r cm-1 ( d e n s i t y o f dark s t a t e s ) from t h e number o f ZAC's p e r kGauss. F o r s e v e r a l v i b r a t i o n a l l e v e l s o f t h e HCCH 1 - s t a t e , t h e d e n s i t y o f ZAC's i m p l i e s an i m p o s s i b l y l a r g e d e n s i t y o f dark s t a t e s if o n l y t h e ab i n i t i o p r e d i c t e d [ I 6 1 t r i p l e t s t a t e s o f a c e t y l e n e a r e considered. Normally one would c o n s i d e r o n l y t r i p l e t s t a t e s because a 'cg+

s t a t e i s n o t expected t o be Zeeman-active. However, t h e a c e t y l e n e 1 - s t a t e i s embedded i n a dense m a n i f o l d o f h i g h l y e x c i t e d v i b r a t i o n a l l e v e l s o f t h e ~ 1 1 ~ ' ground s t a t e . The k - s t a t e v i b r a t i o n a l l e v e l d e n s i t y i s comparable t o t h e dark s t a t e d e n s i t y i n f e r r e d f r o m t h e ZAC spectrum. The Zeeman a c t i v i t y o f t h e s e 2 - l e v e l s i s borrowed f r o m t h e t r i p l e t s ; as a t r i p l e t s t a t e i s Zeeman t u n e d t h r o u g h a dense m a n i f o l d o f ? - l e v e l s , t h e t r i p l e t a c t s l i k e a snowplow pushing t h e ? - l e v e l s ahead o f i t t h r o u g h t h e m o n i t o r e d A-state. The p o i n t o f t h i s d i s c u s s i o n i s t o show t h a t ZAC can p r o v i d e a sample o f a l l d a r k l e v e l s i n a g i v e n energy r e g i o n , n o t m e r e l y t h e n o r m a l l y Zeeman-active ones.

I I I. Examples

The s t a g e i s now s e t f o r b r i e f d i s c u s s i o n s o f f o u r examples which i 11 u s t r a t e t h e power and e l e g a n t simp1 i c i t y o f h i g h r e s o l u t i o n spectroscopy.

A. A Complete Set o f ~i ,xi j f o r H2CO ⁱ ¹

[lo].

^~ ^~Double resonance, i n i t s s i m p l e s t form, s i m p l y by r e d u c i n g t h e r o t a t i o n a l c o n g e s t i o n t y p i c a l o f a s i n g l e resonance spectrum, a l l o w s weak f e a t u r e s t o be r e s o l v e d and assigned and o v e r l a p p i n g bands t o be untangled. SEP spectroscopy has extended t h e energy r e g i o n o f d e f i n i t i v e r o t a t i o n - v i b r a t i o n assignments i n H2CO

X ~ A ~

t o E < 9300 cm-1

[lo].

It appears, a t l e a s t ' f o r n o n r o t a t i n g (J = 0 ) l e v e l s , t h a t t h e H2CO ground s t a t e remains v i b r a t i o n a l l y w e l l organized. I n o t h e r words, t h e v i b r a t i o n a l l e v e l s f o l l o w t h e t e x t b o o k f o r m u l a

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T a b l e I shows t h e complete s e t o f s i x v i b r a t i o n a l f r e q u e n c i e s ( ~ ~ 0 ) and 21 anharmoni c i ti e s (xi j) determined by augmenting p r e v i o u s H2CO X - s t a t e s p e c t r a l d a t a w i t h r e s u l t s f r o m SEP spectroscopy. A s u r p r i s i n g l y l a r g e number o f c o n s t a n t s i s r e q u i r e d (27) t o p r o v i d e t h i s most p r i m i t i v e c h a r a c t e r i z a t i o n o f a four-atom molecule (54 c o n s t a n t s a r e r e q u i r e d f o r 5 atoms), y e t even t h i s 1 eve1 o f c h a r a c t e r i z a t i o n i s i n s u f f i c i e n t t o determine t h e t e n harmonic f o r c e constants, t o say n o t h i n g o f t h e 22 c u b i c and 48 q u a r t i c f o r c e c o n s t a n t s needed t o r e p r e s e n t t h e near e q u i l i b r i u m r e g i o n o f t h e p o t e n t i a l energy surface.

Moreover, H2CO i s one of t h e few f o u r atom o r l a r g e r .molecules f o r which such a complete f u l l y e m p i r i c a l { w i ,xi j} c h a r a c t e r i z a t i o n has been achieved.

Table I

Another advantage of SEP i s i t s J - s e l e c t i v i t y . At low-J t h e H ~ C O s ~ p s p e c t r a d i s p l a y t e x t b o o k s i m p l i c i t y . Yet by J-10, Ka-2 t h e s p e c t r a have become u n i n t e r p r e t a b l y complex [171. T h i s c o n t r o l over J a1 lows t h e e v o l u t i o n o f t h e spectrum t o be f o l l o w e d from t e x t b o o k - a s s i g n a b l e t o i n t r i n s i c a l l y unassignable. I n a s i n g l e resonance spectrum, a l l J ' s would be p r e s e n t and h o p e l e s s l y tangled; even a p a r t i a l assignment o f t h e low-J r e g i o n o f such a spectrum would be p r o h i b i t i v e l y d i f f i c u l t .

6 . PFOODR Spectroscopy o f L i 7 C141. I n a d d i t i o n t o s p e c t r a l s i m p l i f i c a t i o n , PFOODR i n L i 2 e x p l o i t s e i t h e r o f two r a r e , near p e r f e c t , a c c i d e n t a l degeneracies between a b r i g h t l e v e l , 6 L i 2 A'zu+ v = 2, J = 33 o r v = 9, J = 20 and a dark l e v e l b3nu v = 9, J = 33, N = 32 o r v = 15, J = 20, N = 19 [14,18,191, i n o r d e r t o bend t h e AS = 0 s p i n s e l e c t i o n r u l e p r o h i b i t i n g s p e c t r o s c o p i c t r a n s i t i o n s between s i n g l e t and t r i p l e t s t a t e s .

Two v e r y i n t e r e s t i n g phenomena a r e i l l u s t r a t e d w i t h t a n t a l i z i n g s i m p l i c i t y i n PFOODR r e s o l v e d f l u o r e s c e n c e s p e c t r a o f 6 L i 2 [14a]. F i g u r e 7 shows a p o r t i o n of t h e Z3n a3zu+ f l u o r e s c e n c e spectrum. The t o p and bottom t r a c e s r e s p e c t i v e l y show f c o r e s c e n c e o r i g i n a t i n g from N = 33, J = 34 and N = 31, J = 32 r o t a t i o n a l l e v e l s o f Z3n

.

Each v i b r a t i o n a l l e v e l o f t h e weakly bound a3cu+ s t a t e [D0O(a3cu+) = 136 cm-11 appears as a s i n g l e A N = A J = 0 r o t a t i o n a l l i n e . The N = 31 spectrum c o n t a i n s t r a n s i t i o n s i n t o v = 0, 1, and 2 of a3zu+ p l u s a weak, asymmetric f e a t u r e which i s t h e v = 3, N = 31 l e v e l quasibound behind a c e n t r i f u g a l b a r r i e r ,

It i s remarkable t h a t , o n l y two a n g u l a r momentum u n i t s above N = 31, a t N = 33, a l l t r a c e o f v = 3 has disappeared. T h i s i s q u i t e c o n s i s t e n t w i t h t h e a3zu+ s t a t e p o t e n t i a l energy curve, determined from t h e complete

N = 18,20,31,33 d a t a s e t , which i m p l i e s t h a t t h e h i g h e s t quasibound r o t a t i o n a l l e v e l of v = 3 i s N = 3 1 C14aI. The 6 L i 2 a3cu+ s t a t e i s p r e d i c t e d t o support a t o t a l of o n l y 193 bound p l u s 46 quasibound r o t a t i o n - v i b r a t i o n l e v e l s !

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F i g u r e 8 shows a p o r t i o n o f t h e 13hg+b3nU v = 0 f l u o r e s c e n c e spectrum [14bI. Note t h a t t h e R(30) and P(32) l i n e s a r e sharp, b u t Q ( 3 1 ) i s broad [ t h e a r e a under Q(31) i s a p p r o x i m a t e l y equal t o t h e sum o f t h e R(30) and P(32) areas]. Note a l s o t h a t t h e c o l l i s i o n a l s a t e l l i t e l i n e s i n R, Q, and P branches a r e sharp f o r even J and broad f o r odd J. T h i s p a t t e r n i s p r e c i s e l y what i s expected f o r t h e p r e d i s s o c i a t i o n o f b3n, by t h e continuum o f a3cuf

[14a] and i n e x a c t analogy t o t h e p r e d i s s o c i a t i o n o f H2 c3nu by b3cu+ [20].

I n b o t h cases t h e s p i n - o r b i t i n t e r a c t i o n i s n e g l i g i b l e and t h e AN = 0 p e r t u r b a t i o n s e l e c t i o n r u l e [I] l e a v e s h a l f o f t h e 3ny l e v e l s (+ p a r i t y l e v e l s f o r odd N and

-

p a r i t y l e v e l s f o r even N) p r e d i s s o c i a t i o n f r e e , hence m e t a s t a b l e s i n c e t h e y a r e p r o h i b i t e d from r a d i a t i n g t o t h e o n l y l o w e r l y i n g

s t a t e s (3zUf and Xlzg+). T h i s d i r e c t view o f t h e p r e d i s s o c i a t i o n i n t h e b3nu s t a t e p r o v i d e s a complete p i c t u r e o f t h e A1cut-b3nu-a3cu+ a c c i d e n t a l

p r e d i s s o c i a t i o n o f t h e L i ~lz,,' s t a t e [14a]. I n a d d i t i o n , t h e s h a r p l b r o a d p a t t e r n o f t h e c o l l is i o n a ? s a t e l l i t e s i m p l i e s t h a t t h e f i n e s t r u c t u r e p r o p e n s i t y r u l e f o r c o l l i s i o n induced t r a n s i t i o n s w i t h i n t h e 6 L i 2 b3nu s t a t e i s AN = A J [14].

F i g u r e 7 F i g u r e 8

F i u r e 7. PFOODR Resolved Fluorescence Spectrum o f 6 L i 2 2% +a3cUt Bound+Bound T r z n s i t i o n s . Top t r a c e i s from v = 13, N = 33 a t T,J = 32788.963 cm-1. Bottom t r a c e i s f r o m v = 9, N = 3 1 a t Tv 32036.281 cm-1. Note t h e presence o f a weak, broad, and asymmetric peak i c i r r e s p o n d i n g t o a3xut v = 3) i n t h e l o w e r t r a c e and i t s absence i n t h e upper t r a c e .

F i g u r e 8. PFOODR Resolved Fluorescence Spectrum o f 6 L i 2 1 3 ~ ~ + b 3 I I , 0-4 Band.

The upper l e v e l i s N = 31e, J = 32 a t T,J = 31980.136 cm'l. The 3 s t r o n g e s t f e a t u r e s a r e a c o l l i s i o n a l l y u n r e l a x e d RQP t r i p l e t where o n l y t h e Q l i n e i s broadened by t h e J, p a r i t y - s e l e c t i v e b3nu-a3xut p r e d i s s o c i a t i o n .

C. An U e r Bound f o r D O(HCC-H) by Zeeman A n t i c r o s s i n S e c t r o s c o p y ( u n p u b l i s h e d wE:k o f P e t e r ~ r e E n l . Bond d i s s o c i a t i o n enersie: a!e amons t h e most p r i z e d p r o d u c t s o f m o l e c u l a r s p e c t r o s c o p i c s t u d i e s . i n p r i n c i p l e i t i s p o s s i b l e t o l o c a t e t h e onset' o f d i s s o c i a t i o n t o b e t t e r t h a n 1 cm-1 accuracy, which corresponds t o -103 t i m e s h i g h e r p r e c i s i o n t h a n i s t y p i c a l o f most thermochemi c a l measurements o f Ooo values. However, t h e r e a r e s e v e r a l o b s t a c l e s t o s p e c t r o s c o p i c d e t e c t i o n o f t h e t r u e t h r e s h o l d f o r d i s s o c i a t i o n [211: poor Franck-Condon access f r o m t h e X - s t a t e v i b r a t i o n l e s s l e v e l t o t h e

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C7-26 JOURNAL DE PHYSIQUE

t h r e s h o l d r e g i o n o f t h e e x c i t e d s t a t e p o t e n t i a l s u r f a c e , a b a r r i e r t o d i s s o c i a t i o n on t h e e x c i t e d s t a t e s u r f a c e , t h e o p t i c a l l y a c c e s s i b l e e x c i t e d s t a t e d i s s o c i a t e s t o e x c i t e d s t a t e s o f one o r b o t h fragments.

The a c e t y l e n e HCC-H bond energy i s o f c o n s i d e r a b l e i m p o r t a n c e t o models o f hydrocarbon combustion because i t i s r e l a t e d t o t h e h e a t o f f o r m a t i o n o f t h e C2H r a d i c a l by

Measurements o f bond d i s s o c i a t i o n e n e r g i e s o f s t a b l e molecules a r e o f t e n t h e most d i r e c t p a t h t o AHfo f o r r a d i c a l s . R e c e n t l y , upper bounds t o DOo (HCC-H) have been o b t a i n e d f r o m rneasure~rlents o f t r a n s 1 a t i onal energy r e 1 ease

subsequent t o 193 nm ArF l a s e r p h o t o l y s i s of HCCH [22] and t h e extreme UV (-66 nm) t h r e s h o l d f o r HCCbHCC + ti+ t e- 1231. S i n c e HCC has a l o w l y i n g

(E < lOkcal/mole) e x c i t e d e l e c t r o n i c s t a t e and i t i s p l a u s i b l e b u t u n l i k e l y t h a t b o t h p h o t o l y s i s schemes produce p r e d o m i n a n t l y e l e c t r o n i c a l l y e x c i t e d HCC*, a t o o h i g h v a l u e

(g.

a

very

l o o s e upper bound) f o r DoO(HCC-H) m i g h t have been obtained.

ZAC s t u d i e s of t h e HCCH ~ I A , s t a t e suggested t h a t t h e - s t a t e experiences a n t i c r o s s i n g s by components o f t h e two l o w e r l y i n g t r i p l e t s u r f a c e s as w e l l as h i g h v i b r a t i o n a l l e v e l s o f t h e

X1z

⁺s t a t e . S i n c e t h e l o w e s t HCCH

d i s s o c i a t i o n asymptote, H(2S) + HC?(XZZ+), c o r r e l a t e s w i t h t h e HCCH X l c ⁺and one of t h e two l o w e s t t r i p l e t s u r f a c e s , i t i s l i k e l y t h a t ZAC w i l l samp?e t h e onset o f d i s s o c i a t i o n i n t o ground s t a t e fragments. It i s improbable t h a t b a r r i e r s t o d i s s o c i a t i o n e x i s t on b o t h t h e s i n g l e t and t r i p l e t surfaces, t h u s t h e ZAC p r e d i s s o c i a t i o n t h r e s h o l d i s l i k e l y t o be a v e r y t i g h t upper bound t o t h e t r u e thermochemical t h r e s h o l d . The o b s e r v a t i o n o f e s s e n t i a l l y complete mode m i x i n g ("quantum chaos") a t E-28000 cm-1 i n HCCH X l c g + by SEP [24]

i m p l i e s t h a t dynamical b a r r i e r s o r Franck-Condon r e s t r i c t i o n s a r e u n l i k e l y t o i n t e r f e r e w i t h d e t e c t i o n of t h e d i s s o c i a t i o n t h r e s h o l d i n t h e s i n g l e t channel.

I n o r d e r t o o p t i m i z e s e n s i t i v i t y t o a weakly p r e d i s s o c i a t e d d a r k l e v e l , t h e ZAC d e t e c t i o n scheme was m o d i f i e d . Two s i g n a l channels, e a r l y and l a t e f l u o r e s c e n c e , were recorded and r a t i o e d ( e a r l y l l a t e ) as t h e magnetic f i e l d was scanned. A n t i c r o s s i n g s by normal, long-1 i v e d , d a r k l e v e l s r e s u l t i n an

i ncrease i n 1 a t e f l u o r e s c e n c e h e n c e T e s o n a n t decrease i n t h e r a t i oed s i g n a l

,

whereas a n t i c r o s s i n g s by p r e d i s s o c i a t e d d a r k l e v e l s r e s u l t i n a resonant i n c r e a s e i n t h e e a r l y l l a t e r a t i o . T h i s procedure a l l o w s many ZAC1s t o be surveyed r a p i d l y i n search o f a s p e c i a l a n t i c r o s s i n g t h a t d i s p l a y s t h e sought i n c r e a s e i n e a r l y / l a t e f l u o r e s c e n c e r a t i o . The p o s s i b i l i t y o f e s p e c i a l l y e f f i c i e n t quenching of t h e d a r k l e v e l i s t h e n e l i m i n a t e d by a Stern-Volmer p l o t o f 1 / ~

z.

p r e s s u r e w i t h t h e magnetic f i e l d h e l d f i x e d a t t h e c e n t e r o f t h e s p e c i a l a n t i c r o s s i n g . The t y p i c a l A - s t a t e v3 = 4, Ka = 1 f l u o r e s c e n c e r a t e i s y = 2.60(7) x

l o 6

sec-1 and t h e p r e s e n t d e t e c t i o n t h r e s h o l d f o r p r e d i s s o c i a t i o n i s ~y = 2.5 x 105 sec-1. Of more t h a n 50 a n t i c r o s s i n g s observed i n t h e

1

v3 = 4, Ka = 1, J = 1, M = 0 l e v e l , o n l y one dark l e v e l i s d e t e c t a b l y p r e d i s s o c i a t e d w i t h ny = 7.1(10! x

l o 5

sec-1. Prom t h i s an upper bound o f DOO(HCC-H) < 132.3 k c a l / m o l e i s obtained, which agrees w e l l w i t h Refs. [22] and [23].

D. D e t e c t i o n o f Acetylene-Vi n y l i d e n e I s o m e r i z a t i o n by C r o s s - C o r r e l a t i o n o f SEP S p e c t r a ( u n p u b l i s h e d work o f Yongqin Chen). I s o m e r i z a t i o n , e s p e c i a l l y when bonds a r e broken and reformed. i s a l a r a e a m ~ l i t u d e Drocess f o r which t h e s m a l l amplitude, normal mode t r e a t m e n t o f v i g r a t i b n s i s

iil

- s u i t e d .

I n f o r m a t i o n about a dynamical process such as i s o m e r i z a t i o n , a l t h o u g h

u n d o u b t e d l y p r e s e n t i n t h e spectrum, i s u n d e c i p h e r a b l e by s t a n d a r d methods o f s p e c t r a l assignment, f i t t i n g , and i n v e r s i o n t o p o t e n t i a l energy s u r f a c e parameters. The i r r e d u c i b l e simp1 i c i t y o f an SEP spectrum, when combined w i t h a s t a t i s t i c a l ( r a t h e r t h a n d e t e r m i n i s t i c ) a n a l y s i s o f t h e spectrum, m i g h t r e v e a l t h e h i t h e r t o hidden dynamical processes.

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When SEP s p e c t r a a r e recorded from two v i b r a t i o n a l l e v e l s (33 and 2142 o r 2162C121) of t h e HCCH a - s t a t e and SEP s p e c t r a o r i g i n a t i n g f r o m two l e v e l s w i t h i d e n t i c a l J,Ka quantum numbers a r e c r o s s - c o r r e l a t e d , t h e expected and u s u a l l y found r e s u l t i s no c o r r e l a t i o n . The l a c k o f c o r r e l a t i o n owes t o t h e enormous d i f f e r e n c e between t h e v 3 ( t r a n s - b e n d ) and v q o r v 6 ( t o r s i o n o r a n t i s y m m e t r i c i n - p l a n e bend) Vibrations i n t h e 1 - s t a t e . When e i t h e r v 4 o r V6 i s e x c i t e d i n t h e 1 - s t a t e , o n l y l e v e l s o f t h e 1 - s t a t e w i t h nonzero v 5 (=-bend) a r e Franck-Condon a c c e s s i b l e . Conversely f r o m t h e

1

33 l e v e l , o n l y X v5 = 0 l e v e l s a r e a c c e s s i b l e . Thus t h e two k - s t a t e v i b r a t i o n a l l e v e l s sample

d i s j o i n t s e t s o f X - s t a t e v i b r a t i o n a l l e v e l s . Our a b i l i t y t o e x c i t e i n t o t h e 8 2142 o r 2162 l e v e l owes t o a p e r t u r b a t i o n b y t h e 33 l e v e l [12], hence t h i s i s a n o t h e r example o f PFOODR ( o r PFSEP! )

.

A 25 cm-1 r e g i o n has been found, c e n t e r e d a t -15617 cm-1 above t h e HCCH 1 - s t a t e z e r o - p o i n t l e v e l , which e x h i b i t s s t r o n g c r o s s - c o r r e l a t i o n between t h e t w o SEP spectra. The group t h e o r e t i c a l and random m a t r i x arguments s u p p o r t i n g o u r i n t e r p r e t a t i o n o f t h i s c r o s s - c o r r e l a t i o n as due t o a v i n y l i d e n e

v i b r a t i o n a l b a s i s s t a t e which l i v e s f o r -200 f s w i l l be presented elsewhere.

T h i s o b s e r v a t i o n o f a r e l a t i v e l y l o n g - l i v e d v i n y l i d e n e l e v e l p l a c e s an upper bound on t h e energy o f t h e v i n y l i d e n e z e r o p o i n t l e v e l and suggests t h a t t h e v i n y l i d e n e isomer corresponds t o a t r u e . a l b e i t s h a l l o w minimum on t h e SO p o t e n t i a l energy surface.

T h i s SEP c r o s s - c o r r e l a t i o n d i a g n o s t i c r a i s e s t h e p o s s i b i l i t y o f o t h e r , perhaps more s e n s i t i v e and s p e c i f i c s t a t i s t i c a l measures o f dynamical processes.

Acknowl edgments

The SEP s t u d y o f H2C0 was supported by a g r a n t f r o m t h e AFOSR

(AFOSR-85-0381). The PFOODR experiments on L i 2 were supported by g r a n t s from t h e NSF ( 3 ~ g s t a t e s : PHY83-20098, b3xu and a3cu+ s t a t e s : CHE81-12966). The ZAC and SEP experiments on a c e t y l e n e were supported by t h e DOE

(DE-AC02-81ER10831). P e t e r Green i s an NSF P r e d o c t o r a l Fellow. The a c e t y l e n e experiments were c a r r i e d o u t i n t h e NSF-sponsored MIT Laser Research Center i n t h e George R. H a r r i s o n Spectroscopy Laboratory.

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