THE VALENCE ELECTRON EXCITATIONS AND THE OPTICAL PROPERTIES OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES

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THE VALENCE ELECTRON EXCITATIONS AND THE OPTICAL PROPERTIES OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES

E. Burstein, A. Brotman, P. Apell

To cite this version:

E. Burstein, A. Brotman, P. Apell. THE VALENCE ELECTRON EXCITATIONS AND THE OP- TICAL PROPERTIES OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES.

Journal de Physique Colloques, 1983, 44 (C10), pp.C10-429-C10-439. �10.1051/jphyscol:19831086�.

�jpa-00223544�

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JOURNAL DE PHYSIQUE

Colloque C10, supplkrnent a u n012, Tome 44, d6cembre 1983 page Ci0-429

THE VALENCE ELECTRON E X C I T A T I O N S AND THE O P T I C A L P R O P E R T I E S OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES*

E . B u r s t e i n , A. Brotman and P . Apellf

Physics Department and Laboratory for Research on t h e S t r u c t u r e of Matter, U n i v e r s i t y o f Pennsy Zvania, PhiZadeZphia, PA 1 9 2 0 4 , U.S. A.

~ 6 s u m 6

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Nous p r e s e n t o n s une r e v u e s u r l e s e x c i t a t i o n s d e s e l e c t r o n s d e v a l e n c e q u i j o u e n t un r 8 1 e d a n s l e s p r o p r i d t d s o p t i q u e s , p a r exemple d i f f u s i o n Raman, luminescence, e t c . , d e s atomes e t d e s mole'cules a d s o r b &

Les p r o p r i d t 6 s o p t i q u e s elles-m&mes f o u r n i s s e n t un moyen pour d t u d i e r l a s t r u c t u r e i l e c t r o n i q u e d e s complexes a d s o r b a t - s u b s t r a t .

A b s t r a c t

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We p r e s e n t a n overview o f t h e v a l e n c e e l e c t r o n e x c i t a t i o n s t h a t p l a y a r o l e i n t h e o p t i c a l p ~ o p e r t i e s eg., Raman s c a t t e r i n g , luminescence, e t c . , o f adsorbed atoms and molecules. The o p t i c a l p r o p e r t i e s c a n themselves be used a s s u r f a c e - s e n s i t i v e s p e c t r o s c o p i c probes o f t h e e l e c t r o n i c s t r u c t u r e o f t h e a d s o r b a t e - s u b s t r a t e complexes.

INTRODUCTORY REMARKS

The l a c k o f i n f o r m a t i o n about t h e e l e c t r o n i c s t r u c t u r e and, s p e c i f i c a l l y about t h e e n e r g i e s , wavefunctions and widths o f v i r t u a l bound s t a t e s and bonding and a n t i b o n d i n g s t a t e s of t h e adsorbed atoms and m o l e c u l e s , h a s been a major b a r r i e r t o e f f o r t s t o e l u c i d a t e t h e key mechanisms, o t h e r t h a n s u r f a c e roughness enhanced EM f i e l d s , t h a t p l a y a r o l e i n t h e i r o p t i c a l p r o p e r t i e s , e g . , Raman s c a t t e r i n g , second harmonic g e n e r a t i o n , e t c . There h a s been a tendency i n t h e p a s t t o view t h e m e t a l s u b s t r a t e and t h e adsorbed molecules a s s e p a r a t e e n t i t i e s , a l b e i t

p e r t u r b e d by e a c h o t h e r ' s p r e s e n c e , and t o a t t r i b u t e t h e enhanced o p t i c a l phenomena t o enhancement by t h e m e t a l s u b s t r a t e . Thus, t h e enhanced Raman s c a t t e r i n g

by t h e adsorbed molecules on Ag h a s been termed l l s u r f a c e enhanced Raman s c a t t e r i n g " . It i s now c l e a r , from a v a r i e t y of e x p e r i m e n t a l e v i d e n c e , t h a t t h e Raman s c a t t e r i n g c r o s s - s e c t i o n s o f t h e adsorbed molecules i s due, i n p a r t , t o c o n t r i b u t i o n s from

" i n t e r m o l e c u l a r t t (e.g., c h a r g e t r a n s f e r ) e l e c t r o n i c e x c i t a t i o n s o f t h e adsorbed atoms and m o l e c u l e s , t h a t a r e a b s e n t i n t h e f r e e atoms and m o l e c u l e s /1,2,3/.

The a p p r o p r i a t e p o i n t o f view i s t h a t t h e o p t i c a l p r o p e r t i e s o f t h e adsorbed atoms and m o l e c u l e s a r e t h o s e o f a d s o r b a t e - s u b s t r a t e complexes whose "intermolec- u l a r " and " i n t r a m ~ l e c u l a r ' ~ e x c i t a t i o n s i n t e r a c t w i t h t h e e l e c t r o n i c e x c i t a t i o n s o f t h e u n d e r l y i n g m e t a l .

The enhancement o f t h e macroscopic and l o c a l EM f i e l d s a t a n A-S complex by s u r f a c e roughness, image d i p o l e s , e t c . , d o e s correspond t o a n enhancement by t h e m e t a l s u b s t r a t e . On t h e o t h e r hand, t h e enhanced o p t i c a l r e s p o n s e a r i s i n g from " i n t e r m o l e c u l a r l t e x c i t a t i o n s i s n o t a s u r f a c e enhancement. It i s simply t h e m a n i f e s t a t i o n o f t h e f o r m a t i o n o f a n A-S complex. We n o t e i n t h i s c o n n e c t i o n t h a t , t o o b s e r v e t h e Raman s c a t t e r i n g (RS) by a monolayer, o r submonolayer, o f adsorbed molecules o n a metal s u b s t r a t e i n t h e absence o f any " s u r f a c e enhancement" o f t h e i n c i d e n t and s c a t t e r e d EM f i e l d s , i t is advantageous t o c a r r y o u t t h e RS measurements under resonance enhanced c o n d i t i o n s /4,5/ ( e g . , t o u s e e x c i t a - t i o n wavelengths a t which i n t r a m o l e c u l a r o r i n t e r m o l e c u l a r r e s o n a n c e s o c c u r ) a n d , f o r t h i s p u r p o s e , t o extend t h e e x c i t a t i o n wavelengths from t h e v i s i b l e i n t o t h e u l t r a v i o l e t and i n f r a r e d ,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831086

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JOURNAL DE PHYSIQUE

BONDING, ELECTRONIC STRUCTURE AND VALENCE ELECTRON EXCITATIONS

To a d e q u a t e l y understand t h e o p t i c a l p r o p e r t i e s o f adsorbed atoms and molecules on metal s u b s t r a t e s , one needs i n f o r m a t i o n about t h e e l e c t r o n i c s t r u c t u r e and, i n p a r t i c u l a r , i n f o r m a t i o n about t h e e l e c t r o n i c e x c i t a t i o n s , (eg., e n e r g i e s , w i d t h s , o s c i l l a t o r s t r e n g t h s , e t c . ) of t h e A-S complexes t h a t a r e formed.

The d e t e r m i n a t i o n o f t h e e l e c t r o n i c s t r u c t u r e of molecules adsorbed on metal s u r f a c e s (and, t h e r e b y , t h e e l u c i d a t i o n of t h e bonding between a d s o r b a t e s and s u b s t r a t e s ) h a s been a major o b j e c t i v e o f s u r f a c e p h y s i c i s t s and chemists, and t h e r e h a s been c o n s i d e r a b l e experimental and t h e o r e t i c a l p r o g r e s s i n a t t a i n i n g t h i s o b j e c t i v e f o r a number o f adsorbed molecules and metal s u b s t r a t e s /6/.

We p r e s e n t h e r e a n overview o f t h e bonding o f atoms and simple molecules a t m e t a l s u r f a c e s and o f t h e e l e c t r o n i c s t r u c t u r e and e l e c t r o n i c e x c i t a t i o n s o f t h e A-S complexes t h a t a r e formed. I n doing s o , we w i l l l i m i t o u r s e l v e s t o i s o l a t e d , e.g., low coverage, a d s o r b a t e s on smooth metal s u r f a c e s i n u l t r a high vacuum. Our primary o b j e c t i v e i s t o c l a r i f y t h e n a t u r e o f t h e valence e l e c t r o n e x c i t a t i o n s t h a t p l a y a r o l e i n o p t i c a l phenomena e x h i b i t e d by adsorbed atoms and molecules on m e t a l s u r f a c e s /7/.

The m e t a l s u b s t r a t e s f a l l q u a l i t a t i v e l y i n t o s e v e r a l groups. i ) Metals, s u c h a s A l , Na and K , which do n o t have d-bands and a r e f r e e - e l e c t r o n - l i k e . The c o v a l e n t bonding of a d s o r b a t e s by t h e s , p e l e c t r o n s of t h e s e m e t a l s i s r e l a t i v e - l y weak. i i ) Noble m e t a l s (eg., Ag, Cu and Au), whose d-bands l i e w e l l below t h e Fermi l e v e l , f o r which t h e chemisorption of a d s o r b a t e s v i a t h e d e l e c t r o n s i s only moderately s t r o n g . i i i ) T r a n s i t i o n m e t a l s (eg., N i , Pd, P t ) whose d-bands o v e r l a p t h e Fermi l e v e l and, t h e r e b y , c o n t r i b u t e t o a s t r o n g c o v a l e n t bonding o f a d s o r b a t e s .

The a d s o r b a t e s t o be d i s c u s s e d a l s o f a l l i n t o s e v e r a l groups. i ) simple aromatic molecules, eg., benzene, p y r i d i n e , p y r a z i n e , e t c . i i ) Diatomic molecules, eg., CO and N2. i i i ) Atomic a d s o r b a t e s , eg., hydrogen, halogen, a l k a l i and r a r e g a s atoms.

Aromatic Adsorbates

The aromatic molecules, eg., benzene, p y r i d i n e , e t c . , a r e chemisorbed on metal s u r f a c e s v i a c o v a l e n t bonding o f t h e i r n e l e c t r o n s with t h e d e l e c t r o n s o f t h e metal s u b s t r a t e , and g e n e r a l l y l i e f l a t on s u r f a c e . The c o v a l e n t bonding o f benzene i s o n l y moderately s t r o n g , even i n t h e c a s e o f t r a n s i t i o n metal sub- s t r a t e s /8/.

I n t h e c a s e o f p y r i d i n e , t h e p l a n e of t h e adsorbed molecules may be i n c l i n e d , o r p e r p e n d i c u l a r , t o t h e s u r f a c e due t o c o v a l e n t bonding v i a t h e l o n e p a i r of e l e c t r o n s of t h e N / 9 / . A s shown by Demuth and co-workers /10,11/ who have determined t h e e l e c t r o n i c e x c i t a t i o n s o f p y r i d i n e , p y r a z i n e and benzene adsorbed on A g ( l l 1 ) using e l e c t r o n energy l o s s measurements, t h e i n t r a m o l e c u l a r e x c i t a t i o n s o f t h e adsorbed molecules a r e o n l y moderately s h i f t e d and broadened r e l a t i v e t o t h o s e o f t h e f r e e molecules. Thus, t h e r e i s only a moderate mixing of t h e a d s o r b a t e and s u b s t r a t e o r b i t a l s . The e l e c t r o n energy l o s s s p e c t r a f o r p y r i d i n e and p y r a z i n e a l s o e x h i b i t a v e r y broad f e a t u r e with a n o n s e t a t - 2 t o 2.5 eV, which Demuth e t a 1 i d e n t i f y a s corresponding t o A-S c h a r g e - t r a n s f e r e x c i t a t i o n s . The energy l e v e l s o f f r e e p y r i d i n e molecules and o f p y r i d i n e molecules adsorbed on pg, based on t h e e l e c t r o n i c e x c i t a t i o n s e n e r g i e s r e p o r t e d by Demuth e t a1 and on t h e valence e l e c t r o n binding e n e r g i e s obtained from photoemission measurements /12/, a r e shown s c h e m a t i c a l l y i n Fig. l a . The a f f i n i t y l e v e l o f f r e e p y r i d i n e l i e s above t h e vacuum l e v e l (E ) i . e . , t h e n e g a t i v e p y r i d i n e i o n i s u n s t a b l e . When adsorbed on a f r e e - e l e c t r o n - l i k e m e t a l s u b s t r a t e , t h e a f f i n i t y l e v e l i s lowered by t h e coulomb i n t e r a c t i o n o f t h e n e g a t i v e p y r i d i n e i o n w i t h i t s image charge i n t h e metal s u b s t r a t e . The a f f i n i t y l e v e l of t h e adsorbed p y r i d i n e l i e s a t a n energy - 2 eV below EV and corresponds t o a " v i r t u a l bound

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s t a t e " . An e l e c t r o n i n t h e a f f i n i t y l e v e l of t h e adsorbed p y r i d i n e is u n s t a b l e w i t h r e s p e c t t o t h e m e t a l s u b s t r a t e and w i l l "hopff i n t o a n empty l e v e l of t h e m e t a l . We n o t e t h a t t h e e x c i t e d s t a t e s of t h e adsorbed ( n e u t r a l ) p y r i d i n e molecules l i e above t h e Fermi l e v e l (E ) and a l s o correspond t o f f v i r t u a l bound s t a t e s " . The f a c t , t h a t t h e observed F ; i n t r a m o l e c u l a r l p e l e c t r o n i c e x c i t a t i o n s o f adsorbed p y r i d i n e a r e n o t a p p r e c i a b l y broadened, i n d i c a t e s t h a t t h e c o n t r i b u t i o n t o t h e broadening from t h e d e c r e a s e d l i f e t i m e of t h e t f u n s t a b l e f l e x c i t e d s t a t e s is n o t l a r g e . There a r e two p o s s i b l e c h a r g e - t r a n s f e r e x c i t a t i o n s . One i n v o l v e s t h e t r a n s i t i o n o f a n e l e c t r o n i n t h e metal below E t o t h e a f f i n i t y l e v e l of adsorbed p y r i d i n e /13/ w i t h a n o n s e t energy ECT = EA* - F ~ F e q u a l t o

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²^eV. The o t h e r i n v o l v e s t h e t r a n s i t i o n o f a n e l e c t r o n from t h e ground s t a t e o f adsorbed p y r i d i n e t o a n empty l e v e l above EF i n t h e s u b s t r a t e ( a p r o c e s s which c o r r e s p o n d s t o photoemission from t h e a d s o r b a t e i n t o t h e metal s u b s t r a t e ) w i t h a n o n s e t energy ECT = EF

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EI* e q u a l t o

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3 eV.

FREE ADSORBED

co CO

( b )

FREE ADSORBED PYRlDlNE PYRlDlNE

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1

F i g . 1. Schematic energy l e v e l diagram f o r ( a ) p y r i d i n e adsorbed on Ag and ( b ) CO adsorbed on N i .

Diatomic A d s o r b a t e s

CO i s s t r o n g l y chemisorbed by t r a n s i t i o n m e t a l s w i t h t h e C end a t t a c h e d t o t h e m e t a l /11,12/. CO i s a l s o chemisorbed by t h e noble m e t a l s w i t h t h e a x i s of

t h e molecule normal t o t h e s u b s t r a t e . The c o v a l e n t bonding i s , however, a p p r e c i a b l y weaker t h a n on t h e t r a n s i t i o n m e t a l s /13/. CO i s o n l y weakly adsorbed by A 1 /14/.

The molecule l i e s f l a t on t h e A 1 s u r f a c e and i s presumed t o be physisorbed.

CO adsorbed on N i i s probably b e t t e r understood t h a n any o t h e r a d s o r b a t e on a m e t a l s u b s t r a t e . The h i g h e s t occupied l o n e p a i r o r b i t a l ( 5 0 ) and t h e l o w e s t unoccupied o r b i t a l (21r ) a r e a p p r e c i a b l y admixed w i t h t h e d / s o r b i t a l s of N i . The admixture o f adsorbed molecule and m e t a l s u b s t r a t e o r b i t a l s l e a d s t o bonding and a n t i b o n d i n g s t a t e s whose wavefunctions a r e l a r g e l y l o c a l i z e d i n t h e v i c i n i t y o f t h e molecule. The o n e - e l e c t r o n e n e r g y l e v e l s f o r f r e e CO, and f o r CO a d s o r b e d on N i / I ? / , a r e shown s c h e m a t i c a l l y i n F i g . Ib. The occupied 50 -M d / s bonding s t a t e ( 5 0 ) i s predominantly CO i n c h a r a c t e r , and t h e c o r r e s p o n d i n g unoccupied a n t i b o n d i n g s t a t e (Md/s) i s predominantly metal i n c h a r a c t e r . On t h e o t h e r hand, t h e occupied 2m -Md/s bonding s t a t e ( ~ d / s ) i s predominantly metal i n charac- t e r , and t h e c o r r e s p o n d i n g unoccupied a n t i b o n d i n g s t a t e ( 2 % ) i s predominantly CO i n c h a r a c t e r . We n o t e a l s o t h a t t h e o r b i t a l o f t h e CO a f f i n i t y l e v e l , which i n t h e f r e e molecule l i e s - 2 eV above EV, i s a l s o admixed w i t h t h e metal d / s o r b i t a l s . On t h e b a s i s o f r e c e n t i n v e r s e photoemission d a t a /15/, t h e a f f i n i t y l e v e l o f t h e adsorbed CO molecule ( i . e . , t h e energy l e v e l of t h e A-S complex

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C10-432 JOURNAL DE PHYSIQUE

w i t h a n a d d i t i o n a l e l e c t r o n ) l i e s " 4 eV above EF.

The valence e l e c t r o n e x c i t a t i o n s o f t h e adsorbed CO molecule i n c l u d e i n t e r m o l e c u l a r (bonding-antibonding) t r a n s i t i o n s o f t h e A-S complex, c h a r g e - t r a n s f e r t r a n s i t i o n s between t h e e l e c t r o n l e v e l s o f t h e metal s u b s t r a t e and t h e A-S complex, a s w e l l as i n t r a m o l e c u l a r t r a n s i t i o n s between l e v e l s o f t h e A-S complex which a r e predominantly CO i n c h a r a c t ~ r . I n t h i s connection we n o t e t h a t a n e l e c t r o n i c t r a n s i - t i o n from t h e occupied 5 0 bonding s t a t e t o t h e unoccupied Md/s a n t i b o n d i n g state i n v o l v e s a t r a n s f e r o f charge from t h e CO t o t h e metal and, t h u s , corresponds a l s o t o a "charge-transfer" e x c i t a t i o n i n which t h e CO i s t h e donor and t h e m e t a l i s t h e a c c e p t o r . The corre_sponding t r a n s i t i o n from t h e occupied Md/s bonding s t a t e t o t h e unoccupied 2~ a n t i b o n d i n g s t a t e a l s o corresponds t o a c h a r g e - t r a n s f e r e x c i t a t i o n from t h e metal a s t h e donor t o t h e CO a s t h e a c c e p t o r . The c o v a l e n t bonding o f t h e homonuclear molecule N by N i is moderately

s t r o n g /15,19/. The adsorbed N molecule is orienged w i t h i t s a x i s normal t o t h e s u b s t r a t e , and t h e N - N i complex t h a t 2 i s formed l a c k s a c e n t e r o f i n v e r s i o n . The N-N s t r e t c h i n g v i b r a g i o n mode is a c c o r d i n g l y i n f r a r e d a c t i v e and, i n f a c t , h a s a s i z e a b l e e f f e c t i v e charge. The e f f e c t i v e charge o f t h e N-Ni v i b r a t i o n mode i s com- p a r a b l e i n magnitude t o t h a t o f t h e C - N i v i b r a t i o n mode o f CO adsorbed on N i . The valence e l e c t r o n e x c i t a t i o n s o f t h e adsorbed N2 molecule a r e s i m i l a r i n t y p e t o t h o s e o f t h e adsorbed CO molecule.

Atomic Adsorbates

T h e o r e t i c a l i n v e s t i g a t i o n s o f atomic a d s o r b a t e s have been c a r r i e d o u t by Lang and Williams /20/, by Lundqvist and coworkers /21/ and by Flynn and coworkers /22/.

Experimental i n v e s t i g a t i o n s have been c a r r i e d o u t by F l y n n ' s group (e.g., o p t i c a l a b s o r p t i o n i n t h e u l t r a v i o l e t by halogen, a l k a l i and r a r e g a s atoms adsorbed on v a r i o u s m e t a l s u b s t r a t e s i n c l u d i n g t h e a l k a l i m e t a l s and Mg) /19/, and by Walden and Lindgren /22/and Anderson and J o s t e l l /21/(e.g., photoemission and e l e c t r o n energy l o s s measurements f o r a l k a l i atoms adsorbed on noble m e t a l s and t r a n s i t i o n m e t a l s ) .

The charge s t a t e o f t h e monovalent a d s o r b a t e s ( e . g . , hydrogen, halogen, and a l k a l i atoms) on a g i v e n metal s u b s t r a t e i s determined by t h e r e l a t i v e magnitudes o f t h e i o n i z a t i o n and a f f i n i t y e n e r g i e s o f t h e adsorbed atom E

*

and EA*, respec- t i v e l y , and t h e work f u n c t i o n 0 of t h e metal s u b s t r a t e /21/.

BI*

corresponds t o t h e d i f f e r e n c e i n energy o f t h e adsorbed atom and t h a t o f t h e adsorbed i o n i z e d - atom w i t h a n e l e c t r o n a t i n f i n i t y . I n t h e absence o f any s i z e a b l e c o v a l e n t bonding o f t h e a d s o r b a t e by t h e s u b s t r a t e , which i s g e n e r a l l y t h e c a s e when t h e s u b s t r a t e i s a f r e e - e l e c t r o n l i k e m e t a l , EI* w i l l be smaller t h a n EI, t h e i o n i z a t i o n energy o f t h e f r e e atom, because of t h e a t t r a c t i v e coulomb i n t e r a c t i o n o f t h e p o s i t i v e a d s o r b a t e i o n w i t h its image charge i n t h e metal. S i m i l a r l y E

*

corresponds t o t h e d i f f e r e n c e i n energy o f t h e adsorbed atom w i t h a n e l e c t r o n a! i n f i n i t y and t h a t of t h e adsorbed atom w i t h t h e e l e c t r o n a t t a c h e d (e.g., t h a t o f t h e n e g a t i v e a d s o r b a t e i o n ) . Here t o o , i n t h e absence o f any s i z e a b l e c o v a l e n t bonding, EA* w i l l be l a r g e r t h a n EA, t h e a f f i n i t y energy of t h e f r e e atom, because of t h e a t t r a c t i v e coulomb i n t e r a c t i o n between t h e n e g a t i v e a d s o r b a t e i o n and i t s image charge i n t h e metal.

I n tQe c a s e o f atoms adsorbed on f r e e - e l e c t r o n - l i k e metal s u b s t r a t e s , t h e magnitude of U = EI*

-

^EA*i s a p p r e c i a b l y s m a l l e r t h a n U = EI

-

^EA. ( I n t h e c a s e og

L i adsorbed on m e t a l s u b s t r a t e w i t h a high e l e c t r o n d e n s l t y , such a s A l , U m a y , a c t u a l l y be n e g a t i v e , e.g. E

*

< EA*). When t h e work f u n c t i o n 0 i s l a r g e r t h a n EA*, but s m a l l e r t h a n E

*,

t h e adsorbed atom w i l l be n e u t r a l . When 0 i s smaller t h a n E A

*,

which is t$e c a s e f o r halogens adsorbed on A l , t h e a f f i n i t y l e v e l w i l l l i e below EF and be occupied, e.g. t h e a d s o r b a t e w i l l be a n e g a t i v e i o n . When 0 i s l a r g e r t h a n EI*, which i s t h e c a s e f o r a l k a l i atoms adsorbed on A l , t h e atoms w i l l be i o n i z e d , e.g., p o s i t i v e l y charged.

When t h e c o v a l e n t bonding of t h e a d s o r b a t e by t h e metal s u b s t r a t e i s s t r o n g ,

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which i s o f t e n t h e c a s e f o r t r a n s i t i o n metal s u b s t r a t e s , t h e i n c r e a s e i n t h e valence e l e c t r o n binding energy due t o covalent bonding o f f s e t s t h e decrease due t o t h e screening of t h e coulomb i n t e r a c t i o n by t h e metal s u b s t r a t e . Moreover, t h e a f f i n i t y energy EA* of t h e A-S complex does n o t have a simple r e l a t i o n t o t h a t of t h e f r e e adsorbate and is g e n e r a l l y much s m a l l e r than EI* and smaller than t h e 0 o f t h e s u b s t r a t e . The adsorbed atoms on a t r a n s i t i o n metal s u b s t r a t e w i l l accordingly be l a r g e l y n e u t r a l .

Among t h e atomic adsorbates, H h a s been t h e most e x t e n s i v e l y s t u d i e d t h e o r e t i c a l - l y and experimentally /21,25,26/. The f r e e H- i o n which has a He c o n f i g u r a t i o n is s t a b l e with EA = +0.75 eV. When H i s adsorbed on a metal s u b s t r a t e , i t s a _ f f i n i t y l e v e l i s lowered appreciably due t o t h e coulomb i n t e r a c t i o n of t h e H i o n with i t s image charge i n t h e metal. Because of t h e small s i z e of t h e H- i o n , and t h e small 0 of t h e a l k a l i metals, t h e a f f i n i t y l e v e l of H adsorbed on a n a l k a l i metal s u b s t r a t e l i e s below EF. Thus, H adsorbs on t h e a l k a l i metals a s an H- i o n and t h e bonding i s predominantly i o n i c . (H a l s o adsorbs on Ag a s t h e H- i o n ) . On t h e o t h e r hand H i s chemisorbed by t r a n s i t i o n metals (e.g., N i , Pd and P t ) a t low temperature v i a s t r o n g covalent bonding of t h e 1 s e l e c t r o n with t h e d e l e c t r o n s of t h e t r a n s i t i o n metal s u b s t r a t e . (On warming t o 300 K t h e adsorbed H atoms undergo an i r r e v e r s i b l e t r a n s i t i o n t o a sub-surface s t a t e ) . Photoemission d a t a i n d i c a t e t h a t a bonding H l e v e l i s s p l i t o f f from t h e s u b s t r a t e d bands. I n t h e c a s e of H adsorbed on N i a t low coverage, t h e s p l i t - o f f l e v e l l i e s - 8 eV below EF.

A l k a l i and halogen i o n s (e.g. ~ i + and Cl') have r a r e gas c o n f i g u r a t i o n s and a r e bonded t o t h e metal s u b s t r a t e predominantly by t h e coulomb i n t e r a c t i o n of t h e i o n with i t s image charge i n t h e metal. The s e l e c t r o n s of t h e n e u t r a l a l k a l i atoms and t h e p e l e c t r o n s of n e u t r a l halogen atoms form covalent bonds with t h e s , p , d e l e c t r o n s of t h e metal s u b s t r a t e . The covalent bonds a r e , i n g e n e r a l , r e l a t i v e l y weak when t h e a l k a l i and halogen atoms a r e adsorbed on a f r e e - e l e c t r o n - l i k e metal s u b s t r a t e . I n t h e c a s e o f t r a n s i t i o n metal s u b s t r a t e s , however, t h e covalent bonding between t h e s and p e l e c t r o n s of t h e a l k a l i and halogen atoms with t h e metal d e l e c t r o n s can be r e l a t i v e l y strong.

Rare g a s atoms a r e physisorbed by metals v i a t h e van e r Waals i n t e r a c t i o n .

g

A s pointed out by Flynn e t a 1 /22/, when one of t h e p valence e l e c t r o n s of adsorbed r a r e g a s atoms is e x c i t e d i n t o t h e next s l e v e l , t h e e x c i t e d r a r g a s atom has an a l k a l i atom c o n f i g u r a t i n.

8

( I n t h e c a s e of Xe which has a 5p con- f i g u r a t i o n , t h e e x c i t e d Xe has a 5p 6s' C s c o n f i g u r a t i o n ) . A s a consequence, t h e i n t e r a c t i o n of t h e e x c i t e d r a r e g a s atom with t h e metal s u b s t r a t e w i l l be s i m i l a r t o t h a t of t h e corresponding a l k a l i atom (e.g. t h e s e l e c t r o n forms a covalent bond with t h e e l e c t r o n s of t h e s u b s t r a t e ) .

Experimental d a t a on t h e charge-states of a d s o r b a t e s is obtained from t h e dependence of t h e work f u n c t i o n of t h e metal s u b s t r a t e on adsorbate coverage, o p t i c a l a b s o r p t i o n and e l e c t r o n energy l o s s measurements o f t h e e l e c t r o n i c e x c i t a t i o n s , i n f r a r e d d a t a on t h e v i b r a t i o n frequency of t h e A-S bond, and d a t a on Ex* derived from photoemission measurements. The g e n e r a l t r e n d s i n t h e observed charge- s t a t e s of halogen and a l k a l i a d s o r b a t e s on d i f f e r e n t metal s u b s t r a t e s a r e a s follows:

Halogen atoms which have a f f i n i t y e n e r g i e s g r e a t e r than, o r c l o s e t o , t h e work f u n c t i o n of a f r e e - e l e c t r o n - l i k e metal s u b s t r a t e (e.g. C 1 and Br f o r which EA = 3.8 and 3.4 eV, r e s p e c t i v e l y , adsorbed on C s , Mg and A 1 f o r which 0 = 1.8, 3.8 and 4.2 eV, r e s p e c t i v e l y ) a r e adsorbed a s negative i o n s , i . e . , t h e a f f i n i t y energy of t h e adsorbed halogens EA* f s g r e a t e r than t h e 0 o f t h e s u b s t r a t e . I n g e n e r a l t h e image charge coulomb i n t e r a c t i o n , and, t h e r e f o r e , t h e magnitude o f E

* -

EA, f o r a given halogen, i n c r e a s e s with i n c r e a s e i n t h e e l e c t r o n d e n s i t y o f t k e metal. The bonding i s a l s o i o n i c i n t h e c a s e of halogen atoms adsorbed a t low coverage on Ag and Cu. On t h e o t h e r hand, halogen atoms adsorbed on t r a n s i t i o n metal s u b s t r a t e s a r e adsorbed a s "neutral" atoms, i n p a r t , because o f t h e s i z e a b l e covalent bonding which tends t o decrease EA* and, i n p a r t , because

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JOURNAL DE PHYSIQUE

of t h e l a r g e values o f 0 of t h e metal s u b s t r a t e s .

A l k a l i atoms whose EI values range from 3.9 eV (e.g., Cs) t o 5.4 eV (e.g. L i ) a r e adsorbed a s n e u t r a l atoms on a l k a l i metal s u b s t r a t e s (e.g., low e l e c t r o n d e n s i t y metals with small 0 ) . On t h e o t h e r hand, they a r e adsorbed a s p o s i t i v e i o n s on Mg and A 1 which have g r e a t e r 0 values and g r e a t e r e l e c t r o n d e n s i t i e s . C s atoms which have t h e s m a l l e s t E a r e adsorbed a s i o n s on a Cu s u b s t r a t e . On t h e o t h e r hand, t h e a l k a l i atoms a r e adsorbed a s t t n e u t r a l l t atoms on t r a n s i t i o n I metal s u b s t r a t e s , i n l a r g e p a r t , bepause t h e c o n t r i b u t i o n of covalent bonding t o t h e binding energy o f f s e t s t h e decrease i n t h e i o n i z a t i o n energy due t o t h e coulomb i n t e r a c t i o n of t h e i o n i z e d atoms with its image charge.

The atomic adsorbates e x h i b i t t h r e e types o f e l e c t r o n i c e x c i t a t i o n s : i ) Atomic- t y p e e x c i t a t i o n s of t h e adsorbed atoms which a r e t h e c o u n t e r p a r t s of t h e i n t r a - molecular e x c i t a t i o n s of adsorbed molecules. i i ) A-S c h a r g e - t r a n s f e r e x c i t a t i o n s . i i i ) E l e c t r o n t r a n s i t i o n s between bonding and antibonding l e v e l s of t h e adsorbed atom-metal s u b s t r a t e complexes.

Atomic-type e l e c t r o n i c t r a n s i t i o n s a r e observed f o r r a r g a s a s o r b a t e s , e.g.

f o r atoms t h a t a r e weakly adsorbed. The u l t r a v i o l e t (p8+ p5srf s p e c t r a of r a r e gas atoms adsorbed on a v a r i e t y of metal s u b s t r a t e s (e.g. a l k a l i metals, Mg, A l , A u and T i ) have been i n v e s t i g a t e d i n some d e t a i l by Flynn and coworkers /20/.

They observed two types of s p e c t r a a r e observed. I n one type, which i n c l u d e s n e a r l y a l l of t h e r a r e gases adsorbed on C s , K , Mg and A l , t h e s p e c t r a e x h i b i t s t r o n g atom-like e x c i t a t i o n s . I n t h e second type, which i n c l u d e s Xe adsorbed on A l , Au and T i , and K r adsorbed on A l , t h e atom-like e x c i t a t i o n s do n o t appear.

They suggested t h a t t h e absence o f t h e atomic-type e x c i a i o n s i n t h e second

f f

type of s p e c t r a was due t o t h e f a c t t h a t t h e e x c i t e d ( p s ) s t a t e s f o r t h e adsorbed Xe and K r l i e above E F of t h e metal s u b s t r a t e and, were t h e r e f o r e , tlunstabletr t o charge-transfer o f t h e e l e c t r o n s from t h e e x c i t e d l e v e l t o t h e s u b s t r a t e . Lang, e t a 1 /27/, on t h e o t h e r hand f i n d , on t h e b a s i s of d e n s i t y f u n c t i o n a l c a l c u l a t i o n s of t h e e l e c t r o n i c s t r u c t u r e of r a r e g a s adsorbates, t h a t t h e e x c i t e d s t a t e s of t h e r a r e g a s a d s o r b a t e s , f o r both types of s p e c t r a , l i e above EF of t h e metal s u b s t r a t e , and t h a t t h e i n s t a b i l i t y of t h e e x c i t e d s t a t e s toward charge- t r a n s f e r t o t h e s u b s t r a t e was not a v a l i d explanation f o r t h e d i f f e r e n c e s i n t h e two types of s p e c t r a . We have a l r e a d y noted e a r l i e r t h a t t h e i n t r a m o l e c u l a r e x c i t a t i o n s of p y r i d i n e adsorbed on Ag a r e n o t appreciably broadened even though t h e e x c i t e d s t a t e s of t h e adsorbed p y r i d i n e a r e t t v i r t u a l bound s t a t e s t t . Demuth e t a1 /28/have r e c e n t l y obtained e l e c t r o n energy l o s s d a t a on t h e e l e c t r o n i c e x c i t a t i o n s of A r and Xe on A 1 and on t h e noble metals, which, i n f a c t , show t h a t t h e e x c i t a t i o n s do have an atomic-type c h a r a c t e r . Their d a t a i n d i c a t e , moreover, t h a t t h e e x c i t e d s t a t e l i f e t i m e s were 4 times l o n g e r than t h a t pre- d i c t e d by Lang e t a 1 /27/ on t h e b a s i s of t h e i r d e n s i t y f u n c t i o n a l c a l c u l a t i o n s . More r e c e n t l y , Eberhardt and Zangwill /29/ have measured t h e 4d Rydberg s t a t e e x c i t a t i o n s of Xe adsorbed on Au, and f i n d t h a t they e x h i b i t a broad, but well defined s t r u c t u r e whose i n t e g r a t e d o s c i l l a t o r s t r e n g t h i s roughly t h e same a s t h a t f o r f r e e Xe atoms, i . e . t h e o p t i c a l e x c i t a t i o n s a r e atomic-like i n character.

An explanation f o r t h e apparent discrepancy, between t h e d a t a obtained by Flynn's group and those obtained by t h e o t h e r groups, i s s t i l l lacking. It may well r e s i d e i n d i f f e r e n c e s i n t h e p r e p a r a t i o n and c h a r a c t e r of t h e adsorbed r a r e gas samples.

Charge-transfer e x c i t a t i o n s a r e of p a r t i c u l a r i n t e r e s t i n t h e case of hydrogen, halogen and a l k a l i atoms which a r e adsorbed a s i o n s (e.g., H-, C1-, ~ a + ) /30/.

I n t h e c a s e of adsorbed C1- i o n s t h e charge-transfer e x c i t a t i o n s involve t h e t r a n s i t i o n of a n e l e c t r o n from t h e adsorbate a f f i n i t y l e v e l (which l i e s below EF) t o a n empty l e v e l i n t h e metal s u b s t r a t e (Fig. 2a). The charge-transfer e x c i t a t i o n s have a n o n s e t a t a n energy hw = EF

-

EA*.and extend t o a n energy beyond .tfw = EV

-

EA* where they c o a l e s c e with photoemrssion e x c i t a t i o n s i n t o t h e vacuum.

I n t h e c a s e of adsorbed ~ a + i o n s whose energy l e v e l l i e s above EF, t h e charge-

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t r a n s f e r e x c i t a t i o n s i n v o l v e t h e t r a n s i t i o n of an e l e c t r o n from a n occupied l e v e l i n t h e metal s u b s t r a t e t o t h e " v i r t u a l bound s t a t e " o f t h e adsorbed ~ a + (Fig. 2b). The c h a r g e - t r a n s f e r e x c i t a t i o n s have a n o n s e t a t a n e n e r g y f w

= E I

* -

E and extend t o a n energy i l w F = EIt

-

EC.

Fig. 2. Charge-transfer t r a n s i t i o n s of atomic a d s o r b a t e s on a f r e e - e l e c t r o n - l i k e metal ( a ) ~ 1 - ( b ) ~ a + ( c ) xeO.

Charge-transfer e x c i t a t i o n s a l s o occur f o r adsorbed r a r e g a s atomic 6e.g. xeO) and f o r t h e a l k a l i atoms t h a t a r e adsorbed a s n e u t r a l atoms (e.g. L i adsorbed on a C s s u b s t r a t e ) and The a f f i n i t y l e v e l o f t h e s e a d s o r b a t e s l i e below EV and above EF, and, t h e r e f o r e , correspond t o v i r t u a l bound s t a t e s . For t h e s e a d s o r b a t e s , t h e c h a r g e - t r a n s f e r e x c i t a t i o n s i n v o l v e t h e t r a n s i t i o n o f a n e l e c t r o n from t h e m e t a l i n t o t h e empty a f f i n i t y l e v e l , w i t h a n o n s e t a t %m = E

* -

^EF^(Fig.

2 c ) . ^A

F i n a l l y we n o t e t h a t n e u t r a l atoms t h a t a r e c o v a l e n t l y bonded by t h e metal sub- s t r a t e , e.g. H chemisorbed on a t r a n s i t i o n metal s u b s t r a t e , e x h i b i t intermolec- u l a r e l e c t r o n i c e x c i t a t i o n s between bonding and a n t i b o n d i n g s t a t e s which a r e s i m i l a r i n n a t u r e t o t h e i n t e r m o l e c u l a r e x c i t a t i o n s t h a t occur f o r molecular a d s o r b a t e s .

Some comments a r e i n o r d e r r e g a r d i n g t h e widths o f t h e e l e c t r o n i c e x c i t a t i o n s o f A-S complexes. The energy l e v e l s o f A-S complexes have s i z e a b l e widths due t o t h e admixing o f t h e a d s o r b a t e o r b i t a l s w i t h t h e o r b i t a l s of t h e quasi-continuous energy l e v e l s of t h e s u b s t r a t e /18/. These widths depend on t h e o v e r l a p o f

t h e a d s o r b a t e and s u b s t r a t e o r b i t a l s and, t h e r e f o r e , i n c r e a s e w i t h i n c r e a s e i n t h e bonding s t r e n g t h . E f f e c t s t h a t d e c r e a s e t h e l i f e t i m e of t h e e x c i t e d s t a t e a l s o c o n t r i b u t e t o t h e widths of t h e e l e c t r o n i c e x c i t a t i o n s . The coupling o f t h e e l e c t r o n i c e x c i t a t i o n s o f t h e A-S complex w i t h t h e s i n g l e - p a r t i c l e and c o l l e c t i v e e l e c t r o n e x c i t a t i o n s of t h e metal s u b s t r a t e l e a d s t o a broadening, a s w e l l a s s h i f t i n g , of t h e e l e c t r o n i c e x c i t a t i o n s . The v i b r o n i c s t r u c t u r e o f t h e e l e c t r o n i c e x c i t a t i o n s can l e a d t o a n a p p r e c i a b l e broadening o f t h e photo- e x c i t a t i o n (e.g. a b s o r p t i o n ) s p e c t r a when t h e widths of t h e e l e c t r o n i c l e v e l s a r e g r e a t e r t h a n t h e e n e r g i e s ( h a ) o f t h e v i b r a t i o n modes. We n o t e a l s o t h a t t h e valence e l e c t r o n e x c i t a t i o n s o f t h e A-S complex may be accompanied by s i z e a b l e shake-up, and r e l a t e d e f f e c t s , i n v o l v i n g e l e c t r o n - h o l e p a i r e x c i t a t i o n s i n t h e metal s u b s t r a t e which l e a d t o a n a p p a r e n t broadening of t h e e l e c t r o n i c e x c i t a t i o n s . VIBRONIC EXCITATIONS

We have t h u s f a r d i s c u s s e d t h e e l e c t r o n i c l e v e l s and e x c i t a t i o n s of A-S complexes.

We now c o n s i d e r t h e v i b r o n i c n a t u r e o f t h e l e v e l s and, i n p a r t i c u l a r , t h e intermolec- u l a r v i b r o n i c e x c i t a t i o n s which play i m p o r t a n t r o l e s i n t h e Raman s c a t t e r i n g

and luminescence by t h e A-S complexes. For s i m p l i c i t y , we d i s c u s s t h e v i b r o n i c c h a r g e - t r a n s f e r e x c i t a t i o n s of a n atomic a d s o r b a t e (e.g., C1-) on a f r e e - e l e c t r o n - l i k e metal s u b s t r a t e (e.g., Al) /31/.

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Fig. 3. Dependence of t h e energy l e v e l s of C1- and

c1°

on d i s t a n c e from an A 1 s u b s t r a t e .

The v i b r o n i c c h a r a c t e r of t h e charge t r a n s f e r e x c i t a t i o n of an atomic adsorbate a r i s e s from t h e dependence o f t h e ground and e x c i t e d energy l e v e l s on t h e adsor- b a t e s e p a r a t i o n from t h e s u b s t r a t e . I n Fig. 3 we show t h e dependence of t h e energy l e v e l s of ~ 1 - a n d

c1°

on t h e i r d i s t a n c e from t h e metal s u b s t r a t e . E r e p r e s e n t s t h e energy t o remove a n e l e c t r o n from a f r e e C1- t o i n f i n i t y and thereby form

clO.

The charge-transfer e x c i t a t i o n involves t h e t r a n s f e r o f a n e l e c t r o n from C1- t o an empty l e v e l i n t h e metal, l e a v i n g

c1°

adsorbed a t t h e s u r f a c e . The energy o f t h e adsorbed ~ 1 - l e v e l has an a t t r a c t i v e c o n t r i b u t i o n from t h e image charge p o t e n t i a l and, a t s h o r t d i s t a n c e s from t h e s u b s t r a t e ,

a r e p u l s i v e c o n t r i b u t i o n from t h e o v e r l a p of t h e a d s o r b a t e and s u b s t r a t e e l e c t r o n s , e.ej., t h e P a u l i exclusion p r i n c i p l e . On t h e o t h e r hand, t h e energy of t h e adsorbed C 1 l e v e l has an a t t r a c t i v e c o n t r i b u t i o n from t h e van d e r Waals i n t e r a c t i o n s and from weak covalent bonding of t h e

c1°

p e l e c t r o n s with t h e metal s , p e l e c t r o n s , and a r e p u l s i v e c o n t r i b u t i o n , a t s h o r t d i s t a n c e s , due t o t h e overlap o f a d s o r b a t e and s u b s t r a t e e l e c t r o n s . Since t h e i o n i c i n t e r a c t i o n of t h e C1- with t h e sub- s t r a t e i s s t r o n g e r than t h e van d e r Waals and covalent i n t e r a c t i o n s of t h e

c1°

with t h e s u b s t r a t e , t h e equilibrium d i s t a n c e of ~ 1 - , and t h e corresponding s p r i n g c o n s t a n t K ( c ~ - ) which determines t h e s t r e t c h v i b r a t i o n frequency of t h e C1--

s u b s t r a t e bond, i s g r e a t e r than t h e corresponding q u a n t i t i e s f o r

clO.

The v e r t i c a l energy s e p a r a t i o n E - ( d )

-

E o ( d ) = ~ ~ * ( d ) r e p r e s e n t s t h e energy t o photoionize C1- a t constant d. '$he v a r i a f l o n of E o ( d )

-

0 with d i s t a n c e from t h e sub-

C 1

s t r a t e i s a l s o shown i n Fig. 3 . The magnitude of EClo(d)

-

⁰

-

^E - ( d ) = EA*fd)

-

0 r e p r e s e n t s t h e energy t o t r a n s f e r an e l e c t r o n from t h e C1- l e d 1 t o a l e v e l i n t h e metal a t EF, i . e . , t h e tfonsettf energy ECT(d).

The e l e c t r o n i c t r a n s i t i o n s between t h e ground s t a t e , (e.g., EC1-(d)) and t h e e x c i t e d s t a t e (e.g., EClo(d)) correspond t o v i b r o n i c t r a n s i t i o n s , e.g., they involve changes i n t h e v i b r a t i o n a l quantum number of t h e adsorbed C1- and

clO.

The o p t i c a l matrix elements f o r such t r a n s i t i o n s depend on t h e overlap of t h e v i b r a t i o n a l wavefunctions of t h e ground and e x c i t e d s t a t e s , which i n t u r n depend on t h e d i f f e r e n c e i n equilibrium d i s t a n c e s of t h e adsorbed C1- and

clO.

^Since

charge-transfer e l e c t r o n i c e x c i t a t i o n s have a continuum of e n e r g i e s beyond E CT' t h e v i b r o n i c e x c i t a t i o n s do not l e a d t o any observable v i b r a t i o n a l s t r u c t u r e . The v i b r o n i c c h a r a c t e r of t h e charge-transfer e x c i t a t i o n s does play a key r o l e i n t h e Raman s c a t t e r i n g by t h e v i b r a t i o n a l modes of t h e A-S complex. Also, a s shown by Gadzuk e t a 1 /32/ i n t h e i r i n v e s t i g a t i o n of t h e photoemission e x c i t a - t i o n s of Xe adsorbed on Cu(llO), t h e v i b r a t i o n a l l e v e l s of t h e A-S complexes make a s i z e a b l e c o n t r i b u t i o n t o t h e broadening of t h e photoemission s p e c t r a . RAMAN SCATTERING AND LUMINESCENCE

The p o i n t o f view t h a t t h e Raman s c a t t e r i n g and luminescence by adsorbed atoms

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and molecules a r e o p t i c a l phenomena of adsorbate-substrate complexes l e a d s one immediately t o t h e r e a l i z a t i o n t h a t t h e microscopic mechanisms, v i b r a t i o n mode s e l e c t i o n r u l e s , e t c . f o r t h e s e phenomena can be markedly d i f f e r e n t from t h o s e f o r t h e f r e e adsorbates. (This conclusion is, of course, a n obvious one i n t h e case of t h e Raman s c a t t e r i n g by adsorbed atoms, which does not e x i s t f o r t h e f r e e atoms). The emphasis of e f f o r t s t o e l u c i d a t e t h e Raman s c a t t e r i n g and luminescence by adsorbates on metal s u b s t r a t e s , i s , i n p a r t , placed on t h e e l u c i d a t i o n of t h e n a t u r e of t h e A-S complexes. Once t h e n a t u r e o f t h e A-S complexes i s e s t a b l i s h e d , we can make use of t h e e x t e n s i v e knowledge about t h e Raman s c a t t e r i n g /33/ and luminescence of molecules /34/, t h a t a l r e a d y e x i s t s , t o a s c e r t a i n t h e physics underlying t h e Raman s c a t t e r i n g and luminescence by t h e A-S complexes. It i s a l s o evident t h a t t h e Raman s c a t t e r i n g and luminescence o f t h e adsorbed atoms and molecules can, themselves, be used a s s u r f a c e s e n s i t i v e spectroscopic probes of t h e e l e c t r o n i c e x c i t a t i o n s of A-S complexes. The measurement of Raman s c a t t e r i n g e x c i t a t i o n p r o f i l e s is, i n f a c t , a form of modulation spectroscopy which i s p a r t i c u l a r l y advantageous f o r A-S complexes, s i n c e i t enables one t o determine t h e e l e c t r o n i c e x c i t a t i o n s t h a t a r e a s s o c i a t e d with t h e adsorbed atoms and molecules. The measurement of t h e e x c i t a t i o n p r o f i l e s of t h e Raman s c a t t e r i n g by v i b r a t i o n modes of d i f f e r e n t symmetry provides informa- t i o n , not only about t h e e n e r g i e s and widths of t h e e l e c t r o n i c e x c i t a t i o n s t h a t i n t e r a c t with t h e v i b r a t i o n modes, but a l s o , about t h e c h a r a c t e r of t h e e l e c t r o n i c s t a t e s t h a t a r e involved.

I n i t s s i m p l e s t form, t h e microscopic mechanism f o r t h e Raman s c a t t e r i n g by molecules, which i s a l s o a p p l i c a b l e t o c h a r g e - t r a n s f e r e x c i t a t i o n s o f S-A complexes / 2 / , involves a v i r t u a l o p t i c a l t r a n s i t i o n from a v i b r a t i o n a l l e v e l i n t h e ground e l e c t r o n i c s t a t e t o a i n t e r m e d i a t e s t a t e , t h a t corresponds t o a vibra- t i o n a l l e v e l i n t h e e x c i t e d e l e c t r o n i c s t a t e , followed by a v i r t u a l o p t i c a l t r a n s i t i o n from t h e i n t e r m e d i a t e s t a t e t o a d i f f e r e n t v i b r a t i o n a l l e v e l i n t h e ground e l e c t r o n i c s t a t e . A s can be seen i n t h e e l e c t r o n energy-atomic configura- t i o n coordinate diagram shown i n Fig. 4, t h e equilibrium configuration-coordinates and v i b r a t i o n a l frequencies of t h e ground and e x c i t e d e l e c t r o n i c s t a t e s a r e d i f f e r e n t .

The t r a n s i t i o n p o l a r i z a b i l i t y (Raman s c a t t e r i n g matrix element) f o r t h e two- l e v e l , two-step process can, under s i m p l i f y i n g assumptions, be expressed i n t h e following form /32/:

where <e

1

p . A ~

1

O> and < O p - A,

1

e > a r e t h e momentum matrix elements f o r t h e t r a n s i

-

t i o n s between o> and

1

^{e i}^;^<v

^/

^v^>

^,

^and <vto!ve> a r e Franck-Condon type lloverlapfl i n t e g r a l s over t h e wave?un8tions of t h e vibrational l e v e l s involved i n t h e o p t i c a l t r a n s i t i o n , which depends on _Qe

-

Qo and on t h e s t r e n g t h of t h e e l e c t r o n - v i b r a t i o n mode i n t e r a c t i o n s involved; E and E a r e t h e e n e r g i e s of t h e v i b r o n i c l e v e l s i n t h e ground and e x c i t e d states: r e s p e g t i v e l y , and o oe r e p r e s e n t s t h e Lorentzian broadening of t h e e l e c t r o n i c e x c i t a t i o n . The resonance enhancement of t h e Raman s c a t t e r i n g i n t e n s i t y , t h a t occurs a t a photon energy -TIW = _Ee

-

^Eo

depends on t h e width of t h e e l e c t r o n i c e x c i t a t i o n .

Luminescence d i f f e r s from Raman s c a t t e r i n g i n t h a t i t involves a

real

o p t i c a l t r a n s i t i o n from a v i b r a t i o n l e v e l i n t h e ground e l e c t r o n i c s t a t e t o a v i b r a t i o n l e v e l i n t h e e x c i t e d e l e c t r o n i c s t a t e , followed by a

real

t r a n s i t i o n ( a f t e r

dephasing) from a v i b r a t i o n a l l e v e l i n t h e e x c i t e d e l e c t r o n i c s t a t e t o a v i b r a t i o n a l l e v e l i n t h e ground e l e c t r o n i c s t a t e (Fig. 4 ) . The luminescence by adsorbed

atoms and molecules w i l l i n g e n e r a l be q u i t e weak because of i n t e r a c t i o n s of t h e e l e c t r o n i c e x c i t a t i o n s of t h e A-S complexes with t h e s i n g l e p a r t i c l e and c o l l e c t i v e e l e c t r o n e x c i t a t i o n s of t h e metal s u b s t r a t e , which decrease t h e l i f e -

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C10-438 JOURNAL DE PHYSIQUE

time of t h e e x c i t e d s t a t e /35/. When t h e luminescence of an A-S complex i s observable, measurements of t h e luminescence emission spectrum and t h e luminescence e x c i t a t i o n p r o f i l e can provide v i t a l information about t h e v i b r o n i c e x c i t a t i o n s of t h e A-S complex.

le)

lo)

Fig. 4. Energy-configuration c o o r d i n a t e diagrams showing t h e v i b r o n i c t r a n s i t i o n s t h a t a r e involved i n ( a ) Raman s c a t t e r i n g and ( b ) luminescence.

Acknowledgements

We wish t o acknowledge v a l u a b l e d i s c u s s i o n s with C. P. Flynn, J. W. Gadzuk,

N. D. Lang, B. I. Lundqvist, R . P. Messmer, E. W. Plummer, P. Soven and M. S u n j i c . References

*

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