20 Dynamic Processes of Metal Atoms and Small Metal Clusters in Solid Supports G E O F F R E Y A . O Z I N and S. A. M I T C H E L L University of Toronto, Lash Miller Chemistry Laboratories, Toronto, Ontario M5S 1A1 Canada
Major developments in the field of metal vapor synthesis and matrix isolation spectroscopy involving ground state metal atomic and cluster reagents over the past decade are briefly contemplated. A new direction which focuses attention on the chemistry and spectroscopy of these reagents in selected excited electronic states is the subject of this paper. In particular, matrix cage relaxation dynamics, following uv and visible excitation of atomic and small cluster guests of copper and silver in rare gas lattices will be examined. Reactivity patterns of electronically excited metal atomic reagents with methane and dioxygen will also be briefly described. Metal-support effects involving both ground anexcited electronic states of the metal guest-cage unit will feature prominently in these discussions. These interactions play a critical role in determining reactivity patterns and relaxation processes of immobilized metal guests and clearly bear a relationship to metal-support effects known to be important in supported metal catalysts. As the main theme of t h i s meeting i s to assess and c o n s o l i date past achievements i n various key areas of inorganic/organom e t a l l i c chemistry, with the o b j e c t i v e o f gazing deep and hard i n t o the f u t u r i s t i c chemical c r y s t a l b a l l of the 21st century, the purpose of my p r e s e n t a t i o n w i l l be to focus a t t e n t i o n on p i v o t a l developments i n the f i e l d o f t r a n s i t i o n metal atom/metal c l u s t e r chemistry over the past decade and then to attempt to p r o j e c t and f o r e c a s t some of the more promising d i r e c t i o n s that the area i s l i k e l y to f o l l o w i n the years ahead. I f one surveys the e x c i t i n g growth p e r i o d of the e a r l y sevent i e s one cannot help but n o t i c e the n a t u r a l but constrained subd i v i s i o n of the f i e l d of metal vapor (MV) chemistry i n t o a macr o s c a l e s y n t h e t i c s c h o o l , conducting experiments u s u a l l y at 77300K and a matrix s c a l e s p e c t r o s c o p i c school, working i n the lower 0097-6156/83/0211-0303$07.25/0 © 1983 American Chemical Society
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temperature range of 4.2-300K (1). Recently, the two method o l o g i e s have been s u c c e s s f u l l y merged to the great b e n e f i t of both. C u r r e n t l y , both s o l i d s t a t e cocodensation and s o l u t i o n phase MV experiments of a combined s y n t h e t i c / s p e c t r o s c o p i c type are performed on a routine b a s i s i n our l a b o r a t o r y (2). Moreover, matrix and macroscale MVS equipment i s now commercially a v a i l a b l e (3) . These e a r l y studies were aimed at e x p o l i t i n g the o f t e n unique chemical r e a c t i v i t y of naked metal atoms, molecular metal c l u s t e r s and c o l l o i d a l metal p a r t i c l e s i n t h e i r ground e l e c t r o n i c s t a t e s , with each other to form w e l l defined metal aggregates, as w e l l as with organic ligands to produce n o v e l r e a c t i v e i n t e r mediates and products w i t h v a r y i n g degrees of s t a b i l i t y over the cryogenic temperature to room temperature range. The f i e l d of ground s t a t e MVS enjoyed p r o f u s i v e growth i n the seventies and a t t r a c t e d a m u l t i d i s c i p l i n a r y audience of s c i e n tists. This was because of the wide ranging r a m i f i c a t i o n s of the r e s u l t s i n f i e l d s as diverse as n u c l e a t i o n theory, photographic and xerographic s c i e n c e , c l u s t e r and chemisorption model theory, c a t a l y s i s by supported metal c l u s t e r s , o r g a n o m e t a l l i c s y n t h e s i s , homogeneous c a t a l y s i s , organometal polymers, to name but a few (4) . Along with the elegant chemical achievements and impressive instrumental design, one also witnessed a r a p i d expansion i n s p e c t r o s c o p i c and k i n e t i c techniques f o r c h a r a c t e r i z i n g matrix entrapped a t o m i c / c l u s t e r metal vapor r e a c t i o n products. Noteable amongst these pioneering experiments are SIMS (Michl ( 5 ) ) , MCD ( G r i n t e r ( 8 ) , Schatz ( 7 ) ) , UPS (Jacobi ( 8 ) ) , EXAFS (Montano ( 9 ) ) , NMR (Michl (10)), ESR (Weltner, (11), Kasai (12), Lindsay (13)), Mossbauer ( B a r r e t t , Montano (14)), o p t i c a l absorption and f l u o r escence (Kolb (15), Ozin (16)), Laser f l u o r e s c e n c e , Raman, Resonance Raman (Bondybey (17), Nixon (18) Schulze (19), Gruen (20), Moskovits (21)). Molecular e l e c t r o n i c s t r u c t u r e c a l c u l a t i o n s of naked metal c l u s t e r s , with n u c l e a r i t i e s spanning the range from atom to bulk became the focus of intense s c i e n t i f i c i n t e r e s t i n the 70*s (22). Experimental techniques, on the other hand, f o r f a b r i c a t i n g and s p e c t r o s c o p i c a l l y probing s p e c i f i c c l u s t e r s developed more slowly. P r e s e n t l y , molecular beam and matrix i s o l a t i o n methods have emerged as the premier approaches f o r studying l i g a n d - f r e e metal c l u s t e r s i n the gas and condensed phases r e s p e c t i v e l y . Each method d i s p l a y s advantages and l i m i t a t i o n s . In the i d e a l c o l l i s i o n f r e e environment of a molecular beam, the p r o p e r t i e s of a metal c l u s t e r can be considered to be t r u e l y i s o l a t e d from c l u s t e r - s u b s t r a t e e f f e c t s . Therefore, spectros c o p i c methods that can s e l e c t i v e l y e x t r a c t i n f o r m a t i o n from metal c l u s t e r beams hold great promise f o r i l l u m i n a t i n g diverse s i z e dependent p r o p e r t i e s of aggregates of metal atoms i n t h e i r e q u i l i b r i u m c o n f i g u r a t i o n (23). Condensation of metal atoms and/or metal c l u s t e r s , with or i n rare gases at cryogenic temperatures, from e i t h e r j e t s or beams
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represents the other main approach f o r observing n u c l e a t i o n phenomena from i s o l a t e d metal atoms to bulk metal aggregates i n a weakly i n t e r a c t i n g s o l i d support (4,24). In t h i s way, metal d i s p e r s i o n , thermal and p h o t o l y t i c behaviour of p a r t i c u l a r metal c l u s t e r s may be observed. The s p e c t r a obtained are often of low r e s o l u t i o n and observations r e f l e c t not only the p r o p e r t i e s of the guest but a l s o t h e i r i n t e r a c t i o n with the host (25). The matrix method appears to be able to provide i n d i v i d u a l c l u s t e r p r o p e r t i e s up to about s i x atoms (4,24,26). Above t h i s s i z e , s p e c t r a l overlap problems between d i f f e r e n t species i n the matrix u s u a l l y preclude an unambiguous determination of c l u s t e r n u c l e a r i t y . Nevertheless, the important t r a n s i t i o n , atom to molecule to bulk and quantum s i z e e f f e c t s i n small metal aggregates can be studied i n t h i s way (26,27). Because the s i z e regime of n=l-6 atoms i s of great p r a c t i c a l s i g n i f i c a n c e to the s p e c t r o s c o p i c , chemical and c a t a l y t i c prop e r t i e s of supported metal c l u s t e r s i n both weakly and s t r o n g l y i n t e r a c t i n g environments (28), i t i s important to study v e r y small metal c l u s t e r s i n v a r i o u s types of substrate as w e l l as i n the gas phase. In t h i s way, one can hope to develop a s c a l e of metal c l u s t e r - s u p p o r t e f f e c t s (guest-host i n t e r a c t i o n s ) and evaluate the r o l e that they play i n d i v e r s e t e c h n o l o g i c a l phenomena. In the past few years, the f i e l d of metal atom/metal c l u s t e r chemistry has taken an i n t e r e s t i n g turn out of the realm of the ground e l e c t r o n i c s t a t e i n t o the world of the e x c i t e d s t a t e . T h i s promises to be an i n t r i g u i n g new phase and one with considerable p o t e n t i a l f o r new s c i e n t i f i c discovery and t e c h n o l o g i c a l development. The o r i g i n a l swing of emphasis away from ground s t a t e metal atom/metal c l u s t e r chemistry can be traced to i n v e s t i g a t i o n s of the photoprocesses of these reagents embedded i n weakly i n t e r a c t i n g supports, mainly of the rare gas v a r i e t y (24). Studies of t h i s k i n d revealed that the i n t e r a c t i o n s between the e x c i t e d s t a t e s but not the ground s t a t e s of c e r t a i n matrix entrapped metal guests with the surrounding cage, were not q u i t e as innocent as might have been i n i t i a l l y a n t i c i p a t e d . In f a c t , these metal support e f f e c t s were of s u f f i c i e n t magnitude to r e s u l t i n a range of h i t h e r t o f o r e unobserved matrix r e l a x a t i o n processes, i n cluding metal atom p h o t o d i f f u s i o n and aggregation (30), metal c l u s t e r photofragmentation/fluorescence/cage-recombination (31), and metal c l u s t e r photoisomerization (32). These p h o t o e f f e c t s were found to be e x c e p t i o n a l l y s e n s i t i v e to the nature of the support and u n v e i l e d the existence and operation of s u r p r i s i n g l y strong guest-host i n t e r a c t i o n s . I t was as a r e s u l t of i n v e s t i g a t i o n s of the aforementioned kind that a new kind of e x c i t e d s t a t e metal atom/metal c l u s t e r photoprocess was discovered, i n v o l v i n g chemical r e a c t i o n with the support i t s e l f (33). A p r e r e q u i s i t f o r the s u c c e s s f u l e x p l o i t a t i o n of t h i s n o v e l k i n d of chemistry, i s a weakly i n t e r a c t i n g metal atom/metal c l u s t e r - cage ground e l e c t r o n i c s t a t e . Only i n
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t h i s s i t u a t i o n i s i t p o s s i b l e to prepare the metal reagents i n the d e s i r e d e l e c t r o n i c s t a t e f o r i n v e s t i g a t i o n of t h e i r e x c i t e d s t a t e r e a c t i v i t y p a t t e r n s . This phase of development i s very recent and may be considered to have opened up a new era of metal vapor chemistry, c e r t a i n l y worthy of c o n s i d e r a t i o n by chemists of the 21st century. E a r l y i n d i c a t o r s emerging from s t u d i e s of the e x c i t e d s t a t e chemistry of metal atomic and c l u s t e r reagents p o i n t to an i n t e r e s t i n g and b r i g h t future f o r the f i e l d . H i g h l i g h t s over the past year include e x c i t e d s t a t e metal atom agglomeration reac t i o n s (34), s e l e c t i v e p h o t o i n s e r t i o n processes of metal atoms i n t o the CH bonds of p a r a f f i n i c hydrocarbons (33,35), the s p l i t t i n g of dihydrogen (36), dioxygen (37), water and ammonia (38) by photoexcited metal atoms, and photoinduced e l e c t r o n - t r a n s f e r processes (37). A taste of the i n t r i g u i n g o p p o r t u n i t i e s that t h i s new f i e l d has to o f f e r , can be appreciated from the r e c e n t l y discovered s e l e c t i v e , room temperature,atmospheric pressure,photoheterogeneous d i m e r i z a t i o n of p a r a f f i n s to higher saturated a l k a nes on a metal z e o l i t e (39) ,an i n t e r e s t i n g new avenue i n the quest f o r transforming n a t u r a l gas u l t i m a t e l y to gasoline hydrocarbons. For the remainder of t h i s p r e s e n t a t i o n , I w i l l concentrate on some of the more i n t e r e s t i n g photophysical and photochemical p r o p e r t i e s r e c e n t l y observed f o r metal atomic and small c l u s t e r reagents entrapped i n non-reactive and r e a c t i v e s o l i d s u b s t r a t e s . A t t e n t i o n w i l l be d i r e c t e d to matrix-cage r e l a x a t i o n e n e r g e t i c s and dynamics f o l l o w i n g uv and v i s i b l e p h o t o e x c i t a t i o n of atomic and small c l u s t e r guests of copper and s i l v e r i n rare gas,methane and dioxygen l a t t i c e s . Metal-support i n t e r a c t i o n s i n v o l v i n g both 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 of the metal guest-cage u n i t w i l l feature prominently throughout t h i s d i s c u s s i o n as they play a c r i t i c a l r o l e i n c o n t r o l l i n g the observed r e a c t i v i t y p a t t e r n s and r e l a x a t i o n processes of the entrapped metal guests. Results and Discussion The major absorption and emission processes of gaseous Cu and Ag atoms i n the u v - v i s i b l e range are r a t h e r s t r a i g h t f o r w a r d l y i n t e r p r e t e d i n terms of e l e c t r o n i c t r a n s i t i o n s between an i s o t r o p i c a d ( n t l - l ) s Si/ ground s t a t e and n d ( n + l ) p , Ρ / ? , / and n d ( n + l ) s , D / , 5 / 2 e x c i t e d s t a t e s (40) .Only the S χ / + P 1/2 > 3/2 t r a n s i t i o n s are both p a r i t y and spin allowed although the D 5 / 2 ~ ^ S i / 2 t r a n s i t i o n i s seen i n the emission spectra of Ag vapor (41). The P i / 2 > 3 / 2 terms are almost c o i n c i d e n t f o r Cu and Ag atoms .Furthermore,the D /2,5/2 term of s i l v e r i s almost e q u i e n e r g e t i c with the P \ l 2 ^ 3 / 2 term, whereas t h i s term f o r Cu l i e s w e l l below that of the P i / » 3/2 term. The spin o r b i t coupling constant f o r Ag020,7cm- ) i s considerably l a r g e r than f o r 1
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Curtail- ) . When these atomic species are embedded i n a s o l i d rare gas, dramatic a l t e r a t i o n s i n t h e i r s p e c t r o s c o p i c and p h o t o l y t i c pro-
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p e r t i e s ensue which can be traced to s p e c i f i c guest-host i n t e r a c t i o n s operating i n t h e i r ground and e x c i t e d s t a t e s ( 3 0 , 3 1 , 3 4 ) . These perturbations can be described i n terms of a t t r a c t i v e van der Waals i n t e r a c t i o n s and short range r e p u l s i v e f o r c e s due to overlap between charge clouds on the metal and matrix atoms ( 4 2 ) . It i s evident that the p o t e n t i a l energy of i n t e r a c t i o n between a metal atom and a support w i l l i n general be d i f f e r e n t f o r d i s t i n c t e l e c t r o n i c s t a t e s of the metal atom, since the p o l a r i z a b i l i t y , r a d i a l extent and symmetry of the e l e c t r o n i c charge d i s t r i b u t i o n w i l l g e n e r a l l y vary from one e l c t r o n i c s t a t e to another. One of the o b j e c t i v e s of t h i s paper i s to evaluate the spect r o s c o p i c and photochemical consequences of the occurrence of markedly disparate guest-host i n t e r a c t i o n s i n the ground and o p t i c a l l y e x c i t e d s t a t e s of Cu and Ag atoms, and some of t h e i r low n u c l e a r i t y c l u s t e r s , i n rare gas as w e l l as other supports. O r i g i n a l papers should be consulted f o r d e t a i l s . In a l l d i s c u s s i o n s of metal support e f f e c t s , whether weak or strong, a knowledge of the l o c a l s t r u c t u r e and symmetry of the metal s i t e i s mandatory.For the s o l i d rare gases, the a v a i l a b l e spectroscopic (43) and t h e o r e t i c a l (44) evidence leans h e a v i l y towards s u b s t i t u t i o n a l i n c o r p o r a t i o n i n t o a tetradecahedral s i t e with 0 symmetry, as the most probable l o c a t i o n f o r entrapped Cu, Ag and Au atoms. In p r i n c i p l e ^ m a t r i x EXAFS measurements could place t h i s proposal on a concrete f o o t i n g . Comparison of the estimated van der Waals diameters of Cu, Ag, Au atoms ( 4 . 4 0 , 4 . 7 0 , 4.60Â r e s p e c t i v e l y ) with the known s u b s t i t u t i o n a l s i t e diameters f o r Ar, Kr, and Xe s o l i d s ( 3 . 8 3 , 4.05, 4.41Â respectively) suggest that t h i s would be a reasonable trapping s i t e arrangement e s p e c i a l l y f o r Cu atoms i n Xe, although a rather l a r g e mismatch i n s i z e would occur f o r Ag atoms i n Ar matrices. I t i s expected that these comparisons are u s e f u l only as rough q u a l i t a t i v e guidelines. n
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Consider the absorption s p e c t r a f o r a t o m i c a l l y dispersed Ag in Ar, Kr, Xe (30) (Figure 1 ) . Instead of d i s p l a y i n g the gas phase s p i n - o r b i t doublet p a t t e r n with i t s c h a r a c t e r i s t i c 1:2 i n t e n s i t y r a t i o f o r the S \ l i l l > $\l$12 t r a n s i t i o n s , one observes three w e l l resolved components (major blue s i t e ) with roughly equal i n t e n s i t i e s , with the s p l i t t i n g between the low energy component and the mean of the two high energy components being close to the gas phase s p i n - o r b i t s p l i t t i n g energy. E x c i t a t i o n of any one of these three l i n e s r e s u l t s i n r a p i d bleaching at equal rates with concurrent growth of Ag c l u s t e r absorptions s t r a d d l i n g the Ag atomic bands ( 2 9 , 3 0 ) . This phenomenon of photoaggregation has been found to be a s e n s i t i v e f u n c t i o n of the metal d i s p e r s i o n , nature and temperature of the support. During the p h o t o l y s i s of these Ag atom-rare gas f i l m s one cannot help but n o t i c e intense v i s i b l e fluorescence (Figure 2 ) . A time 2
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Figure 1. Optical absorption spectra of well-isolated A g atoms in Ar, Kr, and Xe matrices (Ag/inert gas ^l/ΙΟ ) at 10-12 K. (Reproduced from Ref. 30. Copyright 1980, American Chemical Society.) 5
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Figure 2. Comparison of the major emission bands of A g atoms isolated in Ar, Kr, and Xe matrices. The ordinate represents emission intensity in arbitrary units, and the corresponding excitations are illustrated on the absorption spectra at the left. (Reproduced from Ref. 30. Copyright 1980, American Chemical Society.)
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resolved study of the two main emissions f o r Ag/Xe using a pulsed Ν 2 l a s e r , y i e l d e d corrected r a d i a t i v e l i f e t i m e s (^12ns) w i t h i n a f a c t o r of two of that of the ^3/2 l e v e l of f r e e s i l v e r atoms (6.5±0.6ns at 328nm)(45). T h i s suggests that the f l u o r e s c e n t states i n question decay e x c l u s i v e l y by the observed r a d i a t i v e pathways since l i f e t i m e s s i g n i f i c a n t l y shorter than the f r e e atom values would be expected i f concurrent n o n - r a d i a t i v e decay pro cesses were important. The measurements a l s o demonstrate that both f l u o r e s c e n t s t a t e s were formed w e l l w i t h i n nano seconds f o l l o w i n g e x c i t a t i o n of the s i l v e r atoms, c o n s i s t e n t with the p i c t u r e of a f a s t v i b r a t i o n a l r e l a x a t i o n process and a r e l a t i v e l y long r a d i a t i v e l i f e t i m e such that emission o c c u r s a f t e r the matrix cage has a t t a i n e d i t s f u l l y relaxed c o n f i g u r a t i o n . T h i s r a i s e s the issue of the occurrence of s h i f t s between the matrix and the gas phase e l e c t r o n i c t r a n s i t i o n energies. These can be understood i n terms of d i f f e r e n c e s between the interatomic p o t e n t i a l s which apply f o r ground and e l e c t r o n i c a l l y e x c i t e d states of the metal atom i n i t s matrix cage. I f the net i n t e r a c t i o n energy between the metal atom and the matrix i s not iden t i c a l i n the two s t a t e s involved i n an e l e c t r o n i c t r a n s i t i o n then a matrix s h i f t i n the t r a n s i t i o n energy r e s u l t s as seen f o r Ag in Ar, Kr, and Xe. The i n t e r a c t i o n energy f o r the e x c i t e d s t a t e of the metal atom r e f e r s i n t h i s context to the i n t e r a c t i o n where the c o n f i g u r a t i o n of the rare gas atoms about the metal atom i s unchanged between the ground and e x c i t e d s t a t e s , that i s the l a t t i c e i s " f r o z e n " during the e l e c t r o n i c t r a n s i t i o n . Since the interatomic p o t e n t i a l s change as a r e s u l t of the e l e c t r o n i c t r a n s i t i o n , there w i l l be a tendency f o r the rare gas atoms to r e l a x to a new e q u i l i b r i u m c o n f i g u r a t i o n about the e x c i t e d metal atom. I f the guest-host i n t e r a c t i o n s are appreciably d i f f e r e n t between the ground and e x c i t e d s t a t e s , then the s i t u a t i o n imme d i a t e l y f o l l o w i n g absorption i s one i n which the matrix cage i s i n a s t a t e of high v i b r a t i o n a l p o t e n t i a l energy. In the absence of very f a s t n o n - r a d i a t i v e e l e c t r o n i c r e l a x a t i o n processes the system would tend to v i b r a t i o n a l l y r e l a x and thus the matrix cage would a t t a i n the e q u i l i b r i u m c o n f i g u r a t i o n appropriate f o r the e x c i t e d s t a t e of the metal atom. In t h i s way p a r t of the e l e c t r o n i c energy i s channelled d i r e c t l y i n t o v i b r a t i o n a l e x c i t a t i o n of the host l a t t i c e . I t i s u s e f u l to view o p t i c a l absorption and emission processes i n such a system i n terms of t r a n s i t i o n s between d i s t i n c t v i b r a t i o n a l l e v e l s of the 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 of a metal atom-rare gas complex or quasi-molecule. Since the v i b r a t i o n a l motions o f the complex are coupled with the bulk l a t t i c e v i b r a t i o n s , a complicated p a t t e r n of c l o s e l y spaced v i b r a t i o n a l l e v e l s i s involved and t h i s r e s u l t s i n the appearance of a smooth, s t r u c t u r e l e s s absorption p r o f i l e (25). Thus the homogeneous width of the absorption band a r i s e s from a coupling between the e l e c t r o n i c s t a t e s of the metal atom and the host l a t t i c e v i b r a t i o n s , which i s induced by the d i f f e r e n c e s between the guest-host
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i n t e r a c t i o n s i n the ground and e x c i t e d s t a t e s of the metal atom. Inhomogeneous c o n t r i b u t i o n s to the band width most l i k e l y a r i s e from the occurrence of a d i s t r i b u t i o n of s l i g h t l y d i f f e r e n t matrix trapping s i t e s f o r the metal atom. The fluorescence p r o f i l e s of Ag i n say Kr, b a s i c a l l y shows two major emission bands around 415 and 510 nm which can be traced through t h e i r e x c i t a t i o n dependence to o r i g i n a t e from the three S+ P atomic components. S p e c i f i c a l l y , the lowest energy absroption band gives r i s e to only the lowest energy emission system, whereas the two higher energy absorption bands give r i s e to both emission band systems. The observation of l a r g e Stokes s h i f t s f o r the P - S t r a n s i t i o n of entrapped Ag atoms i n d i c a t e s that the guest-host i n t e r a c t i o n s are markedly d i f f e r e n t f o r the S and P states of t h i s system and can be explained i n terms of matrix cage r e l a x a t i o n effects. An i n t e r e s t i n g way to v i s u a l i z e the o r i g i n of these s h i f t s i s to consider the ways i n which twelve rare gas atoms i n the symmetry of a s u b s t i t u t i o n a l s i t e can approach a s i l v e r atom i n i t s ground and e x c i t e d s t a t e s and to evaluate the van der Waals a t t r a c t i v e forces and r e p u l s i v e i n t e r a c t i o n s that c o n t r i bute to the b i n d i n g energy of the cage-complex (46). It i s w e l l known that one e l e c t r o n wave functions f o r the |j,m. > s p i n - o r b i t l e v e l s a r i s i n g from a np c o n f i g u r a t i o n can be w r i t t e n i n the f o l l o w i n g form (47): 2
2
2
2
2
2
1
I 3/2,
± 3/2
1 > = (1/2)
I 3/2,
± 1/2
> = (2/3)^
11/2,
± 1/2
/ 2
(ρ
± ip )|±l/2>
χ
p J ± l / 2 > + ( l / 6 ) ( p + i p ) | ; l/2> / 2
a
1/2
> = -(l/3) p |±l/2> +(l/3)
1/2
z
y
(p ±ip ) | ; l/2> x
where Ρ , Py and p r e f e r to the r e a l p - o r b i t a l s and |± l/2> i s the spin f u n c t i o n corresponding to m = ± 1/2. Here 13/2, ± 3/2> and |3/2, ± l/2> are the degenerate components of P3/2 I 1/2, ± l/2> corresponds to P i / 2 > where each s t a t e i s a Kramers doublet (48). M u l t i p l i c a t i o n of these wave functions by t h e i r complex conjugates and i n t e g r a t i n g over the spin v a r i able y i e l d s the f o l l o w i n g expressions f o r the associated charge distributions : χ
z
g
a n d
P
P
P
2
2
3/2,
± 3/2
=
1/2(P
X
3/2,
± 1/2
=
1/6(Ρ
χ
l / 2 , ± 1/2
=
1/3(Ρ
χ
2
+
2
P ) y
2
+ P )
+ 2/3
y
2
+ P
2 y
+
p
2 z
2
P ) z
Thus the van der Waals s t a b i l i z a t i o n should be maximized i n the P3/2 312 s t a t e by allowing the rare gas atoms above anà below the xy plane to approach the metal atom more c l o s e l y than those l y i n g i n the xy plane. In terms of the s u b s t i t u t i o n a l s i t e , t h i s corresponds to an a x i a l c o n t r a c t i o n along the z-axis, which 2
±
INORGANIC
312
CHEMISTRY:
TOWARD THE
21 ST
CENTURY
should l e a d t o a d e s t a b i l i z a t i o n o f t h e P 3 / 2 > l / 2 s t a t e because o f t h e o v e r l a p b e t w e e n t h e p c h a r g e d e n s i t y on t h e m e t a l atom and t h e c h a r g e c l o u d s on t h e a x i a l r a r e gas atom. An e x p a n s i o n a l o n g t h e z - a x i s w o u l d r e v e r s e t h i s s i t u a t i o n and c a u s e t h e P 3 / , ± 1/2 s t a t e t o be s t a b i l i z e d . I t c a n b e s e e n i n t h i s way t h a t an a x i a l d i s t o r t i o n f r o m 0^ symmetry g i v e s r i s e t o a s p l i t t i n g of the P / 2 l e v e l i n t o P / 2 > ± 3/2 * 3 / 2 > l / 2 com p o n e n t s s u c h t h a t one o f t h e s e components i s s t a b i l i z e d b y t h e distortion. T h e s e i d e a s c a n be p u t on a more s o l i d f o o t i n g b y d i s c u s s i n g t h e m a t r i x cage r e l a x a t i o n p r o c e s s i n t h e P e x c i t e d s t a t e o f Ag i n terms o f a J a h n - T e l l e r e f f e c t . I t c a n be shown t h a t an a x i a l d i s t o r t i o n i s r e q u i r e d b y symmetry c o n s i d e r a t i o n . The above s i m p l y p r o v i d e s a p h y s i c a l p i c t u r e o f t h i s r e q u i r e m e n t i n terms o f v a n d e r Waals s t a b i l i z a t i o n o f t h e s i l v e r a t o m - r a r e gas c o m p l e x . The a s p e c t o f t h e J a h n - T e l l e r p r o b l e m (49) w h i c h i s o f i n t e r e s t i n the present context i s the form o f the n u c l e a r poten t i a l e n e r g y s u r f a c e f o r a P e l e c t r o n i c s t a t e i n t h e AgXj2 quasi-molecule. This s t a t e transforms as T ± i n 0 ^ symmetry. O r b i t a l a n g u l a r momentum i s n o t quenched i n a T state. I f s p i n - o r b i t c o u p l i n g i s v e r y s t r o n g , as i n t h e case o f Ag, then the J a h n - T e l l e r e f f e c t i s o f s e c o n d a r y i m p o r t a n c e a n d t h e T state s p l i t s into G /2 d Ei/2 s p i n - o r b i t components ( c o r r e l a t i n g d i r e c t l y w i t h the P3/2 and Ρχ/2 states of the f r e e atom) where t h e symmetry l a b e l s d e n o t e 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 s o f t h e d o u b l e g r o u p O^ appropriate f o r handling h a l f a n g u l a r momenta s t a t e s . I n t h e l i m i t o f s t r o n g s p i n - o r b i t coupling (50), i t i s reasonable t o consider J a h n - T e l l e r i n t e r a c t i o n s only i n the G / 2 u s t a t e r a t h e r than i n the T term as a w h o l e , w h i c h seems more a p p r o p r i a t e f o r Cu ( s e e l a t e r ) . The v i b r a t i o n a l modes t h a t c a n c o u p l e i n f i r s t o r d e r w i t h a ^3/2u s t a t e a r e contained i n the antisymmetric square i 3/2 } l g + g + T . C o u p l i n g w i t h e modes s t a b i l i z e s t e t r a g o n a l d i s t o r t i o n s f r o m Oh symmetry w h e r e a s c o u p l i n g w i t h t2g modes s t a b i l i z e s t r i g o n a l d i s t o r t i o n s . These a x i a l d i s t o r t i o n s remove t h e d e g e n e r a c y o f t h e two K r a m e r s d o u b l e t s c o r r e s p o n d i n g t o t h e G / 2 u s t a t e . The s o l u t i o n o f t h e s t a t i c p a r t of the J a h n - T e l l e r problem i n v o l v i n g a G /2u e l e c t r o n i c s t a t e i n t e r a c t i n g w i t h e g o r t 2 g v i b r a t i o n a l modes i s w e l l known ( 5 1 ) . On t h e b a s i s o f t h e d e s c r i p t i o n g i v e n e a r l i e r o f t h e symmetry p r o p e r t i e s o f the charge d i s t r i b u t i o n a s s o c i a t e d w i t h the de generate sublevels of a P /2 s t a t e i ti s suggested that t h e s t r o n g e s t v i b r o n i c c o u p l i n g s h o u l d i n v o l v e e,g s t r e t c h i n g v i brations. The f o r m o f t h e t e t r a g o n a l d i s t o r t i o n w h i c h i s e x p e c t e d t o be s t a b l i l i z e d b y c o u p l i n g w i t h e g s t r e t c h i n g modes ( 5 2 ) i s i l l u s t r a t e d i n F i g u r e 3. T h e f o r m o f t h e p o t e n t i a l e n e r g y s u r f a c e (50,51) f o r t h i s c a s e i n t h e s i m p l e s t t h e o r y i s a l s o illustrated i n F i g u r e 3. The u p p e r a n d l o w e r b r a n c h e s o f t h e p o t e n t i a l e n e r g y s u r f a c e l a b e l l e d U i a n d U2 r e p r e s e n t t h e two e l e c t r o n i c s t a t e s which replace the o r i g i n a l degenerate s t a t e . ±
z
2
2
2
2
3
a n c
3
±
2p
2
2
2
l u
2
l u
2
a
3
n
2
U
u
2
2
1
2
2
3
l u
2
G
u
2
=
A
E
2 g
g
2
3
3
2
3
20.
OZIN A N D
MITCHELL
Dynamic
Processes of
Metals
313
Figure 3. A: Tetragonal distortion of an MX tetradecahedral complex. B: Schematic potential energy surface for a doubly degenerate electronic state in 0 symmetry interacting with a doubly degenerate vibrational mode, neglecting anharmonicity effects. V represents the nuclear potential energy and Q and Q represents the degenerate components of an e vibrational mode. C: Scheme of the ground and excited state potential energy surfaces for AgX cage complexes (X — Ar, Kr, or Xe). The absorption and emission processes are illustrated on this configuration coordinate diagram f49—51Λ 12
h
2
g
12
3
314
INORGANIC C H E M I S T R Y : TOWARD THE
21ST
CENTURY
The separation of the Ui and U2 branches at any p o i n t i n the (Q2> Q3) coordinate space represents the v i b r o n i c s p l i t t i n g of the G 3 / 2 U s t a t e . I f anharmonic terms are included i n the nu c l e a r p o t e n t i a l then the energy surface looses i t s c y l i n d r i c a l character and r e t a i n s only the three f o l d symmetry a r i s i n g from the cubic symmetry of the Hamiltonian (53). In t h i s case, the p o t e n t i a l surface has three e q u i v a l e n t , t r i g o n a l l y disposed minima separated by saddle p o i n t s , the minima corresponding to e q u i v a l e n t t e t r a g o n a l d i s t o r t i o n s along the x,y,z d i r e c t i o n s of the molecule f i x e d c a r t e s i a n coordinate system. The curve l a b e l l e d U3 represents the p o t e n t i a l energy surface f o r t h e P x / 2 s t a t e i n which account has been taken of the e f f e c t s of P 3 / 2 ~ l / 2 mixing by a second order p e r t u r b a t i o n treatment (53). The c o n f i g u r a t i o n coordinate diagram described above can be used to i n t e r p r e t the major s p l i t t i n g s i n the absorption and emission s p e c t r a of entrapped Ag atoms, i n c l u d i n g the o r i g i n of the Stokes s h i f t s , the d i r e c t i o n of the s h i f t s Xe>Kr>Ar, the temperature dependence of the absorption spectrum, fluorescence l i f e t i m e s and provides a b a s i s f o r s p e c u l a t i o n concerning the o r i g i n of photoinduced d i f f u s i o n and agglomeration e f f e c t s described e a r l i e r (30). Thus, o p t i c a l e x c i t a t i o n to the ϋχ, U2 or U l e v e l s should be followed by v i b r a t i o n a l r e l a x a t i o n to the minimum point of the r e s p e c t i v e p o t e n t i a l energy surfaces and subsequent r a d i a t i v e decay to the ground s t a t e surface as i l l u s t r a t e d i n Figure 3. The large Stokes s h i f t s of the emission bands are seen to be the consequence of the tendency f o r d i s t o r t i o n of the e x c i t e d s t a t e complexes. The s h i f t s a r i s e both from the s t a b i l i z a t i o n of the e x c i t e d s t a t e and accompanying d e s t a b i l i z a t i o n of the ground s t a t e and i s expected to follow the order Xe>Kr>Ar as observed(30). I t i s l i k e l y that the d e s t a b i l i z a t i o n caused by producing ground s t a t e complexes i n the relaxed e x c i t e d s t a t e c o n f i g u r a t i o n , provides the d r i v i n g force f o r photoinduced d i f f u s i o n of the Ag atoms (29). Perhaps the a n i s o t r o p i c f o r c e s a r i s i n g from the a x i a l d i s t o r t i o n of the A g X x 2 complex, causes the Ag atom i n i t s ground e l e c t r o n i c s t a t e to be e j e c t e d from i t s trapping s i t e . The e j e c t e d s i l v e r atom could d i s p l a c e one of the surrounding rare gas atoms i n such a way that an exchange of the o r i g i n a l s i t e p o s i t i o n s occur. T h i s would have the e f f e c t of moving the s i l v e r atom one i n t e r s i t e distance and r e s t o r i n g the o r i g i n a l s t r u c t u r a l arrangement of the trapping s i t e . As the Ag atom absorption p r o f i l e s g e n e r a l l y tend to r e t a i n t h e i r w e l l d e f i n e d s t r u c t u r e during photoinduced aggregation experiments t h i s suggests that the Ag atoms migrate between s i t e s which are not very d i f f e r e n t in structure. 2
2
2 p
3
Atomic Copper i n Rare Gas
Supports 2
2
At f i r s t glance, the three f o l d s p l i t t i n g of the P - S absorp t i o n band of Cu i n Ar, Kr and Xe matrices might lead one to
20.
oziN
AND
Dynamic
MITCHELL
Processes of
315
Metals
expect s i m i l a r s p e c t r o s c o p i c and p h o t o l y t i c p r o p e r t i e s to those described f o r Ag atoms (Figure 4). This i s c e r t a i n l y not the case and d i f f e r e n c e s can be traced to diminished importance of s p i n - o r b i t coupling e f f e c t s and to the a c c e s s i b i l i t y of the low l y i n g Ι>3/2>5/2 term f o r Cu compared to Ag atoms (34). These changes t r a n s l a t e experimentally i n t o d i s t i n c t r e l a x a t i o n mecha nisms f o r the e x c i t e d s t a t e cage complexes of Cu and Ag atoms, which manifest themselves f o r example, i n d i f f e r e n t photoaggregation k i n e t i c behaviour (54). This can be seen i n Figure 5 where the M/Kr r a t i o i s lower by a f a c t o r of two f o r Ag compared to Cu. I f s i m i l a r considerations described f o r Ag atoms are a p p l i e d to Cu atoms i n rare gas s o l i d s taking i n t o account that the JahnT e l l e r e f f e c t i n the T e x c i t e d e l e c t r o n i c s t a t e dominates over that of s p i n - o r b i t coupling, then one can proceed with the a n a l y s i s using the o r b i t a l wave functions representing the T s t a t e i n 0^ symmetry (49). By n e g l e c t i n g s p i n - o r b i t coupling and anharmonicity e f f e c t s , the s o l u t i o n to the problem of l u ® e ^ v i b r o n i c coupling, i s an e x c i t e d s t a t e p o t e n t i a l energy surface i n (Q , Q3) normal coordinate space which c o n s i s t s of three d i s j o i n t (mutually orthogonal) paraboloids (49,51). In t h i s case, the T +• A^g absorption band i s expected to be a s i n g l e Gaussian with a s i n g l e relaxed emission band from the minimum of the upper s t a t e paraboloids. This scheme i s not c o n s i s t e n t with the s p e c t r a l observations of a t h r e e f o l d s p l i t t i n g of the P +• S absorption p r o f i l e (34,55). On the other hand,for dominant T ^ t v i b r o n i c coupling, the upper s t a t e p o t e n t i a l surface has been shown to c o n s i s t of three sheets with four equivalent minima i n (Qt+j Q5, Qg) space the l a t t e r being the normal coordinates spanning the t r e p r e s e n t a t i o n (49,51).Such a p o t e n t i a l energy hypersurface can be seen to y i e l d a t h r e e f o l d s t r u c t u r e i n absorption with the expectancy of a complex emis sion p r o f i l e . Thus the T j © t g coupling scheme can a c c u r a t e l y describe the absorption p r o f i l e of C u X i , whereas a strong s p i n o r b i t and weak v i b r o n i c coupling model would appear to be more appropriate f o r A g X (30,34,55). P h o t o e x c i t a t i o n of Cu rare-gas f i l m s i n the region of the P S absorption band produces intense narrow emissions bands showing large s p e c t r a l red s h i f t s as seen i n Figure 4,(34). In contrast to Ag, these emission p r o f i l e s are i n s e n s i t i v e to v a r i a t i o n s of the e x c i t a t i o n wavelength w i t h i n the t h r e e f o l d s t r u c t u r e of the P «- S absorption band. Simultaneous with the p h o t o l y s i s of any of the three P +• S components, one observes gradual bleaching of a l l l i n e s with concurrent formation of C i ^ where η = 2 - 5 (34,56). A f u r t h e r i n t r i g u i n g observation con cerns the appearance of a weak s t r u c t u r e d emission near 420 nm f o r e x c i t a t i o n s centered on the secondary atomic s i t e band of Cu i n a l l three rare gas f i l m s (Figure 4), which has been found from independent s t u d i e s of the absorption and emission s p e c t r a of matrix entrapped Cu to a r i s e from the Α-state of Cu (34). 2
2
l u
2
l u
T
2
2
2
l u
2
2
l u
2
g
2
u
g
2
2
12
2
2
2
2
2
2
2
2
INORGANIC C H E M I S T R Y : TOWARD THE
316
350
400
450
700
800
750 l\(nm)
850
21ST C E N T U R Y
900
Figure 4. Fluorescence spectra of Cu atoms isolated in solid Ar at 12 Κ (Cu/Ar ~ 1/10 ) uncorrected for instrumental factors. The corresponding excitation wave lengths are indicated on the absorption spectrum shown at the upper left. (Repro duced from Ref. 34. Copyright 1982, American Chemical Society.) 4
Figure 5. Agglomeration of Cu and A g atoms in solid Kr at 10-12 Κ induced by P C u ( 2 p )
1
Cu (A n ) ^ C u ( X ^ ^ + h v (415nm) 2
u
2
2
-» (3)
Cu( S)
where the wavelengths i n parenthesis apply to Ar. Here (1) and (3) represent n o n - r a d i a t i v e t r a n s i t i o n s which d i s s i p a t e p a r t , or a l l , r e s p e c t i v e l y of the P +• S e x c i t a t i o n energy i n t o the rare gas l a t t i c e . The occurrence of process (1) can be recognized d i r e c t l y from the fluorescence spectra since P S photoexcit a t i o n i s followed by D S emission with no s i g n of P •> D fluorescence. S i m i l a r l y the absence of P S fluorescence suggests that process (3) i s important, since the quantum y i e l d f o r D •> S fluorescence i s s i g n i f i c a n t l y l e s s than u n i t y as seen from comparing the fluorescence i n t e n s i t i e s f o r Cu atoms trapped i n two d i f f e r e n t matrix s i t e s , where the m a j o r i t y s i t e (>90%) c l e a r l y shows a smaller quantrum y i e l d f o r D -> S f l u o r e scence f o l l o w i n g P «- S p h o t o e x c i t a t i o n . Although the d e t a i l s of these n o n - r a d i a t i v e t r a n s i t i o n s are not p r e s e n t l y c l e a r , i t seems reasonable to suggest that the energy released i n t o the matrix by processes (1) and/or (3) could account f o r the observed photo-induced d i f f u s i o n and aggregation processes of Cu atoms i n rare gas matrices (34,56). Related to t h i s photoinduced d i f f u s i o n e f f e c t i s process (2) of the above scheme, which d e p i c t s an e x c i t e d s t a t e d i m e r i z a t i o n r e a c t i o n i n v o l v i n g Cu( D)+Cu( S) separated atoms, forming Cu i n an e l e c t r o n i c a l l y e x c i t e d s t a t e as seen by the observation of v i b r a t i o n a l ^ relaxed A •> X Cu fluorescence f o l l o w i n g P S Cu atomic e x c i t a t i o n . T h e f o l l o w i n g points can be c i t e d i n support of the e x c i t e d s t a t e r e a c t i o n p r o p o s a l . F i r s t the D Cu atom e x c i t e d s t a t e i s l i k e l y to be q u i t e long l i v e d as a consequence of the forbidden nature of the o p t i c a l t r a n s i t i o n s to the ground s t a t e . Second, the formation of these s t a t e s should be accompanied by a release of considerable energy i n t o the matrix as a r e s u l t of the P •> D n o n - r a d i a t i v e t r a n s i t i o n . This could r e s u l t i n " l o c a l " melting or matrix s o f t e n i n g which would promote d i f f u s i o n of the D e x c i t e d Cu atoms and allow f o r r e a c t i v e encounters with nearby ground s t a t e S Cu atoms. These considerations suggest that photoinduced d i f f u s i o n and aggregation of Cu atoms i n rare gas s o l i d s i s promoted by P ->• S and P •> D n o n - r a d i a t i v e t r a n s i 2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
INORGANIC C H E M I S T R Y :
318
TOWARD T H E
21 ST
CENTURY
tions (34), i n c o n t r a d i s t i n c t i o n to Ag atoms, which i s b e l i e v e d to i n v o l v e P e x c i t e d Ag atom-cage r e l a x a t i o n processes r e s u l t i n g i n strong d e s t a b i l i z a t i o n of ground s t a t e Ag atoms (30). 2
Dicopper and P i s i l v e r i n Rare Gas Supports Studies of the energetics and dynamics of Cu2 and Ag2 i n rare gas s o l i d s have a l s o been completed (31,34). The absorption and fluorescence s p e c t r a are s i m i l a r l y i n d i c a t i v e of strong guesthost i n t e r a c t i o n s i n the low l y i n g s t a t e s of Cu and Ag2» Rather than presenting the s p e c t r o s c o p i c and photolyt ftic details, a summary of the observed r a d i a t i v e r e l a x a t i o n processes of v i s i b l e and uv e x c i t e d Cu2 and Ag2 i n rare gas s o l i d s i s shown below: 2
Cu( D / )+Cu( S /2)^Cu( Si 2)+Cu( S 2
Cu (B) 2
hv(400nm)
2
3
2
2
1
2
/
' ^
1 / 2
)
+hv( 756nm)
Cu ocr
(2)
2
hv(280nm)" Cu (C) 2
(3) >Cu (A) + Cu (X) + hv(415nm) 2
2
It i s proposed that the B-state o f C u (bound i n the gas phase) (57) i s s u f f i c i e n t l y s t r o n g l y d e s t a b i l i z e d i n the matrix to the extent that i t i s unstable with respect to d i s s o c i a t i o n to Cu( D3/2) + C u ( S i / 2 ) fragments f o l l o w i n g p h o t o e x c i t a t i o n of Cu2 from the ground s t a t e , process (1) i n above scheme. The extent to which the d i s s o c i a t i o n a c t u a l l y occurs depends on the l o c a l dynamics f o l l o w i n g p h o t o e x c i t a t i o n and the d e t a i l s of the Cu2~rare gas p o t e n t i a l s f o r the s p e c i f i c trapping s i t e involved. The absorption and fluorescence s p e c t r a o f A g i n rare gas s o l i d s are a l s o c l e a r l y i n d i c a t i v e of strong guest-host i n t e r actions i n v o l v i n g the A ! * and C Instates of Ag2 as summarized below: 2
2
2
2
1
(1) hv (260nn0 J 2 < \ > A
1
1
1
A g ( n ' ) _ ^ A g ( Z + ) + hv(285nm) ( 2 )
2
u
2
l
A g ( 2 S ) z
1
Ag( P)+Ag ( S+) 2
\ hv(390nm)
l ( 3 )
2
1
Ag( S)+Ag ( E+)+hv(326,365,420,460,480nm) 2
1
Ag ( Z+) 2
2
2
Ag( P) + Ag( S) 2
2Ag( S) + hv(460,480nm)
20.
oziN
AND
Dynamic
MITCHELL
Processes of
319
Metals
The emission spectrum produced by A
1
+
C u ( A n )+Cu (X E )+hv(415) 2
2
U
12K
9
i n d i c a t e d that a p r e r e q u i s i t f o r performing condensed phase e x c i t e d s t a t e metal atom chemistry was a weak i n t e r a c t i o n between the ground s t a t e metal atomic species and the matrix. This would allow f o r the preparation of the desired e x c i t e d state metal atomic s p e c i e s , as i l l u s t r a t e d i n the r e c e n t l y discovered a c t i v a t i o n of CH^ by photoexcited metal atoms (33): 2
Cu( S
1 / 2
){CHi } +
h
v
(
3
2
Q
n
1 2
m
)
2
>
Cu( P
l / 2
,
3 / 2
){CHi } +
1 2
+ CH CuH 3
12K weak ground s t a t e cage-complex
strong e x c i t e d state cage-complex
photoinsertion
On the other hand to perform e x c i t e d s t a t e metal atom chemistry i n the M/CH Br system i s not quite so s t r a i g h t f o r w a r d (58): 3
Fe Co Ni
CH Br 3
M{BrCH } hv h i g h l y coloured strong ground state charge t r a n s f e r complex 3
12K
n
^
no
reaction
because the metal atomic reagents tend to form strong ground s t a t e charge t r a n s f e r complexes,thereby precluding the a b i l i t y of achieving the desired metal atomic e x c i t e d s t a t e s . One way to surmount t h i s d i f f i c u l t y i s to work i n d i l u t e matrices as can be seen i n the ground and e x c i t e d s t a t e r e a c t i v i t y patterns of Cu atoms towards dioxygen summarized i n the scheme below (37):
320
INORGANIC C H E M I S T R Y : TOWARD THE
Ρ P-Cu-0
21ST C E N T U R Y
hv(290) ^O-Cu-0
/
0
0 345nm(LMCT)
290nm(LMCT)
1
1090cm" (vOO)
1
1100cm~ (v00)
(photoisomerization) z
Cu( S!/ ) 2
0 /Xe 2
\
(1/10-1/100) 12K ^ O-Cu-0
/
+
2
Cu-0
290nm _ 1100cm"
+ Cu( S / ){Xe}i2 1
2
hv(310) 12K
224nm _ 1000cm" 2
(Cu( P){Xe}
12
I 0··-Cu = 0 "0 2
Λ Cu
4
2
Cu( D){Xe}
(perturbed copper oxide monomer)
12
(mobile)
-»0
0 The above r e a c t i o n scheme was e s t a b l i s h e d by a combination of u v - v i s i b l e absorption and fluorescence, i r i s o t o p i c s u b s t i t u t i o n , esr and k i n e t i c measurements (37). The important point to note here i s that i n 0 r i c h Xe matrices, ground s t a t e C u ( S i / ) can not avoid r e a c t i v e encounters with 0 to form C u ( 0 ) and Cu(0 ) dioxygen complexes,whereas i t i s proposed that the formation of CuO, Cu(03) and 0 i n d i l u t e 0 /Xe matrices a r i s e s from the r e a c t i o n of a long l i v e d mobile e x c i t e d s t a t e Cu( D) with 0 . On the other hand the r e a c t i o n s of photoexcited Ag( P) with 0 are d i f f e r e n t (37), e l e c t r o n t r a n s f e r being favoured to form Ag*0 ~". I t i s thought that t h i s d i f f e r e n c e between the GS/ES r e a c t i v i t y of Cu and Ag atoms with 0 o r i g i n a t e s i n the a c c e s s i b i l i t y of the r e a c t i v e Cu( D) s t a t e from the p h o t o l y t i c a l l y prepared Cu( P) s t a t e which i s not p o s s i b l e f o r Ag( P) as i l l u s t r a t e d i n Figure 6. R e a c t i v i t y d i f f e r e n c e s between P · D n o n - r a d i a t i v e r e l a x a t i o n i s favoured f o r apex p o s i t i o n s , then t h i s could lead to H-atom a b s t r a c t i o n , whereas the edge or face o r i e n t a t i o n could lead to d i r e c t i n s e r t i o n of C u ( P ) . In e i t h e r case, photoexcited Cu( P) i s e f f i c i e n t l y chemically quenched by CH^. On the other hand,the D s t a t e of Ag appears not to be a c c e s s i b l e from the P state,so cage r e l a x a t i o n of apex o r i e n t e d CH^ molecules around the Ag( P) s t a t e could lead to ground s t a t e Ag i n a relaxed CH^ cagf, This would cause photoinduced d i f f u s i o n of A g ( S ) atoms f o r reasons s i m i l a r to those proposed f o r the rare gases, r e s u l t i n g i n Ag atom photoagglomeration as a competing pathway to CH^ a c t i v a t i o n . L i k e Cu, a Ag( P) atom l y i n g on an edge or face of CH^ i s proposed to be r e a c t i v e g i v i n g d i r e c t i n s e r t i o n to CH AgH. 2
+
2
2
2
2
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
Conclusion From the above d i s c u s s i o n , i t should be p o s s i b l e to apprec i a t e how extremely subtle d i f f e r e n c e s i n guest-host i n t e r a c t i o n s i n the ground and e x c i t e d s t a t e s of Cu and Ag atoms and dimers i n both non-reactive and r e a c t i v e supports can lead to d r a m a t i c a l l y d i s t i n c t chemical r e a c t i v i t y patterns and dynamical processes. Photochemical and photophysical phenomena of t h i s kind should provide chemists of the 21st century with a r i c h f i e l d f o r fundamental and a p p l i e d research, o f f e r i n g considerable scope f o r experimental challenges and i n t e l l e c t u a l s t i m u l a t i o n .
20.
OZIN A N D M I T C H E L L
Dynamic
323
Processes of Metals
A g / C H ( ^ = ' 3 3 2 n m ; A = 3PO nm) 4
ex
Ο t(min) 2
2
Figure 7. First-order kinetic plots for the quenching of P
2
^2,3 (photoaggregation)
( non-radiative 2_ retaxation,mobile AgCS)) CI-^AgH (insertion)
Figure 8.
2
2
Proposed relaxation schemes for P CH CuH (insertion) {CH CuH -> C H C u H (secondary p h o t o l y s i s ) Thermal annealing of the CH ,,,CuH i n t e r a c t i n g p a i r at 30-35K leads t o e f f i c i e n t generation of the i n s e r t i o n product CH CuH, which suggests that t h i s r e a c t i o n i s to some degree exothermic. 3
3
3 t t f t
3
3
