Desorption Mass Spectrometry - ACS Publications - American

2 Particle Bombardment as Viewed by Molecular Dynamics Barbara J. Garrison

Downloaded by UNIV OF IOWA on September 2, 2016 | http://pubs.acs.org Publication Date: October 17, 1985 | doi: 10.1021/bk-1985-0291.ch002

Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

A classical dynamics model is used to investigate nuclear motion in solids due tobombardmentby energetic atoms and ions. Of interest are the mechanisms of ejection and cluster formation both of elemental species such as Ni and Ar and molecular species where we have predicted intact ejection of benzene-C H , pyridine-C H N, napthalene-C H , biphenyl-C H and coronene-C H . The results presented here show that the energy distributions of the parent molecular species, e.g. benzene, are narrower than those of atomic species, even though the ejection processes in both cases arise from energetic nuclear collisions. The bonding geometry also influences the ejection yield and angular distribution. The specific case of π-bonded and σ-bonded pyridine on a metal surface is discussed with comparisons between the calculated results and experimental data. n

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12

6

10

5

5

24

12

The b o m b a r d m e n t o f s o l i d s by e n e r g e t i c p a r t i c l e beams h a s a t t r a c t e d i n t e r e s t due t o t h e e j e c t i o n o f l a r g e and n o v e l s p e c i e s . These s p e c i e s c a n be m o l e c u l e s t h a t a r e p r e s e n t i n t h e o r i g i n a l sample s u c h a s a d o d e c a n u c l e o t i d e (V) o r c l u s t e r s t h a t a r e f o r m e d d u r i n g t h e bombardment e v e n t , f o r e x a m p l e [ Ν Ο ζ ^ Ο β ^ ] " * " e j e c t e d from s o l i d n i t r o u s o x i d e ( 2 ) . Numerous o t h e r e x a m p l e s a p p e a r i n t h e s e proceedings. Our g o a l has b e e n t o u n d e r s t a n d t h e e j e c t i o n mechanisms a n d t h e r e l a t i o n s h i p o f t h e c l u s t e r s t o t h e o r i g i n a l c o n f i g u r a t i o n o f atoms i n the sample. Many m e c h a n i s m s i n v o l v i n g b o t h t h e m o t i o n o f t h e a t o m i c n u c l e i a n d / o r o f e l e c t r o n s c a n be p r o p o s e d t o be r e s p o n s i b l e for e j e c t i n g the molecules. However, i f a s o l i d ( o r l i q u i d ) sample i s b o m b a r d e d b y a h e a v y p a r t i c l e w i t h e n e r g y i n t h e 1 0 0 - 1 0 0 0 0 eV r a n g e t h e r e must be e n e r g e t i c c o l l i s i o n s b e t w e e n t h e a t o m i c n u c l e i .

0097-6156/85/0291-0043S06.00/0 © 1985 American Chemical Society

Lyon; Desorption Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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DESORPTION MASS SPECTROMETRY


Thus as a s t a r t i n g p o i n t f o r u n d e r s t a n d i n g t h e bombardment process we h a v e d e v e l o p e d a c l a s s i c a l d y n a m i c s p r o c e d u r e t o m o d e l t h e m o t i o n o f a t o m i c n u c l e i . The p r e d i c t i o n s o f t h e c l a s s i c a l m o d e l f o r t h e o b s e r v a b l e s c a n be c o m p a r e d t o t h e d a t a f r o m s p u t t e r i n g , s p e c t r o m e t r y ( S I M S ) , f a s t a t o m b o m b a r d m e n t mass s p e c t r o m e t r y ( F A B M S ) , a n d p l a s m a d e s o r p t i o n mass s p e c t r o m e t r y (PDMS) experiments. I n the c i r c u m s t a n c e s where t h e r e i s f a v o r a b l e a g r e e m e n t b e t w e e n the r e s u l t s f r o m t h e c l a s s i c a l model and e x p e r i m e n t a l d a t a i t c a n be c o n c l u d e d t h a t c o l l i s i o n c a s c a d e s are important. The c l a s s i c a l m o d e l t h e n c a n be u s e d t o l o o k a t t h e m i c r o s c o p i c processes which are not a c c e s s i b l e from experiments i n o r d e r t o g i v e us f u r t h e r i n s i g h t i n t o t h e e j e c t i o n m e c h a n i s m s . B r i e f l y , the t h e o r e t i c a l model c o n s i s t s of a p p r o x i m a t i n g the s o l i d a n d p o s s i b l e a d s o r b e d m o l e c u l e s by a f i n i t e a r r a y o f a t o m s ( 3 - 1 2 ) . A s s u m i n g a p a i r w i s e i n t e r a c t i o n p o t e n t i a l among a l l t h e atoms, H a m i l t o n ' s equations of motion are i n t e g r a t e d to y i e l d the p o s i t i o n s a n d momenta o f a l l p a r t i c l e s a s a f u n c t i o n o f t i m e d u r i n g the c o l l i s i o n cascade. The f i n a l p o s i t i o n s a n d momenta c a n be u s e d t o d e t e r m i n e t h e e x p e r i m e n t a l o b s e r v a b l e s s u c h as t o t a l y i e l d o f e j e c t e d p a r t i c l e s , e n e r g y d i s t r i b u t i o n s , a n g u l a r d i s t r i b u t i o n s and possible cluster formation. One a d v a n t a g e o f t h e c l a s s i c a l p r o c e d u r e i s t h a t one c a n m o n i t o r t h e c o l l i s i o n e v e n t s and a n a l y z e m i c r o s c o p i c mechanisms o f v a r i o u s p r o c e s s e s . Mechanisms of

Cluster Formation

From the c l a s s i c a l d y n a m i c a l t r e a t m e n t , i t i s p o s s i b l e to examine t h e c l u s t e r f o r m a t i o n m e c h a n i s m i n d e t a i l and t o p r o v i d e s e m i q u a n t i t a t i v e i n f o r m a t i o n about c l u s t e r y i e l d s . In general, t h e s e c a l c u l a t i o n s s u g g e s t t h a t t h e r e a r e t h r e e b a s i c mechanisms o f c l u s t e r f o r m a t i o n ( 1 2 ) . F i r s t , f o r systems w i t h atomic i d e n t i t y s u c h as m e t a l s , o r a t o m i c a d s o r b a t e s o n a s o l i d , t h e e j e c t e d a t o m s c a n i n t e r a c t w i t h each o t h e r i n the n e a r - s u r f a c e r e g i o n above the c r y s t a l t o f o r m a c l u s t e r by a r e c o m b i n a t i o n t y p e o f p r o c e s s (3-5). T h i s d e s c r i p t i o n would apply to c l u s t e r s of the type M j ^ observed i n many t y p e s o f SIMS e x p e r i m e n t s . I n t h i s c a s e t h e atoms i n t h e c l u s t e r do n o t n e e d t o a r i s e f r o m c o n t i g u o u s s i t e s o n t h e s u r f a c e , , a l t h o u g h i n the absence of long-range i o n i c f o r c e s the c a l c u l a t i o n s i n d i c a t e t h a t most o f them o r i g i n a t e f r o m a c i r c u l a r r e g i o n o f radius ~ 5 angstroms. This rearrangement, h o w e v e r , c o m p l i c a t e s any s t r a i g h t f o r w a r d d e d u c t i o n of the s u r f a c e s t r u c t u r e from the c o m p o s i t i o n of the observed c l u s t e r s . We h a v e o b s e r v e d a n A r 2 5 c l u s t e r to e j e c t from s o l i d argon v i a t h i s mechanism ( 1 3 ) . We would a l s o s p e c u l a t e t h a t the a l k a l i h a l l d e c l u s t e r s ( C s I ) C s w i t h η as l a r g e a s 70 ( 1 4 ) a l s o f o r m by t h i s b a s i c m e c h a n i s m . +

n

A second type of c l u s t e r emission i n v o l v e s molecular species w h i c h c a n be a s s i m p l e a s c a r b o n m o n o x i d e o r a s c o m p l i c a t e d a s t h e dodecanucleotlde mentioned above. I n t h e f i r s t c a s e , t h e CO b o n d s t r e n g t h i s ~ 11 e V , b u t t h e i n t e r a c t i o n w i t h t h e s u r f a c e i s o n l y about 1 eV. C a l c u l a t i o n s i n d i c a t e that t h i s energy d i f f e r e n c e i s s u f f i c i e n t t o a l l o w e j e c t i o n o f CO m o l e c u l e s , a l t h o u g h ~ 15 p e r c e n t o f t h e m c a n be d i s s o c i a t e d by t h e i o n beam o r by e n e r g e t i c m e t a l atoms ( 6 ) . F o r such m o l e c u l a r systems i t i s easy to i n f e r the o r i g i n a l a t o m i c c o n f i g u r a t i o n s o f t h e m o l e c u l e and t o d e t e r m i n e t h e



2.

GARRISON

Particle Bombardment

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surface chemical state. I f CO w e r e d i s s o c i a t e d i n t o o x y g e n a n d c a r b o n a t o m s , f o r e x a m p l e , t h e c a l c u l a t i o n s s u g g e s t t h a t t h e amount o f CO o b s e r v e d s h o u l d d r o p d r a m a t i c a l l y . Although the basic p r i n c i p l e s behind t h i s i n t a c t e j e c t i o n mechanism c a n be i l l u s t r a t e d w i t h c a r b o n m o n o x i d e , t h e e x t r a p o l a t i o n to l a r g e b i o o r g a n i c molecules i s not n e c e s s a r i l y obvious. C a l c u l â t i o n s have been performed f o r a s e r i e s o f o r g a n i c molecules adsorbed on a Ni(00I) surface to understand the mechanisms o f m o l e c u l a r e j e c t i o n ( 8 - 1 2 ) . The f i r s t m o l e c u l e s w h i c h h a v e more t h a n j u s t a few atoms e x a m i n e d a r e b e n z e n e w h i c h π - b o n d s on a m e t a l s u r f a c e and p y r i d i n e w h i c h c a n e i t h e r π - b o n d o r σ - b o n d on a m e t a l s u r f a c e . L a r g e r s t r u c t u r e s , whose s i z e s a p p r o a c h t h e d i a m e t e r o f b i o o r g a n i c m o l e c u l e s , a r e n a p h t h a l e n e , b l p h e n y l and c o r o n e n e whose a d s o r p t i o n s t r u c t u r e s a r e unknown. A l lthe m o l e c u l e s e x c e p t p y r i d i n e a r e assumed t o π - b o n d o n t h e s u r f a c e . I n a l l c a s e s we f i n d t h a t t h e m o l e c u l a r s p e c i e s may be e j e c t e d intact. From o u r t h e o r e t i c a l c a l c u l a t i o n s , t h r e e f a c t o r s f a v o r t h i s process ( 8 - 9 ) . F i r s t , a l a r g e m o l e c u l e h a s many i n t e r n a l degrees o f freedom and c a n absorb energy from an e n e r g e t i c c o l l i s i o n without d i s s o c i a t i n g . S e c o n d , i n t h e more m a s s i v e f r a m e w o r k o f a l a r g e o r g a n i c m o l e c u l e , i n d i v i d u a l a t o m s w i l l be s m a l l i n s i z e compared to a m e t a l atom; t h u s , i t i s p o s s i b l e t o s t r i k e s e v e r a l p a r t s o f t h e m o l e c u l e i n a c o n c e r t e d manner s o t h a t t h e e n t i r e m o l e c u l e moves i n o n e d i r e c t i o n . F i n a l l y , by t h e time the o r g a n i c molecule i s s t r u c k , the energy o f the primary p a r t i c l e has been d i s s i p a t e d so t h a t t h e k i n e t i c e n e r g i e s a r e t e n s o f eVs r a t h e r than hundreds o r thousands o f e V s . These t h r e e f a c t o r s a r e e q u a l l y v a l i d f o r t h e e j e c t i o n o f e i t h e r c a r b o n monoxide, benzene or coronene. H o w e v e r , i n t h e c a s e s o f t h e l a r g e r m o l e c u l e s , we f o u n d t h a t o f t e n 2 - 3 m e t a l atoms w o u l d s t r i k e d i f f e r e n t p a r t s o f the molecule d u r i n g the e j e c t i o n process. The t i m e f o r t h e m o l e c u l e s t o e j e c t a f t e r the p r i m a r y p a r t i c l e has h i t t h e sample i s l e s s t h a n 200 femtoseconds ( f s ; I f s - l x l 0 " ^ s ) . T h i s i n t a c t e j e c t i o n m e c h a n i s m f o r m o l e c u l e s c a n b e a p p l i e d t o m o l e c u l a r s o l i d s . We f i n d f o r t h e b o m b a r d m e n t o f i c e shows t h a t t h e w a t e r m o l e c u l e s a l s o eject intact (15). I t i s d i f f i c u l t t o make q u a n t i t a t i v e d e t e r m i n a t i o n s o f t h e fragment y i e l d s because the f o r c e s that govern a l l the rearrangement channels a r e not known. However, t h e r e i s one i n t e r e s t i n g f e a t u r e r e l a t e d t o f r a g m e n t a t i o n t h a t we h a v e o b s e r v e d . Most o f t h e fragments formed from d i r e c t c o l l i s i o n s w i t h i n ~ 0 . 2 ps a r e t h e p a r e n t m o l e c u l e m i n u s a n H , C H , o r fy^l* T h e s e a r i s e f r o m an e n e r g e t i c c o l l i s i o n t h a t r i p s o f f part of the molecule. In the c a s e o f b i p h e n y l h o w e v e r , a s e v e r i n g b e t w e e n t h e two r i n g s i s o b s e r v e d t o o c c u r w i t h some f r e q u e n c y . Thus t h e s t r u c t u r e o f t h e molecule influences the nature of the d i r e c t fragmentation process. T h e s e s m a l l CH t y p e s p e c i e s w i l l u n d o u b t e d l y be f o r m e d d u r i n g t h e i o n bombardment p r o c e s s . To be d e t e c t e d , h o w e v e r , i n a c o n v e n t i o n a l SIMS o r FABMS a p p a r a t u s t h e y must b e f o r m e d a s a n i o n . W i t h i n t h i s c l a s s i c a l m o d e l we a r e u n a b l e t o p r e d i c t t h e c h a r g e fraction. The f i n a l mechanism f o r c l u s t e r e j e c t i o n i s e s s e n t i a l l y a h y b r i d mechanism i n v o l v i n g b o t h i n t a c t e j e c t i o n and r e c o m b i n a t i o n . I n t h e c a s e o f CO o n N i 3 F e , we f i n d t h a t t h e o b s e r v e d N i C O , Ni2CO



46

a n d N i F e C O c l u s t e r s f o r m by a r e c o m b i n a t i o n o f e j e c t e d N i a n d Fe a t o m s w i t h e j e c t e d CO m o l e c u l e s . T h e r e i s a p p a r e n t l y no d i r e c t r e l a t i o n between t h e s e m o i e t i e s and l i n e a r and b r i d g e - b o n d s u r f a c e states. I n t h e c a s e o f c a t l o n i z e d s p e c i e s s u c h a s NiCfcHfc* i o n s , we propose a r e a c t i o n of the type


N

i

+

+

C

N

6«6

1

C

H

6 6

+

The p r e s u m p t i o n t h a t the N i s u p p l i e s t h e c h a r g e i s based on the f a c t t h a t no C ^ H ^ * i s o b s e r v e d ( 1 6 ) a n d t h a t t h e i o n i z a t i o n p o t e n t i a l of N i i s l e s s than that of benzene. T h i s f i n a l h y b r i d m e c h a n i s m may be r e s p o n s i b l e f o r t h e f o r m a t i o n o f t h e d i m e r i o n o f t h e d o d e c a n u c l e o t i d e (JL) o r o f w a t e r c l u s t e r s ( 1 7 ) . E a c h m o l e c u l a r u n i t e j e c t s i n t a c t and t h e n i n t e r a c t s w i t h o t h e r m o l e c u l e s i n the near s u r f a c e r e g i o n to form the c l u s t e r entities. I n the case o f (H20)2 c l u s t e r s our c a l c u l a t i o n s i n d i c a t e t h a t t h e two H2O m o l e c u l e s o r i g i n a t e f r o m m o s t l y a d j a c e n t s i t e s o n the surface ( 1 5 ) . T h i s i s a c o n s e q u e n c e o f t h e r e l a t i v e l y weak H2O-H2O i n t e r a c t i o n . I o n i c c l u s t e r s s u c h as ( H 2 0 ) H , h o w e v e r , c a n f o r m f r o m a n H2O m o l e c u l e a n d a n H"*" i o n t h a t w e r e f u r t h e r a p a r t o n the s u r f a c e . The f a c t t h a t t h e c o m p o s i t i o n o f t h e e j e c t e d c l u s t e r s may be d i f f e r e n t f r o m t h e o r i g i n a l a r r a n g e m e n t o f s u r f a c e atoms i s somewhat d i s c o u r a g i n g . As i t t u r n s o u t , h o w e v e r , t h e r e a r e s i t u a t i o n s w h e r e t h e p r e c i s e n a t u r e o f t h e r e a r r a n g e m e n t c a n be predicted theoretically. One e x a m p l e i n v o l v e s t h e m e a s u r e d O 2 ~ 7 0 " r a t i o as a f u n c t i o n o f o x y g e n c o v e r a g e o n N i ( O O l ) . This r a t i o i s f o u r t i m e s h i g h e r f o r 50 p e r c e n t o x y g e n c o v e r a g e [ c ( 2 x 2 ) c o v e r a g e ] t h a n f o r 25 p e r c e n t o x y g e n c o v e r a g e [ p ( 2 x 2 ) s u r f a c e ] , a change t h a t i s a l s o c a l c u l a t e d w i t h the model (18). The r e a s o n f o r t h i s e f f e c t i s t h a t t h e r e a r e no c l o s e l y n e i g h b o r i n g o x y g e n atoms o n t h e p ( 2 x 2 ) s u r f a c e , a n d t h e O2 f o r m a t i o n p r o b a b i l i t y i s much l o w e r . C o n c e p t s o f t h i s s o r t may be u s e f u l i n t e s t i n g f o r i s l a n d - g r o w t h mechanisms and d i s t i n g u i s h i n g them from t h o s e t h a t p r o c e e d t h r o u g h s e v e r a l d i s t i n c t p h a s e s . +

Energy

Distributions

T h e e n e r g y d i s t r i b u t i o n o f a t o m i c s p e c i e s e j e c t e d I n bombardment e x p e r i m e n t s a r e c h a r a c t e r i z e d by a p e a k a t l - 5 e V a n d a h i g h e n e r g y t a i l t h a t g o e s a p p r o x i m a t e l y as E " where n * 2 . This d i s t r i b u t i o n Is c h a r a c t e r i s t i c of a n o n - e q u i l i b r i u m c o l l i s i o n cascade. The e n e r g y d i s t r i b u t i o n s o f t h e p a r e n t m o l e c u l a r s p e c i e s a r e much n a r r o w e r , however, and o f t e n t e r m i n a t e a t ~ 1 0 e V . Shown i n F i g u r e l a a r e e x p e r i m e n t a l e n e r g y d i s t r i b u t i o n s f o r A g , C6H5"*" a n d C 2 H 2 i o n s e j e c t e d from a s y s t e m w i t h a monolayer o f benzene adsorbed on a A g ( l l l ) c r y s t a l face (19). S i n c e the m o l e c u l a r s p e c i e s i s e j e c t i n g d u r i n g t h e same c o l l i s i o n c a s c a d e as t h e A g i o n s a n d o n t h e same t i m e s c a l e o n e w o u l d e x p e c t t h e d i s t r i b u t i o n o f c o l l i s i o n e n e r g i e s t h a t c a u s e e j e c t i o n t o be t h e same f o r t h e A g a t o m s a n d n

+

+

+


2.

GARRISON

41


t h e CfcHfc m o l e c u l e s . However, the e n e r g e t i c c o l l i s i o n s w i t h the m o l e c u l a r s p e c i e s c a n a n d do c a u s e f r a g m e n t a t i o n . Thus t h e e n e r g e t i c benzene m o l e c u l e s a r e d e p l e t e d . The fragments then s h o u l d h a v e a d i s t r i b u t i o n a t h i g h e r e n e r g i e s a s i s i l l u s t r a t e d by t h e C2H2"*" f r a g m e n t e n e r g y d i s t r i b u t i o n s h o w n i n F i g u r e l a . N o t e t h a t t h e peak o f the C 2 H 2 d i s t r i b u t i o n i s a t a h i g h e r energy t h a n t h a t o f t h e C5H5"*" d i s t r i b u t i o n . S i n c e t h e peak p o s i t i o n c a n be c o r r e l a t e d to the b i n d i n g energy of the s p e c i e s to the s u r f a c e , the p e a k o f t h e C 2 H 2 d i s t r i b u t i o n s h o u l d be h i g h e r s i n c e i t s b i n d i n g e n e r g y i n c l u d e s two C - C b o n d e n e r g i e s . The e n e r g y d i s t r i b u t i o n s f r o m t h e c a l c u l a t i o n s ( F i g u r e l b ) i l l u s t r a t e t h e same p h y s i c a l phenomena. I t i s tempting to use the energy d i s t r i b u t i o n s o f the e j e c t e d p a r t i c l e s as a k e y t o u n d e r s t a n d i n g t h e mechanisms r e s p o n s i b l e f o r the d e e o r p t i o n . C a r e must be t a k e n , h o w e v e r , as c o l l i s i o n cascades can g i v e r i s e to at l e a s t t h r e e d i s t i n c t i v e shapes o f energy d i s t r i b u t i o n s a s s h o w n i n F i g u r e 1. (The c a l c u l a t i o n s a l s o p r e d i c t t h e d i s t r i b u t i o n o f m e t a l atoms t o have a h i g h e n e r g y t a i l . ) In f a c t t h e c a l c u l a t e d C5H5 d i s t r i b u t i o n o f F i g u r e l b c a n b e r e a s o n a b l y a p p r o x i m a t e d by a M a x w e l l - B o l t z m a n n f o r m e v e n though a thermal e q u i l i b r i u m i s not present i n the s o l i d during the e j e c t i o n event. The c a l c u l a t i o n s i n d i c a t e t h a t e n e r g y d i s t r i b u t i o n s o f e l e m e n t a l (and p r e f e r a b l y b o t h t h e n e u t r a l and charged) species c o u l d p o s s i b l y be t h e m o s t u s e f u l f o r c o m p a r i n g t o t h e d i f f e r e n t e x p e r i m e n t a l mechanisms as t h e s e p a r t i c l e s c a n n o t be f r a g m e n t e d i n energetic c o l l i s i o n s . Even h e r e , h o w e v e r , one c a n o b t a i n energy d i s t r i b u t i o n s f r o m SIMS e x p e r i m e n t s t h a t f a l l o f f more r a p i d l y t h a n E" i f l o w e n e r g y ( 6 i n t h e c a l c u l a t i o n s i s due t o t h e low d e n s i t y o f benzene m o l e c u l e s on the N l s u r f a c e .

American Chemical Society Library 1155 16th N.W. Lyon; Desorption Mass St., Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985. Washington, D.C. 2003S

Lyon; Desorption Mass Spectrometry ACS Symposium Series; American Chemical Society: Washington, DC, 1985. +

Energy (eV)

F i g u r e . 1. E n e r g y D i s t r i b u t i o n s , a ) E x p e r i m e n t a l Ag , C ^ h ^ * a n d C2H2"*" i o n i n t e n s i t i e s f r o m o n e m o n o l a y e r o f b e n z e n e adsorbed on A g ( l l l ) p l o t t e d versus the v o l t a g e on the sample. The p r i m a r y p a r t i c l e I s A r w i t h energy 1 keV i n c i d e n t on t h e sample at a p o l a r angle of 4 5 ° · The s e c o n d a r y p a r t i c l e s a r e c o l l e c t e d a t a p o l a r a n g l e o f 6 0 ° . The raw d a t a has been smoothed. T h i s d a t a h a s b e e n g r a c i o u s l y p r o v i d e d by D . W. Moon, R. J . B l e i l e r and N . Winograd p r i o r t o p u b l i c a t i o n , b) C a l c u l a t e d C5H5 a n d C2H2 e n e r g y d i s t r i b u t i o n s f r o m o n e monolayer o f benzene on N i ( 0 0 1 ) . A l l e j e c t e d p a r t i c l e s have been counted. The energy r e s o l u t i o n i s 5 e V . The c a l c u l a t i o n s are described i n r e f . 9. Reproduced w i t h p e r m i s s i o n from R e f . 1 2 . Copyright 1983, E l s e v i e r Science Publishing Co.

Bias voltage (eV)


2.


GARRISON

100% -

49

Ni

C H 6

e

χ 1/2 H

CH


il

NL

C5H5 1

100% -

Mass

100%-

NiC H * e

e

100%-

Mass F i g u r e 2 · B e n z e n e mass s p e c t r a . The most i n t e n s e peak i n e a c h g r o u p i n g has been i d e n t i f i e d , a) C a l c u l a t e d , (9). b) E x p e r i m e n t a l SIMS, 3 l a n g m u i r s o f ^ Η on N i ( O O l ) , ( 1 6 ) . c ) E x p e r i m e n t a l SIMS, 2100 l a n g m u i r s o f C H o n N i ( O O l ) , (JL6). d ) E x p e r i m e n t a l SIMS, s o l i d benzene ( 1 7 ) . Reproduced w i t h p e r m i s s i o n from R e f . 1 2 . C o p y r i g h t 1933, E l s e v i e r S c i e n c e Publishing Co. 6

6

6



50


I t i s o b v i o u s from F i g u r e 2b-d t h a t the sample p r e p a r a t i o n s t r o n g l y a f f e c t s t h e mass s p e c t r u m . The l o w c o v e r a g e s t u d y a p p e a r s t o b e t h e o n e w h e r e t h e p a r e n t s p e c i e s c a n b e most e a s i l y i d e n t i f i e d a s l o n g a s t h e r e i s a n e n e r g e t i c a l l y f a v o r a b l e means o f ionization, e.g., cationizatlon. The s o l i d b e n z e n e s t u d i e s a r e i n t e r e s t i n g i n t h a t a v a r i e t y of l a r g e c l u s t e r s are observed. H o w e v e r , i f t h e s a m p l e w e r e o f a n u n k n o w n c o m p o u n d , i t w o u l d be d i f f i c u l t to e x t r a c t the parent s p e c i e s from F i g u r e 2d. The c a l c u l a t e d s p e c t r u m ( F i g u r e 2a) p r e d i c t s the p a r e n t m o l e c u l e , C5H5, t o be t h e m o s t a b u n d a n t o r g a n i c s p e c i e s . The c o m p a r a b l e e x p e r i m e n t a l d a t a , F i g u r e 2 b , h o w e v e r , h a s no C5H5"*" p e a k b u t a l a r g e NiC^H^"*" p e a k . Here t h e n , the e l e c t r o n i c environment influences which species are observed. Molecular

Orientation Effects;

Benzene v s .

Pyridine

I t i s o f i n t e r e s t to compare the e j e c t i o n mechanisms f o r m o l e c u l e s bonded to the s u r f a c e w i t h d i f f e r e n t o r i e n t a t i o n s . In benzene, the i n t e r a c t i o n w i t h t h e s u r f a c e i s s h a r e d among s i x c a r b o n a t o m s v i a the π - e l e c t r o n c l o u d . In p y r i d i n e , however, the bonding occurs a l m o s t t o t a l l y t h r o u g h the n i t r o g e n atom w h i l e the remainder of the m o l e c u l e i s p o i n t i n g away f r o m t h e s u r f a c e . The m o s t s t r i k i n g d i f f e r e n c e b e t w e e n t h e two c a s e s i s t h a t t h e c o m p u t e d y i e l d o f m o l e c u l a r s p e c i e s f o r the p y r i d i n e system i s e x t r e m e l y low ( 9 ) . T h e r e a s o n s f o r t h e m a j o r d i f f e r e n c e i n y i e l d s f o r t h e s e two s t r u c t u r e s i s c l e a r from an a n a l y s i s o f the t r a j e c t o r i e s t h a t l e a d to molecular e j e c t i o n of p y r i d i n e . Very s i m p l y , p y r i d i n e e j e c t i o n r e q u i r e s t h e s p e c i f i c c l e a v a g e o f a N - N i bond d u r i n g a s i n g l e collision. When a c a r b o n a t o m i s s t r u c k , t h e m o l e c u l e e i t h e r stays on the s u r f a c e or tends to d i s s o c i a t e . T h e r e a p p e a r s t o be no e f f i c i e n t modes o f t r a n s f e r r i n g t h e e n e r g y o f c o l l i s i o n s w i t h t h e m o l e c u l e i n t o t r a n s l a t i o n away f r o m t h e s u r f a c e . O b v i o u s l y the o r i g i n a l s t r u c t u r e of the o r g a n i c m o l e c u l e s , then, a f f e c t s the e j e c t i o n and f r a g m e n t a t i o n p r o c e s s e s . One w o u l d n o t n e c e s s a r i l y expect s i m i l a r s p e c t r a from a sample of a monolayer of o r g a n i c m o l e c u l e s on a m e t a l , a l i q u i d , o r an o r d e r e d s o l i d . These o r i e n t a t i o n a l e f f e c t s have r e c e n t l y been c o n f i r m e d i n SIMS m e a s u r e m e n t s o f p y r i d i n e a n d b e n z e n e a d s o r b e d o n A g ( l l l ) ( 2 2 ) . I n t h i s system the benzene π - b o n d s to the s u r f a c e w h i l e the p y r i d i n e π - b o n d s at low coverages but r e a r r a n g e s at h i g h e r coverages to σ - b o n d to the s u r f a c e . T h e i n t e n s i t y o f t h e AgC^H^"*" i o n m o n o t o n i c a l l y i n c r e a s e s as t h e b e n z e n e c o v e r a g e on t h e s i l v e r s u r f a c e i s i n c r e a s e d to one m o n o l a y e r . The A g C 5 H 5 N a n d C5H5NH"*" i o n i n t e n s i t i e s , h o w e v e r , i n i t i a l l y i n c r e a s e and t h e n d e c r e a s e as t h e m o l e c u l e r e a r r a n g e s on the s u r f a c e , and f i n a l l y i n c r e a s e a g a i n as t h e p y r i d i n e c o v e r a g e i s i n c r e a s e d to one m o n o l a y e r . The arrangement o f the m o l e c u l e s on the s u r f a c e a l s o I n f l u e n c e s the a n g u l a r d i s t r i b u t i o n s of the e j e c t e d s p e c i e s ( 2 2 ) . The p o l a r a n g l e d i s t r i b u t i o n s o f v a r i o u s e j e c t e d i o n s f o r f o u r systems - 2 . 5 L benzene ( m o n o l a y e r ) , 4 . 5 L p y r i d i n e ( m o n o l a y e r , σ - b o n d e d ) 0 . 1 5 L p y r i d i n e ( π - b o n d e d ) , a n d 12 L t h i o p h e n e ( m o n o l a y e r ) on A g ( l l l ) have been measured. The r e s u l t s o f t h e s e d i s t r i b u t i o n measurements are i l l u s t r a t e d i n F i g u r e 3. For +


2.

GARRISON

51


m o n o l a y e r benzene and f o r low c o v e r a g e p y r i d i n e where t h e m o l e c u l e s are b e l i e v e d to l i e f l a t on A g ( l l l ) , the polar angle d i s t r i b u t i o n o f ( M - H ) ( b e n z e n e ) a n d (M+H)+ ( p y r i d i n e ) a r e b r o a d w i t h a p e a k a t θ - 2 0 ° measured w i t h r e s p e c t t o the s u r f a c e n o r m a l . At the onset of t h e change i n bonding c o n f i g u r a t i o n , however, the p o l a r a n g l e d i s t r i b u t i o n o f t h e C5H5NH i o n s h a r p e n s d r a m a t i c a l l y and t h e peak moves t o θ 1 0 ° · I t appears that the array of σ-bonded p y r i d i n e m o l e c u l e s p r o v i d e s a means o f f o c u s i n g t h e d i r e c t i o n o f e j e c t i o n o f the p y r i d i n e m o l e c u l e s . F u r t h e r , the p o l a r angle d i s t r i b u t i o n of the h i g h k i n e t i c energy i o n s (6-10eV) e j e c t e d from the σ - b o n d e d p y r i d i n e s t r u c t u r e i s 20-30% w i d e r t h a n t h e d i s t r i b u t i o n o f t h e l o w k i n e t i c e n e r g y i o n s ( 3 - 7 e V ) as i s shown i n F i g u r e l b . T h i s t r e n d toward wider p o l a r angle d i s t r i b u t i o n s f o r f a s t e r moving p a r t i c l e s i s c o u n t e r to t h a t o b s e r v e d f o r atom e j e c t i o n . The p o l a r a n g l e d i s t r i b u t i o n o f t h i o p h e n e , i s narrow w i t h a peak a t θ - 1 0 ° C , i n d i c a t i n g that i t also i s ο-bonded to the surface. +

+


β

In t h i s case i t appears that the σ-bonded p y r i d i n e molecules are channeling the e j e c t i n g p y r i d i n e molecules into the v e r t i c a l d i r e c t i o n ( 2 3 ) . One e x a m p l e o f how t h i s b l o c k i n g c a n s i g n i f i c a n t l y a f f e c t the t r a j e c t o r y o f an e j e c t i n g p y r i d i n e molecule i s i l l u s t r a t e d i n Figure 4. O n l y t h e s p e c i e s ( o n e A r i o n a n d two pyridine molecules) d i r e c t l y involved In this p a r t i c u l a r ejection p r o c e s s a r e shown. I n t h i s e x a m p l e t h e m e t a l s u b s t r a t e p l a y s no d i r e c t role i n ejecting the molecule. The g r i d l i n e s a r e drawn b e t w e e n t h e n e a r e s t - n e i g h b o r atoms i n t h e f i r s t p l a n e o f t h e mlcrocryetallite. The e l a p s e d t i m e d u r i n g t h e c o l l i s i o n p r o c e s s i s shown I n f s (1 f s - l x l O " ^ s ) . The i n i t i a l p o s i t i o n s o f t h e a t o m s a r e d r a w n i n F i g u r e 2 ( 0 f s ) . A t 33 f s t h e A r I o n , w h i c h has b a c k s c a t t e r e d from the s u r f a c e , I s c o l l i d i n g w i t h 3 c a r b o n atoms I t h e t a r g e t p y r i d i n e m o l e c u l e . The k i n e t i c e n e r g y o f t h e c e n t e r o f mass o f t h i s p y r i d i n e m o l e c u l e i s 1 1 . 6 eV a n d i t s m o l e c u l a r a x i s i s o r i e n t e d at a p o l a r angle o f 6=66° from the surface normal. A t 85 f s t h e e j e c t i n g p y r i d i n e m o l e c u l e c o l l i d e s w i t h a n e i g h b o r i n g p y r i d i n e m o l e c u l e and d i s s i p a t e s a f r a c t i o n o f i t s momentum. At the f i n a l stage o f the s p u t t e r i n g process (120 f s ) , the p y r i d i n e m o l e c u l e e j e c t s m o l e c u l a r l y , even though d i s t o r t e d , a t a p o l a r a n g l e o f 9 - 3 1 ° w i t h 1.40 eV o f k i n e t i c energy. Due t o t h e b l o c k i n g b y a n e i g h b o r i n g p y r i d i n e m o l e c u l e , the p o l a r angle of the e j e c t e d p y r i d i n e molecule i s a l t e r e d from 6 6 ° t o 3 1 ° . The w a l l s c r e a t e d by p y r i d i n e m o l e c u l e s a r e n o t c o m p l e t e l y r i g i d as i n d i c a t e d by t h e d i s t o r t e d m o l e c u l e shown o n t h e l e f t i n t h e 120 f s f r a m e . Therefore, a pyridine molecule e j e c t i n g w i t h a l a r g e k i n e t i c energy w i l l n o t f e e l a s t r o n g enough f o r c e t o c h a n n e l i t c o m p l e t e l y i n t o t h e upward d i r e c t i o n . The p o l a r angle d i s t r i b u t i o n o f the h i g h energy e j e c t e d p a r t i c l e s i s thus broader than t h a t of the low energy e j e c t e d p a r t i c l e s . This mechanism i s d i s t i n c t from t h a t found w i t h atom e j e c t i o n . In this l a t t e r case, the energy dependence o f the a z i m u t h a l d i s t r i b u t i o n i s r e l a t e d t o t h e t i m e o f e j e c t i o n a n d c o n s e q u e n t l y t o t h e amount o f s u r f a c e s t r u c t u r e p r e s e n t when t h e a t o m e j e c t s . Note t h a t f o r the π - b o n d e d b e n z e n e s y s t e m , t h e r e a r e no c h a n n e l s t o o r i e n t t h e ejecting molecules. +

1

+




τ — I — I — I — I — I — I — I — Γ

θ (degrees) F i g u r e 3· Normalized p o l a r angle d i s t r i b u t i o n of molecular i o n y i e l d s for 4.5 L p y r i d i n e ( , (M+H) ), 0.15 L p y r i d i n e ( , ( M + H ) + ) , 2 . 5 L b e n z e n e ( · · · · , ( M - H ) ) , a n d 12 L t h i o p h e n e ( - · - · - · - · - , M+) o n A g ( l l l ) a t 1 5 3 K . The p y r i d i n e and benzene d a t a i s f r o m (22) and t h i o p h e n e d a t a has been s u p p l i e d by t h e same a u t h o r s . +

+

F i g u r e 4. Change o f t h e e j e c t i o n a n g l e o f a s p u t t e r e d p y r i d i n e m o l e c u l e ( r i g h t o n e ) due t o t h e b l o c k i n g by a n e i g h b o r i n g pyridine molecule ( l e f t one). The l a b e l s a r e i n f s w h e r e 1 f s lxlO" second. (0 f s ) I n i t i a l p o s i t i o n s o f the a t o m s . (33 f s ) The b a c k s c a t t e r e d A r i o n c o l l i d e s and e j e c t s the p y r i d i n e m o l e c u l e at a p o l a r a n g l e of 9 - 6 6 ° . (85 f s ) The e j e c t i n g p y r i d i n e m o l e c u l e I s b l o c k e d by a n e i g h b o r i n g p y r i d i n e m o l e c u l e . (120 f s ) F i n a l l y , the e j e c t i o n p o l a r a n g l e i s changed to θ - 3 1 ° . B o t h t h e s p u t t e r e d m o l e c u l e and t h e b l o c k i n g m o l e c u l e a r e 1 5

+

distorted. Reproduced w i t h p e r m i s s i o n from Ref. 2 3 . 1985, E l s e v i e r S c i e n c e P u b l i s h i n g Co.


Copyright

2.

GARRISON

53


Fragmentation There has been c o n s i d e r a b l e s p e c u l a t i o n as t o whether the observed fragments form p r i m a r i l y from d i r e c t c o l l i s i o n s at the surface (i.e. w i t h i n ~ 0 . 2 χ 10~*2 f t e r the primary p a r t i c l e has s t r u c k ) or from d i s s o c i a t i o n of l a r g e r species d u r i n g the f l i g h t to the d e t e c t o r ( o f t e n as l o n g as 1 0 " ^ s ) . T h e c a l c u l a t i o n s show t h a t i t i s d e f i n i t e l y p o s s i b l e t o f o r m numerous f r a g m e n t s i n d i r e c t c o l l i s i o n s a t t h e s u r f a c e ( F i g u r e 2 a ) . F r o m t h e c a l c u l a t i o n s we have e s t i m a t e d t h a t a p p r o x i m a t e l y t h r e e q u a r t e r s o f the e j e c t e d b e n z e n e m o l e c u l e s h a v e l e s s t h a n 5 eV o f i n t e r n a l e n e r g y ( 9 ) . There i s a reasonable p r o b a b i l i t y that these v i b r a t i o n a l l y c o l d e r m o l e c u l e s w i l l r e m a i n i n t a c t . The e n e r g e t i c m o l e c u l e s , on t h e o t h e r hand, w i l l undoubtedly d i s s o c i a t e .


8

a

At t h i s stage i t i s necessary to d e s i g n c l e v e r experiments o r t h e o r e t i c a l approaches to help e l u c i d a t e the d i f f e r e n t p o s s i b l e modes o f f r a g m e n t a t i o n . R e c e n t l y Moon ( 2 4 ) h a s p r o p o s e d a m e t h o d o f e x a m i n i n g t h e p o l a r a n g l e d i s t r i b u t i o n s a s a means o f d i f f e r e n t i a t i n g b e t w e e n t h e f r a g m e n t a t i o n s c h e m e s . He f i n d s t h a t for chlorobenzene adsorbed on A g ( l l l ) that the C ^ l ^ * and C l ' i o n s p r o b a b l y f o r m by d i r e c t c o l l i s i o n s o n t h e s u r f a c e . For the c h l o r o b e n z e n e as w e l l as benzene and p y r i d i n e adsorbed o n s i l v e r , he found t h a t n e i t h e r m o l e c u l a r o r fragment i o n s formed by gas phase decomposition o f a c a t i o n i z e d s p e c i e s . In the case o f t h e a l k a l i h a l l d e c l u s t e r s ( 1 4 ) , r e c e n t work h a s shown t h a t t h e o s c i l l a t i o n s I n i o n i n t e n s i t y w i t h c l u s t e r s i z e a r e due t o d i s s o c i a t i o n o f m e t a s t a b l e s p e c i e s d u r i n g t h e f l i g h t t o t h e d e t e c t o r ( 2 5 ) . S p e c t r a t a k e n 0 . 2 p s a f t e r bombardment e x h i b i t a monotonie decrease i n i o n i n t e n s i t y w i t h i n c r e a s i n g c l u s t e r s i z e . S p e c t r a t a k e n a f t e r 7 0 u s , h o w e v e r , show a n i n c r e a s e i n t h e ( C s I ) i 3 C s i o n i n t e n s i t y and a d e c r e a s e i n the ( C e I ) i 4 C s and (CsI)i5Ce ion intensities. Here then, decomposition o f l a r g e r s p e c i e s d u r i n g the f l i g h t to the d e t e c t o r has a n o t i c e a b l e e f f e c t on t h e c l u e t e r y i e l d s . T h e s e e x p e r i m e n t s t h o u g h make n o s t a t e m e n t a s t o how t h e c l u s t e r s a r e i n i t i a l l y f o r m e d n e a r t h e s u r f a c e . +

+

+

Closing

Statements

A c l a s s i c a l dynamics model has been d e v e l o p e d t o i n v e s t i g a t e t h e i m p o r t a n c e o f c o l l i s i o n a l p r o c e s s e s i n heavy p a r t i c l e bombardment experiments. This procedure i s very powerful f o r d e s c r i b i n g c o l l i s i o n a l events and provides a working hypothesis against which e x p e r i m e n t a l d a t a c a n be c o m p a r e d . We h a v e s h o w n n u m e r o u s e x a m p l e s f r o m SIMS e x p e r i m e n t s w h e r e t h e c a l c u l a t i o n s h a v e f i t e x p e r i m e n t a l data very w e l l . T h e t i m e h a s come f o r t h e e x p e r i m e n t a l i s t s t o c o n c e i v e and execute experiments aimed at u n c o v e r i n g t h e f u n d a m e n t a l p r o c e s s e s I n v o l v e d I n t h e SIMS a n d FABMS p r o c e d u r e s . I t s h o u l d be n o t e d t h a t v a r i o u s r e s e a r c h e r s have d i f f e r e n t g o a l s f o r u s i n g and u n d e r s t a n d i n g t h e i o n bombardment process. T h e r e a r e t h o s e who a r e u s i n g t h e t e c h n i q u e t o o b t a i n i n f o r m a t i o n about a m o l e c u l e t h a t they have p l a c e d o n t h e s u r f a c e . That i s , t h e y w a n t a mass a n d p o s s i b l y a s t r u c t u r e d e t e r m i n a t i o n . Other researchers are p r i m a r i l y concerned w i t h determining the elemental c o m p o s i t i o n o f t h e sample w h i l e o t h e r s use the t e c h n i q u e t o measure


54


t h e g e o m e t r i c a l a r r a n g e m e n t o f t h e atoms and m o l e c u l e s on t h e surface. Another area of i n t e r e s t i s to probe the e l e c t r o n i c p r o c e s s e s i n v o l v e d when a n atom o r m o l e c u l e i s i n t h e n e a r s u r f a c e region. Although these goals are q u i t e v a r i e d the fundamental processes are intermingled. To u n d e r s t a n d o u r own a r e a o f i n t e r e s t we n e e d t o u n d e r s t a n d a l l o f t h e e x p e r i m e n t a l r e s u l t s a n d t h e d e t a i l e d events o c c u r r i n g on the m i c r o s c o p i c l e v e l .


Acknowledgment T h e i n t e r a c t i o n w i t h t h o s e who h a v e s u p p l i e d t h e e x p e r i m e n t a l d a t a » D . W. M o o n , R . J . B l e i l e r , E . J . K a r w a c k i a n d N . W i n o g r a d , h a s g r e a t l y h e l p e d i n s o l i d i f y i n g many o f t h e i d e a s p r e s e n t e d h e r e . I t h a n k t h e m f o r a l l o w i n g me t o u s e t h e i r d a t a a s w e l l a s f o r many stimulating conversations. The f i n a n c i a l s u p p o r t o f t h e N a t i o n a l S c i e n c e F o u n d a t i o n , the O f f i c e o f N a v a l R e s e a r c h and the C a m i l l e and Henry D r e y f u s F o u n d a t i o n i s g r a t e f u l l y a c k n o w l e d g e d .

Literature Cited

1. McNeal, C. J.; Macfarlane, R. D. J. Am. Chem. Soc. 1981, 103, 1609. 2. Orth, R. G.; Jonkman, H. T.; Michl, J. J. Am. Chem. Soc. 1981, 103, 1564. 3. Garrison, B. J.; Winograd, N.; Harrison Jr., D. E. J. Chem. Phys. 1978, 69, 1440. 4. Winograd, N.; Harrison Jr., D. E.; Garrison, B. J. Surface Science 1978, 78, 467. 5. Garrison, B. J.; Winograd, N.; Harrison Jr., D. E. Phys. Rev. Β 1978, 18 6000. 6. Winograd, N.; Garrison, B. J.; Harrison Jr., D. E. J. Chem. Phys. 1980, 73, 3473. 7. Foley, Κ. E.; Garrison, B. J. J. Chem. Phys. 1980, 72, 1018. 8. Garrison, B. J. J. Am. Chem. Soc. 1980, 102, 6553. 9. Garrison, B. J. J. Am. Chem. Soc. 1982, 104, 6211. 10. Garrison, B. J. J. Am. Chem. Soc. 1983, 105, 373. 11. Winograd, N.; Garrison, B. J. Accts. of Chem. Res. 1980, 13, 406. 12. Garrison, B. J.; Winograd, N. Science 1982, 216, 805; Garrison, B. J. Int. J. Mass Spec. and Ion Phys. 1983, 53, 243. 13. Garrison, B. J.; Winograd, N. Chem. Phys. Lett. 1983, 97, 381. 14. Barlak, T. M.; Wyatt, J. R.; Colton, R. J.; Decorpo, J. J.; Campana, J. E.; Campana, J. E.. J. Am. Chem. Soc. 1982, 104, 1212. 15. Brenner, D. W.; Garrison, B. J. unpublished results. 16. Karwacki, E. J.; Winograd, N. Anal. Chem. 1983, 55, 79. 17. Lancaster, G. M.; Honda, F.; Fukuda, Y.; Rabalais, J. W. J. Am. Chem. Soc. 1979, 101, 1951. 18. Winograd, N.; Garrison, B. J.; Fleisch, T.; Delgass, W. N.; Harrison Jr., D. E. J. Vac. Sci. Tech. 1979, 16, 629. 19. Moon, D. W.; Bleiler, R. J.; Winograd, N. unpublished results.


2. GARRISON


20. Hart, R. G.; Cooper, C. B. Surf. Sci. 1979, 82, 549; Karwacki, E. J. Ph.D. thesis, The Pennsylvania State University, 1982. 21. Jonkman, H. T.; Michl, J.; King, R. N.; Andrade, J. D. Anal. Chem. 1978, 50 2078. 22. Moon, D. W.; Bleiler, R. J.; Karwacki, E. J.; N. Winograd. J. Am. Chem. Soc. 1983, 105, 2916. 23. Moon, D. W.; Winograd, N.; Garrison, B. J. Chem. Phys. Lett., in press. 24. Moon, D. W.; Winograd, N. Int. J. Mass Spec. and Ion Phys. 1983, 52, 217. 25. Ens, W.; Beavis. R.; Standing, K. G. Phys. Rev. Lett. 1983, 50, 27. Downloaded by UNIV OF IOWA on September 2, 2016 | http://pubs.acs.org Publication Date: October 17, 1985 | doi: 10.1021/bk-1985-0291.ch002

RECEIVED

August

2 3 , 1985


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