INNER-SHELL PROCESSES AS PROBES OF MULTICHARGED ION NEUTRALIZATION AT SURFACES

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INNER-SHELL PROCESSES AS PROBES OF

MULTICHARGED ION NEUTRALIZATION AT

SURFACES

F. Meyer, C. Havener, S. Overbury, K. Reed, K. Snowdon, D. Zehner

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au nOl; Tome 50, janvier 1989

INNER-SHELL PROCESSES AS PROBES OF MULTICHARGED ION NEUTRALIZATION AT SURFACES

F.W. MEYER, C.C. HAVENER, S.H. OVERBURY, K.J. REED(^), K.J. SNOW DON(^) and D.M. ZEHNER

Oak Ridge National laboratory, Oak Ridge, TN 37831-6372, U.S.A.

A b s t r a c t

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We have measured energy d i s t r i b u t i o n s f o r e l e c t r o n s e m i t t e d d u r i n g grazing c o l l i s i o n s o f m u l t i c h a r g e d i o n s w i t h Au and Cu s i n g l e c r y s t a l s i n t h e p r o j e c t i l e energy range 20-90 keV. The measured e l e c t r o n spectra a r e c h a r a c t e r i z e d by d i s c r e t e peaks r e s u l t i n g from Auger t r a n s i t i o n s , superimposed on a broad continuum e l e c t r o n d i s t r i b u - t i o n . Auger e l e c t r o n emission from b o t h p r o j e c t i l e and t a r g e t atoms has been observed. The p r o j e c t i l e Auger e l e c t r o n s r e s u l t from t h e decay o f inner-she1 1 vacancies e i t h e r c a r r i e d i n t o t h e c o l l i s i o n o r produced b y vacancy t r a n s f e r from empty o u t e r p r o j e c t i l e l e v e l s v i a pseudocrossings o r r o t a t i o n a l c o u p l i n g o f molecular o r b i t a l s . The l a t t e r mechanism may a l s o l e a d t o t h e c r e a t i o n o f t a r g e t i n n e r - s h e l l vacancies which g i v e r i s e t o Auger e l e c t r o n emission c h a r a c t e r i s t i c o f t h e t a r g e t . An a n a l y s i s o f these Auger e l e c t r o n f e a t u r e s i s presented which i s aimed, on t h e one hand, a t e l u c i d a t i n g d e t a i l e d p r o d u c t i o n mechanisms, and, on t h e other, a t o b t a i n i n g i n f o r m a t i o n on the time s c a l e s d e s c r i b i n g t h e n e u t r a l i z a t i o n o f m u l t i c h a r g e d i o n s d u r i n g t h e i r i n t e r a c t i o n w i t h metal surfaces.

1

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INTRODUCTION

The study o f m u l t i c h a r g e d ion-surface i n t e r a c t i o n s has received s i g n i f i c a n t a t t e n t i o n i n recent years. One reason f o r t h e increased i n t e r e s t i s t h e l a r g e n e u t r a l i z a t i o n p o t e n t i a l energy t h a t t h e mu1 t i c h a r g e d p r o j e c t i l e s c a r r y i n t o t h e c o l l is i o n . The d e t a i l e d mechanisms by which t h e d i s s i p a t i o n o f t h i s p o t e n t i a l energy a f f e c t s i o n n e u t r a l i z a t i o n , s p u t t e r i n g , or p a r t i c l e r e f l e c t i o n a r e s t i l l incompletely understood, making t h i s a f e r t i l e area o f experimental as w e l l as t h e o r e t i c a l i n v e s t i g a t i o n .

One way o f i n v e s t i g a t i n g t h e n e u t r a l i z a t i o n o f m u l t i c h a r g e d i o n s d u r i n g i n t e r a c t i o n s w i t h surfaces i s through t h e measurement o f e l e c t r o n s e m i t t e d d u r i n g t h e i n t e r a c t i o n . Multicharged i o n n e u t r a l i z a t i o n and e l e c t r o n emission a r e i n f a c t c l o s e l y r e l a t e d , as evidenced by the l a r g e f r a c t i o n o f t h e p o s s i b l e n e u t r a l i z a t i o n processes (see Refs. 1 and 2 f o r reviews) t h a t a l s o r e s u l t i n e l e c t r o n emission. I n o r d e r f o r complete n e u t r a l i z a t i o n (i.e., complete d i s s i p a t i o n o f t h e mu1 t i c h a r g e d i o n ' s i n i t i a l p o t e n t i a l energy) t o occur, long i n t e r a c t i o n times w i t h t h e s u r f a c e a r e required, which can be a t t a i n e d by t h e use o f very low p r o j e c t i l e k i n e t i c energies. T h i s approach has been used by de Zwart 131, Varga e t a l . /4,5/, and by Delaunay e t a l . 16-91, who have i n v e s t i g a t e d ' p o t e n t i a l " e l e c t r o n emission f o r i o n s i n c i d e n t on metal surfaces a t normal o r c l o s e t o normal angles w i t h energies as low as 6 eV. A t ORNL, measurements 110-131 o f e l e c t r o n emission have been performed f o r higher-energy multicharged

i o n s ( t y p i c a l l y lOxq keV) i n c i d e n t on metal surfaces a t g r a z i n g angles ( t y p i c a l l y 5'). By

(l)~awrence Livermore National Laboratory. Livermore. CA 94550, U.S.A.

("permanent Address : Department of Physics; University of OsnabrUck, D-4500 OsnabrUck. F.R.G.

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Cl-264 JOURNAL

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t h e use o f such small angles o f incidence, s i m i l a r long i n t e r a c t i o n times w i t h t h e surface can be a t t a i n e d . By v a r y i n g t h e angle o f incidence, and thereby t h e p e r p e n d i c u l a r component o f p r o j e c t i l e v e l o c i t y , t h e i n t e r a c t i o n times can be v a r i e d over a l a r g e range. Furthermore, by v a r y i n g b o t h t h e p r o j e c t i l e energy. and angle o f incidence, t h e p e n e t r a t i o n depth o f the i o n s can be varied; s p e c i f i c a l l y , t h e regime o f specular r e f l e c t i o n can be approached, i n which e s s e n t i a l l y no p e n e t r a t i o n o f t h e surface occurs. I n t h i s regime, e l e c t r o n emission above t h e s u r f a c e can be s t u d i e d i n i s o l a t i o n , i.e., w i t h o u t t h e complications a r i s i n g from e l e c t r o n emission below t h e t a r g e t surface. A t t h e h i g h e r p r o j e c t i l e energies used i n the g r a z i n g incidence measurements, i n n e r - s h e l l b i n a r y - c o l l i s i o n processes become p o s s i b l e during t h e ion-surface i n t e r a c t i o n , which can be used as probes o f t h e n e u t r a l i z a t i o n process. At t h e same time, however, i n t e r p r e t a t i o n o f t h e measured e l e c t r o n energy d i s t r i b u t i o n s becomes more d i f f i c u l t , s i n c e a t these energies I 1 k i n e t i c l 1 e l e c t r o n emission processes 1141 may make s i g n i f i c a n t c o n t r i b u t i o n s t o t h e t o t a l e l e c t r o n y i e l d . The c o n t r i b u t i o n o f " k i n e t i c u emission i s c o n f i n e d t o t h e p r o d u c t i o n o f low energy continuum e l e c t r o n s (energies <50 eV). Consequently, t h e d i s c u s s i o n below w i l l be r e s t r i c t e d t o t h e d i s c r e t e f e a t u r e s i n the measured e l e c t r o n energy d i s t r i b u t i o n s observed a t h i g h e r e l e c t r o n energies, where " k i n e t i c N

emission e f f e c t s a r e expected t o be n e g l i g i b l e . S p e c i f i c a l l y , a q u a l i t a t i v e d i s c u s s i o n o f t h e observed t a r g e t and p r o j e c t i l e Auger f e a t u r e s w i l l be presented, based on molecular o r b i - t a l (MO) energy l e v e l c a l c u l a t i o n s . I n a d d i t i o n , a comparative a n a l y s i s o f p r o j e c t i l e K-Auger f e a t u r e s w i l l be presented t o i l l u s t r a t e , again i n a q u a l i t a t i v e way, t h e use o f inner-she1 1 processes as probes o f t h e time scales over which n e u t r a l i z a t i o n o f mu1 t i c h a r g e d i o n s near surfaces occur. The d i s c u s s i o n w i l l i n p a r t be based on e l e c t r o n energy d i s t r i b u - t i o n measurements o b t a i n e d u s i n g an improved experimental apparatus, a b r i e f d e s c r i p t i o n o f which i s a l s o p r o v i d e d below.

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:TARGET AND PROJECTILE INNER-SHELL EXCITATION

F i g u r e 1 shows e l e c t r o n energy d i s t r i b u t i o n s measured f o r 60 keV N* i o n s i n c i d e n t on Au and Cu s i n g l e c r y s t a l s a t 5O u s i n g a l a r g e acceptance-angle c y l i n d r i c a l m i r r o r analyzer (CMA). D e t a l l s o f t h e experimental apparatus have been p r e v i o u s l y described 111,131. The Au t a r g e t e l e c t r o n s p e c t r a f o r H - l i k e and H e - l i k e i n c i d e n t i o n s a r e c h a r a c t e r i z e d by a broad f e a t u r e around 350 eV. As has been p r e v i o u s l y discussed 111-131, t h i s f e a t u r e i s due t o p r o j e c t i l e KLL t r a n s i t i o n s which f i l l t h e K - s h e l l vacancies c a r r i e d i n t o t h e c o l l i s i o n by each i n c i d e n t i o n ( i n t h e case o f t h e H - l i k e ions), o r by those i o n s i n t h e metastable 2s2 3S s t a t e ( i n the case o f t h e H e - l i k e ions). Note t h e absence o f t h i s KLL f e a t u r e f o r t h e L i - l i k e i n c i d e n t p r o j e c t i l e s . T u r n i n g now t o t h e Cu t a r g e t e l e c t r o n energy spectra, i t i s seen t h a t even f o r t h e L i - 1 i k e i n c i d e n t p r o j e c t i l e s , which have f i l l e d K-shells, t h e p r o j e c t i l e KLL f e a t u r e i s present. T h i s o b s e r v a t i o n i s i n t e r p r e t e d as evidence f o r p r o j e c t i l e K - s h e l l e x c i t a t i o n i n t h i s c o l l i s i o n system. P r o j e c t i l e K-shell e x c i t a t i o n i s a l s o observed f o r F* i o n s i n c i d e n t on Cu 1151, as can be seen from t h e e l e c t r o n energy s p e c t r a shown i n Fig. 2. The l a t t e r spectra were a l s o obtained using a CMA e l e c t r o n spectrometer.

I n order t o o b t a i n i n s i g h t i n t o t h e reasons u n d e r l y i n g t h i s i n t e r e s t i n g d i f f e r e n c e observed f o r Au and Cu t a r g e t s , a d i a b a t i c MO energy l e v e l c a l c u l a t i o n s were c a r r i e d o u t on t h e N

+

Au and N

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Cu c o l l i s i o n systems u s i n g t h e v a r i a b l e screening model developed by E i c h l e r and W i l l e 1161. Representative r e s u l t s o f t h e c a l c u l a t i o n s f o r N4+ i o n s a r e shown i n Fig. 3. The r e l e v a n t curves, which represent t h e p u r e l y e l e c t r o n i c eigenenergies o f the N

+

Au and N

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Cu quasimolecules, r e s p e c t i v e l y , as a f u n c t i o n o f i n t e r n u c l e a r separation, a r e l a b e l le d by t h e atomic l e v e l s t h a t they c o r r e l a t e t o i n t h e separated atom l i m i t . I n t h e f i g u r e , a s i g n i f i c a n t d i f f e r e n c e between t h e sequence o f energy l e v e l s f o r Au and Cu t a r g e t s i s imne- d i a t e l y apparent. F o r t h e N

+

Au system, t h e p o t e n t i a l curve c o r r e l a t e d t o the 1s l e v e l o f t h e p r o j e c t i l e i s separated from those c o r r e l a t i n g w i t h t h e 2s and 2p p r o j e c t i l e l e v e l s by t h e f i l l e d 4s, 4p, and 4d Au t a r g e t l e v e l s . T r a n s f e r o f an e l e c t r o n o u t o f t h e p r o j e c t i l e K-shell (i.e., p r o j e c t i l e K - s h e l l e x c i t a t i o n ) , which proceeds v i a pseudocrossings o r r o t a - t i o n a l c o u p l i n g s o f t h e i n i t i a l s t a t e and adjacent MO's i s t h e r e f o r e i n h i b i t e d , since a l l the neighboring o r b i t a l s c o r r e l a t e t o t a r g e t l e v e l s which a r e a l r e a d y f i l l e d . On t h e o t h e r hand, t r a n s f e r o f an i n i t i a l K - s h e l l vacancy t o neighboring n=4 t a r g e t l e v e l s i s f a c i l i t a t e d . As has been discussed p r e v i o u s l y 111-131, such p r o j e c t i l e - t o - t a r g e t vacancy t r a n s f e r reactions may g i v e r i s e t o t h e NNV and NVV t a r g e t Auger t r a n s i t i o n s a t 220 and 70 eV, r e s p e c t i v e l y , e v i d e n t i n t h e NG

+

Au e l e c t r o n energy spectrum shown i n Fig. 1.

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ORNL-DWG 88.14817 t o 0

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Cu

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0 100 200 300 400 500 600 700

ELECTRON ENERGY

(eV)

Fig. 1

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E l e c t r o n energy d i s t r i b u t i o n s f o r 60 kev N p r o j e c t i l e s i n c i d e n t on Au and Cu t a r g e t s a t 5".

p r o j e c t i l e - c o r r e l a t e d o r b i t a l s , and thus i n d i c a t e a s i m i l a r mechanism f o r p r o j e c t i l e K-shell e x c i t a t i o n f o r those c o l l i s i ' o n systems.

Due t o t h e l a r g e Doppler broadening ( t y p i c a l l y about 90 eV f o r t h e above p r o j e c t i l e N KLL f e a t u r e s ) r e s u l t i n g from t h e 90° o r i e n t a t i o n o f the CMA spectrometer a x i s w i t h respect t o the p r o j e c t i l e beam d i r e c t i o n , t h e r e s o l u t i o n o f t h e p r o j e c t i l e KLL f e a t u r e s i n t h e above spectra i s s i g n i f i c a n t l y degraded, making i n t e r p r e t a t i o n d i f f i c u l t . D e s p i t e t h e 1 im i t e d r e s o l u t i o n , however, a systematic d i f f e r e n c e i n peak p o s i t i o n s i s observed between t h e KLL features observed f o r H - l i k e i n c i d e n t ions, which c a r r y an i n i t i a l K-vacancy, and those observed as a r e s u l t o f K-shell e x c i t a t i o n d u r i n g t h e i n t e r a c t i o n w i t h t h e surface. I n comparing the s p e c t r a f o r F8+ and F6+ i n c i d e n t on Cu i n t h e bottom o f Fig. 2, i t i s seen t h a t t h e KLL f e a t u r e r e s u l t i n g from K-shell e x c i t a t i o n (i.e., f o r i n c i d e n t F ~ + ) , w h i l e c h a r a c t e r i z e d by a s i m i l a r f a l l - o f f toward h i g h e r energies, does n o t extend as low i n e l e c t r o n energy as t h a t observed f o r i n c i d e n t ~ 8 + . As a r e s u l t , t h e KLL "peak" a r i s i n g due t o K-shell e x c i t a t i o n appears t o l i e about 25 eV h i g h e r i n energy than t h a t observed f o r H - l i k e i n c i d e n t ions. A s i m i l a r d i f f e r e n c e i n KLL "peak" p o s i t i o n s i s observed i n t h e case o f N6+ and N4+ ions i n c i - dent on Cu (see Fig. 1). I t i s noted, on t h e o t h e r hand, t h a t t h e KLL "peak" f o r N5+ ions i n c i d e n t on Au, a l s o shown i n Fig. 1, i s n o t s h i f t e d r e l a t i v e t o t h a t observed f o r i n c i d e n t

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JOURNAL

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ORNL-DWG 88-15208

r

I

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I

1

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ELECTRON ENERGY (eV)

ZOO

450 500 550 600 650 700 750 800

ELECTRON ENERGY (eV)

Fig. 2

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E l e c t r o n energy d i s t r i b u t i o n s f o r F i o n s i n c i d e n t on Cu;

Fa+

and Fs+ p r o j e c t i l e energies a r e 80 keV; energies o f F4+ and ~ 2 + i o n s a r e 52 and 26 keV, r e s p e c t i v e l y .

measured f o r an i n c i d e n t charge s t a t e beam

known

t o c a r r y t h e a p p r o p r i a t e i n i t i a l i n n e r - s h e l l vacancy, i t can be i n f e r r e d whether s i m i l a r "peaks" observed f o r lower charge s t a t e i o n s ( i n whose ground s t a t e s t h e i n n e r s h e l l o f i n t e r e s t i s f i l l e d ) a r e t h e r e s u l t o f metastables present i n t h e i n c i d e n t beam o r p r o j e c t i l e i n n e r - s h e l l e x c i t a t i o n d u r i n g t h e i n t e r a c t i o n w i t h t h e surface.

Returning t o t h e e a r l i e r d i s c u s s i o n o f t a r g e t i n n e r - s h e l l e x c i t a t i o n , i t i s noted t h a t de Zwart has a l s o observed t a r g e t Auger t r a n s i t i o n s i n 20 keV

~ r ~ + ,

+

W c o l l i s i o n s , as d i s - cussed i n h i s Ph.D. t h e s i s . He suggests t h a t i n n e r - s h e l l vacancy t r a n s f e r v i a MO pseudocrossings may be t h e u n d e r l y i n g e x c i t a t i o n mechanism f o r t h i s c o l l i s i o n system also.

It

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ORNL-OWG 88-15209

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on Au

INTERNUCLEAR SEPARATION (ad.)

Fig. 3

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A d i a b a t i c MO energy l e v e l c a l c u l a t i o n s f o r t h e N4+

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Au, CU c o l l i s i o n systems; s o l i d l i n e s a r e a o r b i t a l s , dot-dashed l i n e s a r e

r

o r b i t a l s .

atomic numbers o f t h e c o l l i s i o n partners. T h i s makes N an i d e a l probe o f C surface contami- n a t i o n , w h i l e F i o n s a r e a s e n s i t i v e probe f o r s u r f a c e contamination by

0.

The s e n s i t i v i t y o f a N beam probe t o t h e presence o f C on a Au surface i s i l l u s t r a t e d i n Fig. 5, where an e l e c t r o n energy spectrum i s shown f o r N6+ i o n s i n c i d e n t on a Au s u r f a c e having a -20% mono- l a y e r coverage o f carbon. The spectrum was obtained u s i n g a LEED system i n a manner pre- v i o u s l y d e s c r i b e d

/lo/.

The area o f t h e C KVV f e a t u r e i s about 30% o f t h a t o f t h e N KLL feature. Since t h e t o t a l y i e l d o f N KLL e l e c t r o n s has p r e v i o u s l y been determined 1111 t o be on t h e o r d e r o f u n i t y , t h e a b s o l u t e y i e l d o f C KVV e l e c t r o n s i s seen t o be q u i t e h i g h (-30%). For comparison, Fig. 5 a l s o shows a 500 eV e l e c t r o n induced energy spectrum acquired f o r the same s u r f a c e c o n d i t i o n s . As can be seen, on t h e s c a l e o f t h e e l a s t i c s c a t t e r i n g peak, the C KVV peak i s n o t d i s c e r n i b l e . The p r o d u c t i o n o f "target1' Auger e l e c t r o n emission by i n n e r - s h e l l vacancy t r a n s f e r i s thus i n t h i s case considerably more e f f i c i e n t than t h a t produced by electron-impact i o n i z a t i o n .

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APPARATUS IMPROVEMENTS

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JOURNAL DE PHYSIQUE ORNL-DWG 87-43374 SEPARATED UNITED ATOM LIMIT C t N ATOM LIMIT I

Fig. 4

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D i a b a t i c c o r r e l a t i o n diagram f o r t h e N

+

C c o l l i s i o n system.

section, which p r o v i d e s f o r independent, r e a l -time m o n i t o r i n g o f i n c i d e n t i o n beam current, and thus p e r m i t s more convenient n o r m a l i z a t i o n o f t h e measured e l e c t r o n energy d i s t r i b u t i o n s t o the i n c i d e n t i o n f l u x . The beam c o l l i m a t o r / m o n i t o r c o n s i s t s o f a s e r i e s o f apertures which a r e used t o l i m i t t h e divergence o f t h e beam i n c i d e n t on t h e f i n a l c o l l i m a t i o n aper- t u r e , where a small f r a c t i o n o f t h e i n c i d e n t i o n beam i s intercepted. Due t o t h e upstream c o l l i m a t i o n , t h e f r a c t i o n o f i n t e r c e p t e d t o t r a n s m i t t e d beam c u r r e n t (which i s equal t o the c u r r e n t i n c i d e n t on t h e t a r g e t c r y s t a l ) i s almost independent o f upstream beam tuning, i.e., i s s o l e l y determined by t h e c o l l i m a t o r geometry. I n p r e v i o u s v e r s i o n s o f t h e apparatus, the e l e c t r o n s p e c t r a were normalized t o t h e unbiased t a r g e t c u r r e n t , which, due t o t h e l a r g e t o t a l e l e c t r o n y i e l d s , represented m a i n l y t h e e j e c t e d e l e c t r o n f l u x and n o t i n c i d e n t i o n f l u x .

The geometry o f the p r e s e n t setup i s s c h e m a t i c a l l y i n d i c a t e d i n Fig. 6. Note t h a t angle o f incidence as w e l l as e l e c t r o n e j e c t i o n angle can now be independently varied, p r o v i d i n g g r e a t l y increased experimental f l e x i b i 1 i ty. With t h e exception o f these two improvements, t h e apparatus i s i d e n t i c a l t o t h a t p r e v i o u s l y described

/13/.

To i l l u s t r a t e the r e s o l u t i o n a t t a i n a b l e w i t h t h e new spectrometer, Fig. 6 a l s o shows a s c a t t e r e d e l e c t r o n energy spectrum f o r -500 eV e l e c t r o n s i n c i d e n t on Au a t 45".

4

-

PROJECTILE KLL DECAY MEASUREMENTS

(8)

0

100 200 300 400

500

600

ORNL-DWG 87-43357 200 400 600 L A B ENERGY ( e V )

I

-

_{e-+ A u (METAL), 5 0 0 e V}

-

Fig.

5 -

Top

-

e l e c t r o n energy d i s t r i b u t i o n f o r N" i o n s i n c i d e n t a t

5"

on Au having a

20%

monolayer coverage o f carbon. The peaks around

250

eV and

350

eV a r e i d e n t i f i e d as C KVV, and N KLL peaks, r e s p e c t i v e l y . The h o r i z o n t a l b a r i n d i c a t e s t h e broadening i n t r o d u c e d by the numerical d i f f e r e n t i a t i o n o f t h e LEED transmission current. Bottovn

-

e l e c t r o n energy spectrum f o r

500

eV e - ' s i n c i d e n t normally on Au a l s o w i t h

20%

monolayer

C

coverage.

-

t h e i n c i d e n t beam. The s p e c t r a shown a l l have t h e same n o r m a l i z a t i o n , and were acquired w i t h a s e l e c t e d spectrometer-pass-energy o f

100

eV, r e s u l t i n g i n an e l e c t r o n energy-independent instrumental r e s o l u t i o n o f about 5 eV. Doppler broadening i n t h e v i c i n i t y o f t h e p r o j e c t i l e KLL peaks l o c a t e d a t

-500

eV i s about

10

eV. The t o t a l i n s t r u m e n t a l w i d t h due t o these two e f f e c t s combined i s thus small compared t o the observed w i d t h o f t h e p r o j e c t i l e KLL peaks

(-100

eV). By v a r y i n g t h e angle o f incidence, the time i n t e r v a l between t h e s t a r t o f p r o j e c - t i l e n e u t r a l i z a t i o n and pt,.:tration o f t h e s u r f a c e can be varied. Assuming t h a t n e u t r a l i z a - t i o n comnences a t t h e c r i t i c a l d i s t a n c e

1181

above t h e surface a t which t h e Coulomb b a r r i e r between p r o j e c t i l e and metal f a l l s below t h e Fermi l e v e l , equal t o about

21

a.u. f o r

07+

ions, t h e above-surface i n t e r a c t t o n times cover t h e range

1

t o

7

x

10-l5

s,

t h e s h o r t e s t time corresponding t o t h e l a r g e s t angle o f incidence

(80').

The f o l l o w i n g trends a r e noted i n the dependence o f t h e p r o j e c t i l e KLL peak on time spent above t h e surface. The s i z e o f the KLL peak decreases m o n o t o n i c a l l y w i t h decreasing i n t e r a c t i o n time, and becomes b a r e l y d i s c e r n i b l e above t h e continuum background i n t h e spectrum f o r t h e

80"

angle o f incidence. The p o s i t i o n

(9)

JOURNAL

DE

PHYSIQUE

ORNL- DWG 88-45213

400

Fig. 6

-

Top

-

schematic diagram o f t h e new experimental apparatus, showing beam c o l l i m a t i o n / m o n i t o r , i n g section; n o t e t h a t b o t h angle o f incidence and observation angle can be varied. Bottan

-

sample e' induced e l e c t r o n spectrum acquired w i t h new spectrometer, obtained a t constant pass energy o f 50 eV.

o f t h e KLL peak, however, does n o t appear t o change as a f u n c t i o n o f t h e above-surface i n t e r - a c t i o n time. These t r e n d s a r e c o n s i s t e n t w i t h t h e f o l l o w i n g p i c t u r e . A t t h e a l r e a d y defined c r i t i c a l d i s t a n c e from t h e surface, m u l t i p l e resonant n e u t r a l i z a t i o n from t h e valence band o f t h e metal occurs (on a t i m e scale o f t h e o r d e r 5 x 10-l6 s / l 9 / ) , which populates h i g h l y e x c i t e d Rydberg l e v e l s o f the i n c i d e n t i o n (e.g., n=9 f o r i n c i d e n t 07+). A cascade o f e l e c t r o n s t o lower n l e v e l s occurs as t h e m u l t i p l y e x c i t e d i o n a u t o i o n i z e s , i n p a r a l l e l , o f course, w i t h f u r t h e r resonant n e u t r a l i z a t i o n processes t o p r o g r e s s i v e l y lower n as t h e i o n continues t o approach t h e surface. As soon as a t l e a s t two e l e c t r o n s have reached t h e L- s h e l l , KLL t r a n s i t i o n s w i l l occur. I f t h e r a t e o f f i l l i n g t h e L - s h e l l i s much l e s s than the KLL r a t e f o r t h e L i - l i k e i o n , no f u r t h e r f i l l i n g o f t h e L - s h e l l i s possible, and t h e observed KLL peak should be t h a t f o r O5+. I f , on t h e o t h e r hand, t h e L - s h e l l f i l l i n g r a t e exceeds t h a t o f KLL decay, then t h e observed KLL peak should be a composite s t r u c t u r e , c o n s i s t i n g o f KLL c o n t r i b u t i o n s from a l l charge s t a t e s s5. I n t h i s p i c t u r e t h e KLL peak p o s i t i o n i s thus argued t o depend o n l y on t h e r e l a t i v e magnitudes o f two i n t r i n s i c t i m e scales c h a r a c t e r i z i n g t h e n e u t r a l i z a t i o n process, and n o t t o have a dependence on t h e e x t r i n s i c i n t e r a c t i o n time scale s e l e c t e d by a p a r t i c u l a r angle o f incidence, i n accord w i t h observation.

(10)

ORNL DWG 88-15207

o+'

KLL AUGER PEAK

TRANSITIONS

ELECTRON ENERGY (eV)

Fig. 7

-

E l e c t r o n energy d i s t r i b u t i o n s f o r 70 keV 07+ i o n s i n c i d e n t on Au, f o r s i x d i f f e r e n t angles o f incidence. H o r i z o n t a l l i n e s under t h e 0 KLL peaks g i v e c a l c u l a t e d ranges o f KLL t r a n s i t i o n energies f o r a l l charge s t a t e s

s5+.

Table I. C a l c u l a t e d c o n f i g u r a t i o n average KLL t r a n s i t i o n energies and Auger r a t e s f o r d i f f e r e n t charge s t a t e s o f oxygen.

I n i t i a l I n i t i a l F i n a l C o n f i g u r a t i o n Average Rate Charge S t a t e C o n f i g u r a t i o n C o n f i g u r a t i o n Auger E l e c t r o n Energy (loi3 S - l )

(11)

C1-272 JOURNAL DE PHYSIQUE

a " s u p e r - f i l l e d " L - s h e l l , since i t can b i n d an e x t r a e l e c t r o n due t o t h e K-shell vacancy. The p o s s i b i l i t y o f K-Auger t r a n s i t i o n s from a ' s u p e r - f i l l e d " L - s h e l l i s one mechanism t h a t may e x p l a i n t h e e x t e n t o f t h e o b s e r v g KLL f e a t u r e t o energies above t h e KLL peak a t about 495 eV observed subsequent t o K-shell p h o t o i o n i z a t i o n o f n e u t r a l

0.

An a d d i t i o n a l mechanism f o r y i e l d i n g h i g h e r than "normal" Auger t r a n s i t i o n energies, based on i n t e r a c t i o n o f the Auger e l e c t r o n w i t h t h e image charge o f t h e ( i n c o m p l e t e l y screened) e m i t t e r has been discussed by F o l b e r t s and Morgenstern

1211.

A consequence o f t h i s i n t e r p r e t a t i o n o f t h e w i d t h o f t h e observed KLL s t r u c t u r e i s t h a t the f i l l i n g r a t e o f t h e L - s h e l l s i g n i f i c a n t l y exceeds t h e r a t e o f t h e f a s t e s t KLL t r a n s i t i o n s , as discussed e a r l i e r . R e f e r r i n g t o Table I, t h e f a s t e s t KLL decay r a t e i s c a l c u l a t e d f o r n e u t r a l

0

( w i t h t h e " s u p e r - f i l l e d " L - s h e l l ) where i t i s l a r g e r than 2 x

1014

s-l. The l i m i t - i n g r a t e being t h a t o f t h e KLL t r a n s i t i o n , i t i s t h e t i m e s c a l e d e f i n e d by t h i s r a t e t h a t must be compared t o t h e n e u t r a l i z a t i o n time a v a i l a b l e above t h e surface. I t i s seen t h a t a t

10'

angle o f incidence, t h e i n t e r a c t i o n time corresponds t o about

1.2

KLL decay l i f e t i m e s , w h i l e a t

80°,

t h e a v a i l a b l e time corresponds t o o n l y about

0.2

KLL decay l i f e t i m e s . This e x p l a i n s t h e gradual disappearance o f t h e observed KLL peaks w i t h i n c r e a s i n g angle o f i n c i - dence. To v e r i f y t h a t t h e observed KLL t r a n s i t i o n s indeed occur on t h e i n c i d e n t i o n t r a j e c - t o r y , and n o t l a t e i n t h e i n t e r a c t i o n subsequent t o t h e reemergence o f (some o f ) t h e ions from the s u r f a c e along some d i f f e r e n t d i r e c t i o n , t h e Doppler s h i f t o f t h e KLL peak was inves- t i g a t e d as a f u n c t i o n o f viewing angle. F i g u r e 8 shows t h e KLL peak f o r

07+

.incident on Au a t

10"

as observed a t t h r e e d i f f e r e n t observation angles. The w e l l - d e f i n e d Doppler s h i f t o f t h e observed KLL peaks shows c l e a r l y t h a t t h e Auger e l e c t r o n emission occurs w h i l e t h e pro- j e c t i l e i s s t i l l t r a v e l i n g along t h e i n c i d e n t beam d i r e c t i o n , i.e., p r i o r t o any d e f l e c t i o n o f t h e p r o j e c t i l e by c l o s e encounters w i t h t a r g e t atoms. A s i m i l a r conclusion was reached by de Zwart

131

i n a n a l y z i n g A r LMM t r a n s i t i o n s d u r i n g

500

eV Ar9+ c o l l i s i o n s w i t h a W surface. I n t h e l i g h t o f t h e above i n t e r p r e t a t i o n o f t h e observed KLL widths, a reason f o r t h e d i f f e r - ence, noted i n t h e p r e v i o u s section, between t h e KLL peak p o s i t i o n s observed subsequent t o K-shell e x c i t a t i o n and those observed f o r H - l i k e i n c i d e n t p r o j e c t i l e s can now be suggested. Since t h e e x c i t a t i o n process r e q u i r e s a small impact parameter b i n a r y c o l l i s i o n w i t h a t a r g e t atom, and thus occurs l a t e i n t h e multicharged i o n - s u r f a c e i n t e r a c t i o n , t h e i o n w i l l be

ORNL-DWG 88-14819

20.000

100

200

300

400

500

600 ELECTRON ENERGY (eV)

(12)

almost completely n e u t r a l i z e d a t t h e i n s t a n t o f i n n e r - s h e l l e x c i t a t i o n . The subsequent KLL t r a n s i t i o n s w i l l thus occur i n a narrower e l e c t r o n energy range, c h a r a c t e r i s t i c o f , say, n e u t r a l o r s i n g l y charged 0, than t h a t observed f o r H - l i k e i n c i d e n t ions, where KLL t r a n s i - t i o n s from a l l p o s s i b l e charge s t a t e s occur. The h i g h energy s i d e s o f t h e KLL peaks f o r the two cases, which correspond t o KLL t r a n s i t i o n s from " s u p e r - f i l l e d " L-she1 l s , thus coincide, as can be seen from t h e comparison i n Fig. 2 between F h and Fa+. The mechanism f o r obtain- i n g a " s u p e r - f i l l e d " L - s h e l l i n t h e case o f K-shell e x c i t a t i o n i s t h e e x c i t a t i o n i t s e l f , i n t h a t t h e nearest (and most l i k e l y ) u n f i l l e d o r b i t a l t o which a p r o j e c t i l e K - s h e l l e l e c t r o n can be promoted i s t h e o r b i t a l c o r r e l a t i n g t o t h e p r o j e c t i l e 2p l e v e l (see Fig. 3).

To determine t h e e x t e n t t o which t h e shape o f t h e KLL peaks i s p e r t u r b e d by e l e c t r o n emission o f t h e p r o j e c t i l e a f t e r p e n e t r a t i o n o f t h e surface, t h e measurements o f Fig. 7 were extended t o lower angles o f incidence. Results f o r N& i o n s i n c i d e n t on Au a t

l o 0 ,

3", and lo are shown i n Fig. 9. The e l e c t r o n energy s p e c t r a shown cover e l e c t r o n energies i n t h e v i c i n i t y o f t h e N KLL peak; t h e s p e c t r a were scaled t o have t h e same KLL peak height, i n order t o i l l u s t r a t e t h e r e l a t i v e c o n t r i b u t i o n o f t h e broad f e a t u r e below t h e p r o j e c t i l e KLL peak as a f u n c t i o n o f angle o f incidence. C l e a r l y , t h e r e l a t i v e importance o f t h i s f e a t u r e decreases w i t h decreasing angle o f incidence, i n d i c a t i n g t h a t i t i s n o t r e l a t e d t o p r o j e c t i l e KLL emis- s i o n o c c u r r i n g p r i o r t o p e n e t r a t i o n o f t h e surface. The t r e n d shown i n these measurements can be due t o e i t h e r t h e increased i n t e r a c t i o n t i m e above t h e s u r f a c e o b t a i n e d w i t h the s m a l l e r g r a z i n g angles ( r e s u l t i n g i n a l a r g e r KLL peak), o r t h e decreased p e n e t r a t i o n o f the p r o j e c t i l e s , o r both. The l a t t e r p o s s i b i l i t y i s i l l u s t r a t e d i n Fig. 10, where c l a s s i c a l t r a j e c t o r y Monte C a r l o c a l c u l a t i o n s u s i n g t h e MARLOWE code 1221 a r e shown f o r 60 keV N6+ ions i n c i d e n t on Au f o r two d i f f e r e n t angles o f incidence. The s i m u l a t i o n was c a r r i e d o u t using t h e ZBL screened-Coulomb i n t e r a t o m i c s c a t t e r i n g p o t e n t i a l 1231, and assuming an unrecon- s t r u c t e d metal surface. The c a l c u l a t i o n s show t h a t a t t h e present energies, t h e c o n d i t i o n of specular r e f l e c t i o n i s approached f o r -lo g r a z i n g angles o f incidence. A s i m i l a r conclusion can a l s o be reached by c a l c u l a t i n g t h e " h a l f - d i p " angle f o r p l a n a r channeling ( o f which specular r e f l e c t i o n i s t h e s u r f a c e analog) u s i n g t h e formalism presented i n t h e review by Gemnel 1241. F o r 60 keV N i o n s i n c i d e n t on Au, t h i s angle i s c a l c u l a t e d t o be 2.6O, which i s i n good agreement w i t h t h e r e s u l t obtained using t h e Monte C a r l o simulation. Further measurements a r e planned t o i n v e s t i g a t e t h i s i n t e r e s t i n g s c a t t e r i n g regime, i n which the multicharged i o n - s u r f a c e i n t e r a c t i o n can be s t u d i e d i n i s o l a t i o n w i t h o u t having t o consider t h e complications a r i s i n g from surface p e n e t r a t i o n e f f e c t s .

ORNL- DWG 88- 4 5214

2 0 0

300

400 5 0 0

ELECTRON ENERGY

(eV)

(13)

JOURNAL

DE

PHYSIQUE ORNL-DWG 88M-15210 W = 2 O 0.3 - [ I 101 AZIMUTH

-

El = 85 eV RN = 86%

-

I 0 5 1'5

t t t t t t t y t

DEPTH

(A)

Fig. 10

-

MARLOWE c a l c u l a t i o n o f mean p e n e t r a t i o n depth f o r 70 keV

0

p r o j e c t i l e s i n c i d e n t on Au at: 5' and 2'. V e r t i c a l arrows along abscissa correspond t o c r y s t a l l a t t i c e planes i n the

(110) o r i e n t a t i o n ; RN i n t h e f i g u r e r e f e r s t o p a r t i c l e r e f l e c t i o n c o e f f i c i e n t .

5

-

SUMMARY

A q u a l i t a t i v e d i s c u s s i o n has been presented o f i n n e r - s h e l l vacancy p r o d u c t i o n mechanisms r e l e v a n t t o c o l l i s i o n s o f m u l t i c h a r g e d N, 0, and F ions i n c i d e n t on c l e a n Au and Cu surfaces as w e l l as those having fractional-monolayer coverages o f C and/or 0. The d i s c u s s i o n was based on c a l c u l a t e d a d i a b a t i c MO energy l e v e l s , which g i v e an i n s i g h t i n t o t h e p o s s i b l e pathways along which i n n e r - s h e l l e x c i t a t i o n may occur, and i n t o t h e s e n s i t i v i t y o f these pathways t o t h e c o l l i s i o n p a r t n e r s making up t h e quasimolecule. A comparison was made o f p r o j e c t i l e KLL peaks r e s u l t i n g from t h e f i l l i n g o f K-shell vacancies c a r r i e d i n t o t h e c o l l i s i o n , and those observed subsequent t o p r o j e c t i l e e x c i t a t i o n . A n a l y s i s o f t h e former provides i n f o r - mation about t h e r a t e o f f i l l i n g o f t h e L - s h e l l d u r i n g t h e n e u t r a l i z a t i o n process, w h i l e ana-

l y s i s o f t h e l a t t e r g i v e s i n f o r m a t i o n on t h e degree o f f i l l i n g o f t h e L - s h e l l a t the conclusion o f t h e n e u t r a l i z a t i o n o c c u r r i n g above t h e surface. A c h a r a c t e r i s t i c s i g n a t u r e o f K-shell e x c i t a t i o n apparent i n t h e measured e l e c t r o n energy spectra was discussed. This c h a r a c t e r i s t i c can be used t o d i s t i n g u i s h i n n e r - s h e l l t r a n s i t i o n s o c c u r r i n g i n metastable i o n s from those o c c u r r i n g subsequent t o i n n e r - s h e l l e x c i t a t i o n o c c u r r i n g d u r i n g the c o l l i - sion.

(14)

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