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Submitted on 1 Jan 1984
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H+3 FORMATION DURING THE FIELD DESORPTION OF HYDROGEN
N. Ernst, G. Bozdech, S. Kato, J. Block
To cite this version:
N. Ernst, G. Bozdech, S. Kato, J. Block. H+3 FORMATION DURING THE FIELD DES- ORPTION OF HYDROGEN. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-231-C9-237.
�10.1051/jphyscol:1984939�. �jpa-00224419�
H+ FORMATION DURING THE F I E L D DESORPTION OF HYDROGEN
N. E r n s t , G. Bozdech, S. Kato* and J . H . Block
Fritz-Habep-Institut der Max-Planck-Geseilsehaft, Faradayweg 4-6, D-1000 Berlin 33, F.R.G.
Résumé - La formation d ' H+, d H2 et d'H, est étudiée pour dif- férentes surfaces métalliques (Nb, W et A u ) . Le potentiel d'apparition dans la distribution énergétique de ces ions est déterminé au moyen d'une analyse par potentiel retardateur dans un spectromëtre de masse à secteur magnétique. Dans le cas de surfaces de Nb et W, des états précurseurs dans les distribu- tions énergétiques des ions Ho et H,, furent détectés pour un champ de surface supérieur à 20 V/nm.
La dêsorption de champ stimulée électroniquement d" Hp et d'H, adsorbés est l'origine la plus plausible de ces états pré- curseurs. La comparaison des distributions énergétiques d'ions Ne obtenus par dêsorption de champ électroniquement stimulée et d'ions H, obtenus par description de champ, suggère la pré- sence d'espèces H-, linéaires, adsorbées verticalement sur la surface de l'émetteur.
Nb et Au montrent moins de stabilité. L'êvaporation de champ assistée par hydrogène restreint les domaines accessibles pour les forces du champ. Avec une face (111) d'Au, seulement de petites quantités d ions H, purent être obtenues.
+ + +
Abstract - The formation of H , H2» and H3 is investigated for different metal surfaces (Nb, W, and A u ) . The onset potential in the energy distribution of these ions is determined by means of retarding potential analysis in a magnetic sector, mass spec- trometer. For Nb and W surfaces, precursor structures in the energy distributions of high energy wt and Ho field ions were detected at a surface field strength above 20 V/nm.
The electron-stimulated field desorption of adsorbed H0 and H3
is the most plausible origin of these precursors. Comparison of
•Visiting scientist at the Fritz-Haber-Institut, permanent address : Department of Nuclear Engineering, Hokkaido University, Sapporo 060, Japan
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984939
C9-232 JOURNAL DE PHYSIQUE
e n e r g y 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 t i m u l a t e d f i e l d d e s o r b e d ~ e + a n d f i e l d d e s o r b e d H;, t a k e n on W(111) a t t h e same f i e l d
s t r e n g t h a n d s u r f a c e t e m p e r a t u r e , s u g g e s t s t h e p r e s e n c e o f a l i n e a r H3, a d s o r b e d i n an u p r i g h t p o s i t i o n on t h e f i e l d e m i t t e r s u r f a c e .
Nb a n d Au show l e s s s t a b i l i t y . H y d r o g e n p r o m o t e d f i e l d e v a p o r a - t i o n r e s t r i c t s t h e a c c e s s i b l e f i e l d s t r e n g t h r a n g e s . A t a ( 1 1 1 ) Au s u r f a c e , o n l y s m a l l a m o u n t s o f H3 c o u l d be o b t a i n e d .
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I
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INTRODUCTIONThe f i e l d i o n i z a t i o n o f h y d r o g e n h a s b e e n e x t e n s i v e l y i n v e s t i g a t e d u s i n g m a s s - a n d e n e r g y - a n a l y z i n g m e t h o d s /1,2/. F i e l d i o n e m i s s i o n t e c h n i q u e s h a v e a l s o b e e n a p p l i e d t o t h e s t u d y o f h y d r o g e n s u r f a c e i n - t e r a c t i o n s s u c h a s t h e f o r m a t i o n o f s u r f a c e h y d r i d e s / 3 / .
The f o r m a t i o n o f t r i a t o m i c h y d r o g e n i o n s H;, d u e t o a f i e l d - i n d u c e d s u r f a c e r e a c t i o n , was p r o p o s e d t o h a v e t h e f o l l o w i n g r e a c t i o n p a t h w a y / 4 / 3
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HH 2 ( f - a d ) ( a d ) * H 3 ( a d ) ';(g) + e ; ~ e ) ' ( 1 ) An i s o t o p e e f f e c t i n t h e t e m p e r a t u r e d e p e n d e n c e o f H; a n d D; d e s o r p - t i o n r a t e s was c o n s i d e r e d a s a c o n s e q u e n c e o f f i e l d e n h a n c e d d i f f u s i o n of c h e m i s o r b e d h y d r o g e n a t o m s , H ( a d ) , t o w a r d s t h e r e a c t i o n s i t e s / 4 , 5 / . New e x p e r i m e n t a l e v i d e n c e w i l l b e p r e s e n t e d f o r t h e f i e l d e n h a n c e d s u r - f a c e d i f f u s i o n a n d f o r t h e e x i s t e n c e o f a d s o r b e d H E a n d H3 f i e l d d e - s o r p t i o n .
I 1
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EXPERIMENTAL PROCEDUREThe m a g n e t i c s e c t o r f i e l d mass s p e c t r o m e t e r w i t h f i e l d i o n i z a t i o n s o u r c e a n d r e t a r d i n g g r i d e n e r g y a n a l y z e r , as d e c r i b e d b e f o r e / 4 / , was a d d i t i o n a l l y e q u i p p e d w i t h a p r o b e - h o l e c h a n n e l p l a t e . The p r o b e - h o l e s i z e c o r r e s p o n d s t o a s u r f a c e a r e a o f 50
8
d i a m e t e r . The s u r f a c e a r e a a c t u a l l y p r o b e d b y t h e mass a n d e n e r g y a n a l y s i s i s e s t i m a t e d t o b e o n e o r d e r o f m a g n i t u d e s m a l l e r t h a n t h e s i z e o f t h e p r o b e - h o l e i n t h e c h a n n e l p l a t e , d u e t o t h e i n s t r u m e n t ' s a p e r t u r e .I 1 1 - EXPERIMENTAL RESULTS AND DISCUSSIONS
R e s u l t s o f r e c e n t e n e r g y d i s t r i b u t i o n m e a s u r e m e n t s a r e s u m m a r i z e d i n f i g . 1. A c o m p a r i s o n o f t h e e x p e r i m e n t a l r e t a r d a t i o n c u r v e s o f e l e c -
m E
D i s t a n c e from E f f e c t i v e S u r f a c e X
Fig.
1,(a)(b) - Differential energy distributions (full lines), the dotted curves in (a) represent retarding curves o f electron-stimulated field desorbed ~ e + and field desorbed H;, the curves in (b) were de- rived f r o m retarding curves o f hydrogen field ions; upper distances are calculated from measured retarding voltages /10/.(c) - Potential energy diagram for adsorbed H3 (according to data in
( a ) ) ,including the hypothetical potential o f H-H-H because of instability in the free space.
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tron-stimulated field desorption o f Ne ions and field desorbed H3
C9-234 JOURNAL DE PHYSIQUE
i o n s i s g i v e n i n t h e f o r m o f d o t t e d l i n e s i n ( a ) . The s o l i d l i n e s i n ( a ) , a s w e l l a s i n ( b ) , a r e d i f f e r e n t i a l e n e r g y d i s t r i b u t i o n s d e r i v e d f r o m e x p e r i m e n t a l i n t e g r a l d a t a . The e x p e r i m e n t a l E m i t t e r - R e t a r d e r V o l t a g e v a l u e s i n t h e l o w e r a b s c i s s a s c a l e c a n b e t r a n s f e r r e d i n t o t h e u p p e r d i s t a n c e s c a l e w i t h t h e a i d o f a p r o p e r p o t e n t i a l f u n c t i o n / 6 , 7 / . The a t o m i c r a d i u s o f Ne, w h i c h d e t e r m i n e s t h e e n e r g y d i s t r i b u t i o n i n t h e e l e c t r o n - s t i m u l a t e d f i e l d d e s o r p t i o n (ESFD) o f n e o n / 8 , 9 / , s e r v e s f o r t h e d i s t a n c e c a l i b r a t i o n f r o m t h e e f f e c t i v e s u r f a c e .
I n f i g . l ( a ) H 3 - i n t e n s i t i e s d i s p l a y a p r e c u r s o r i n e n e r g y d i s t r i b u t i o n . t These p r e c u r s o r s t a t e s a r e shown w i t h h i g h e r c o u n t i n g r a t e s f o r H3/ t N b ( l l 1 ) a n d ~ ; / ~ ( 1 1 1 ) i n f i g . l ( b ) . The a p p e a r a n c e o f m o r e e n e r g e t i c
t t
H3 a n d H2 p r e c u r s o r p e a k s i s c o r r e l a t e d w i t h t h e o c c u r r e n c e o f l o w e n - e r g y H2. T h i s H:
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mass s i g n a l g e n e r a t e d b y s p a c e i o n i z a t i o n o f H 2 , p r o - v i d e s i n f o r m a t i o n o n t h e e n e r g y a n d r a t e o f i m p i n g i n g e l e c t r o n s . The a p p e a r a n c e o f h i g h e n e r g y H 2 a n d H; t i s t h e r e f o r e u n d e r s t o o d i n t e r m s o f e l e c t r o n - s t i m u l a t e d f i e l d d e s o r p t i o n o f a d s o r b e d H2 a n d H3 / 1 0 / . From e n e r g y d i s t r i b u t i o n s o f ESFD o f n e o n a n d f i e l d d e s o r b e d Hg i o n s , w h i c h h a v e b e e n m e a s u r e d a t t h e i d e n t i c a l s u r f a c e s i t e s a n d a t t h e same s u r f a c e f i e l d s t r e n g t h , t h e p r e s e n c e o f a l i n e a r H3 c o n f i g u r a t i o n a d s o r b e d i n an u p r i g h t p o s i t i o n was i n f e r r e d / 1 0 / . O n s e t a p p e a r a n c e e n e r g y v a l u e s p r o v i d e a m e a s u r e f o r t h e b i n d i n g e n e r g y A o f l i n e a r H 3 i f t h e t h e o r e t i c a l i o n i z a t i o n e n e r g y / 1 1 , 1 2 / o f t h e n e u t r a l i s d e d u c e d . A t W - a n d N b - e m i t t e r s A - v a l u e s a r e f o u n d t o b e e q u a l a n d a m o u n t t o( 1 . 8
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0 . 1 ) eV a s d e r i v e d f r o m o n s e t v o l t a g e s o f o r d i n a r y H3 r e t a r d a - t t i o n c u r v e s .From e x p e r i m e n t a l d a t a we c a n d e r i v e e v i d e n c e o f t h e o c c u r r e n c e o f f i e l d e n h a n c e d d i f f u s i o n o f c h e m i s o r b e d h y d r o g e n a t o m s d u r i n g H: f o r - m a t i o n . T h i s e v i d e n c e a r i s e s f r o m t h e t e m p e r a t u r e d e p e n d e n c e a n d f r o m t h e FWHM ( f u l l w i d t h a t h a l f maximum) o f e n e r g y d i s t r i b u t i o n s o f H;.
I n f i g . 2 t h e H; r a t e on n i o b i u m a n d t u n g s t e n s u r f a c e s i s l i n e a r l y p l o t t e d a s a f u n c t i o n o f t h e e m i t t e r t e m p e r a t u r e . A t t h e r e l a t i v e l y h i g h f i e l d s t r e n g t h o f t h e s e m e a s u r e m e n t s t h e maximum t o 1 e r a b l e t e m - p e r a t u r e f o r Nb was 3 0 0 K , o t h e r w i s e h y d r o g e n p r o m o t e d f i e l d e v a p o r a - t i o n w o u l d p r e v a i l . F o r W, t h e H; r a t e i n c r e a s e s w i t h i n c r e a s i n g t e m - p e r a t u r e a b o v e 3 0 0 K , y i e l d i n g a t 5 0 0 K f o u r t i m e s m o r e i o n s t h a n a t 78 K . T h i s b e h a v i o u r o f H; s t r o n g l y d e p e n d s on t h e f i e l d s t r e n g t h . The FWHM o f e n e r g y d i s t r i b u t i o n of H; ( f i g . 3 ) m e a s u r e d a t 7 8 K i s s i g - n i f i c a n t l y l a r g e r t h a n a t 500 K . T h i s b e h a v i o u r i s s c h e m a t i c a l l y e x - p l a i n e d i n f i g . 4 . Space i o n i z a t i o n o f h y d r o g e n m o l e c u l e s i n t h e f r e e
F i g . 2
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H2 i o n c o u n t i n g r a t e a s t F i g . 3-
E n e r g y d i s t r i b u t i o n o f a f u n c t i o n o f t h e e m i t t e r tem-H;
a t 78 K a n d 5 0 0 K, h i g h e l e c -p e r a t u r e . t r o - s t a t i c f i e l d s t r e n g t h o f
28 ~ ' / n m .
s p a c e a t l o w t e m p e r a t u r e s i n o n e c a s e , a n d f i e l d d e s o r p t i o n o f h y d r o g e n m o l e c u l e s f r o m t h e s u r f a c e a t h i g h t e m p e r a t u r e s i n t h e o t h e r c a s e a r e r e s p o n s i b l e a t h i g h f i e l d - s t r e n g t h r e g i m e . On t h e l e f t - h a n d s i d e ( f i g . 4 ) i m p i n g i n g h y d r o g e n m o l e c u l e s a r e m a i n l y f i e l d i o n i z e d f a r a b o v e t h e c r i t i c a l d i s t a n c e X,. A d d i t i o n a l l y e l e c t r o n - s t i m u l a t e d f i e l d d e s o r p t i o n o f v i b r a t i o n a l l y e x c i t e d H 2 o c c u r s a b o v e X,
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d u e t o e l e c t r o n s r e l e a s e d b y t h e s p a c e i o n i z a t i o n . A r a t h e r b r o a d e n e r g y d i s t r i b u t i o n i s t h e c o n s e q u e n c e . A t h i g h e r t e m p e r a t u r e s t h e r e c o m b i n a t i o n o f h y d r o g e n a t o m s w i t h t h e i m m e d i a t e t h e r m a l d e s o r p t i o n o f H2 m o l e c u l e s a n d s u b s e q u e n t f i e l d i o n i z a t i o n j u s t a b o v e X p r e d o m i n a t e s o v e r t h e w i d e - r a n g e d s p a c et C
i o n i z a t i o n o f H 2 . A l s o t h e H3 s p e c i e s a r e no l o n g e r f o r m e d o n a c c o u n t o f t h e s m a l l d w e l l t i m e o f h y d r o g e n m o l e c u l e s a t t h e s u r f a c e . T h e r e f o r e a t h i g h t e m p e r a t u r e a n d h i g h f i e l d s t r e n g t h a r e l a t i v e l y n a r r o w e n e r g y d i s t r i b u t i o n i s t o be e x p e c t e d .
F r o m t h e s e e x p e r i m e n t a l r e s u l t s a s w e l l a s t h e r e s u l t s p r e v i o u s l y r e - p o r t e d / 4 / we c o n c l u d e t h a t t h e f i e l d e n h a n c e d d i f f u s i o n o f c h e m i s o r b e d h y d r o g e n on t h e s u r f a c e p l a y s an i m p o r t a n t r o l e i n t h e f o r m a t i o n o f H3
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a t l o w t e m p e r a t u r e s .
I n g e n e r a l , i t i s c o n c e i v a b l e t h a t t h e b i n d i n g e n e r g y o f H3 d e p e n d s on t h e s u b s t r a t e m a t e r i a l . The p r e s e n t m o d e l r e q u i r e s t h e d i s s o c i a t i v e a d s o r p t i o n .of H 2 . From e x p e r i m e n t s on m a c r o s c o p i c s a m p l e s i t h a s t o be c o n c l u d e d t h a t g o l d d o e s n o t d i s s o c i a t e H2, a t l e a s t w i t h o u t an e l e c -
JOURNAL DE PHYSIQUE
ionization r a t e X
4 X A ionization r a t e
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.
.
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.
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F1 of incoming H2 F1 of desorbing- H 2
F 0 of H3 no H3 formation
E S F D of H2 and H3 no E S F D
F i g . 4
-
M e c h a n i s m s o f i o n f o r m a t i o n a n d e n e r g y d i s t r i b u t i o n s .t r o - s t a t i c f i e l d . We h a v e , t h e r e f o r e , p e r f o r m e d m e a s u r e m e n t s w i t h g o l d e m i t t e r t i p s . T h e s e a r e c h a r a c t e r i z e d b y a r e l a t i v e l y s m a l l e v a p o r a t i o n f i e l d o f 3 5 V/nm /l/. F i g . 5 r e p r e s e n t s c o u n t i n g r a t e s o f h y d r o g e n i o n s p e c i e s o f a ( 1 1 1 ) o r i e n t e d A u - e m i t t e r a t 7 8 K f o r v a r i o u s f i e l d s t r e n g t h v a l u e s ( U = v o l t a g e b e t w e e n t i p a n d c o u n t e r e l e c t r o d e ) . The v o l t a g e r a n g e f o r Au was l i m i t e d d u e t o t h e o c c u r r e n c e o f s t r o n g h y d r o - gen p r o m o t e d f i e l d e v a p o r a t i o n . N e v e r t h e l e s s , e n e r g y d i s t r i b u t i o n s c o u l d b e m e a s u r e d a t t h e f i e l d s t r e n g t h o f l e s s t h a n 2 0 V/nm. T h e y a r e p r e s e n t e d i n f i g . 6 . The o n s e t a p p e a r a n c e e n e r g y f o r H p c o r r e s p o n d s t o
+
t h e i o n i z a t i o n e n e r g y o f H2, v a l u e s f o r
H+
e q u a l t h o s e m e a s u r e d a t t u n g s t e n . A l t h o u g h t h e H3 r a t e s a r e i n f e r i o r , t h e y s t i l l a l l o w u s t o+
d e t e r m i n e t h e o n s e t a p p e a r a n c e e n e r g y ( ~ ' " 1 2 . 7 e V ) . T h i s v a l u e f o r HQ
+
on AU i s s l i g h t l y h i g h e r t h a n v a l u e s on W o r o t h e r m e t a l s . The a p p e a r - a n c e e n e r g y d a t a l e a d t o t h e c o n c l u s i o n t h a t t h e m e c h a n i s m o f H;-forma- t i o n w i l l h a v e a m e c h a n i s m s i m i l a r t o t h a t p r o p o s e d f o r t u n g s t e n . T h i s w o u l d i n v o l v e t h e e l e c t r o - s t a t i c f i e l d i n d u c i n g t h e d i s s o c i a t i o n o f H2 m o l e c u l e s o n g o l d e m i t t e r s u r f a c e s .
I V
-
ACKNOWLEDGEMENTSM r . J . K n o b l o c h was h e l p f u l i n d e v e l o p i n g c o m p u t e r p r o g r a m s . T h i s w o r k was i n p a r t s u p p o r t e d b y t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t / S o n d e r -
F i g . 5
-
H y d r o g e n - i o n c o u n t i n g r a t e F i g . 6-
R e t a r d a t i o n c u r v e s o f a s a f u n c t i o n o f t i p - c o u n t e r e l e c - h y d r o g e n i o n s m e a s u r e d a t t r o d e v o l t a g e a t A u ( l l 1 ) s u r f a c e . A u ( l l 1 ) s u r f a c e .f o r s c h u n g s b e r e i c h 6 V - REFERENCES
1. E . W . M u l l e r a n d T . T . T s o n g , i n " F i e l d I o n M i c r o s c o p y , P r i n c i p l e s a n d A p p l i c a t i o n s " , E l s e v i e r , New Y o r k , 1 9 6 9 , P r o g . S u r f . S c i .
4,
1 ( 1 9 7 3 ) .
2 . A . J . J a s o n , P h y s . Rev. 1 5 6 , 266 ( 1 9 6 7 ) .
3 . S.V. K r i s c h n a s w a m y a n d
m.
M u l l e r , Z . P h y s . Chem.104,
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