Demountable field emission microscope tube

Letters

to the Editor

Omegatron Investigation of Hydrogen in an Ultra-High Vacuum System* PREVIOUS w o r k e r s x-v h a v e a n a l y s e d t h e residual gases in u n b a k e d a n d b a k e d v a c u u m s y s t e m s c o n s t s t m g o f radio t u b e s a n d television picture tubes, a n d h a v e also studied t h e a d s o r p t i v e properties o f getter films. H ] c k m o t t e h a s recently p u b l i s h e d a n r e f o r m a t i v e p a p e r o n t h e interaction o f a t o m i c h y d r o g e n with glass W e h a v e e n c o u n t e r e d residual gases m a n u l t r a - h i g h v a c u u m s y s t e m o f s u c h a c h a r a c t e r as to accord well with Hlckm o t t ' s findings In a d d i t i o n we h a v e n o t e d a slgmficant m a s s 3(He +) p e a k m t h e m a s s s p e c t r a o f h y d r o g e n - c o n t a i n i n g residual gases a n d a fast i o n g a u g e p u m p i n g o f t h e h y d r o g e n p r e s e n t m t h e residual gases T h i s n o t e is a brief s u m m a r y o f t h e s e findings

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FIG 2

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Spectrum of hydrogen-containing residual gases m v a c u u m system with the Bayard-Alpert ion gauge on

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FIG 1

VI and V2 magnetically controlled ball joint valves F 5 m d m d m m filament. T hqmd mtrogen trap I G Bayard-Alpert 1on gauge M magnet (3250G) O omegatron G gas supply G P Granvllle-Phllhps valve

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T h e v a c u u m s y s t e m was similar to t h a t u s e d by A r t h u r a n d H a n s e n x° with s o m e m o & f i c a t l o n s (Fig 1) T h e i o n g a u g e w a s a B a y a r d - A l p e r t Ionization gauge, W L - 5 9 6 6 , w h i c h was a t t a c h e d to t h e reaction cell t h r o u g h 25 m m t u b u l a t l o n , h e n c e i n c r e a s i n g t h e c o n d u c t a n c e f r o m t h e reaction cell to t h e ion gauge. T h e partial p r e s s u r e a n a l y s e r was a L e y b o l d - O m e g a t r o n n , z2 m a s s s p e c t r o m e t e r tube, w]th f o u r g u a r d rings to e n h a n c e the u n i f o r m i t y o f t h e variable r a d i o - f r e q u e n c y field T h e h y d r o g e n u s e d was R e a g e n t R e s e a r c h G r a d e o b t a i n e d f r o m the M a t h e s o n C o m p a n y m o n e htre flasks with b r e a k - o f f tips, a n d t h e gas s u p p l y was s e p a r a t e d f r o m t h e w o r k i n g area by a b a k a b l e G r a n v f l l e - P h d h p s valve T h e ion c u r r e n t f r o m t h e o m e g a t r o n w a s m e a s u r e d with a n A p p h e d P h y s i c s v i b r a t i n g reed electrometer, m o d e l 31-CV, a n d recorded o n a M o s e l y x - y recorder, m o d e l 3S T h e s y s t e m was b a k e d o u t at 4 5 0 ° C for 8 h a n d all t h e electrodes were extensively o u t g a s s e d until t h e p r e s s u r e in t h e s y s t e m was 2 × 10 -~° torr T h e b r e a k - o f f seal was b r o k e n by m e a n s o f a glass e n c a p s u l a t e d m a g n e t , a n d h y d r o g e n was leaked ]nto t h e s y s t e m u n t d t h e p r e s s u r e was 6 × 10 -7 t o r r ( e q m v a l e n t N2 pressure) T h e partial p r e s s u r e o f h y d r o g e n was d e t e r m i n e d u s i n g a value o f 4 t o r t -~ for the sensitivity o f t h e o m e g a t r o n for h y d r o g e n x3 W i t h t h e ion g a u g e o p e r a t i n g at 1 0 m A e m i s s i o n c u r r e n t a n d t h e o m e g a t r o n o p e r a t i n g at a b e a m c u r r e n t o f 5tLamps, t h e m a r e p e a k s recorded were H2 +, C O +, H 2 0 +, H3 + a n d CHa + (Fig 2) T h e o t h e r p e a k s o b s e r v e d were smaller a n d were d u e to t h e i o n f r a g m e n t s o f t h e p a r e n t Ions W i t h t h e 1on g a u g e off a n d t h e o m e g a t r o n o p e r a t i n g at a b e a m c u r r e n t o f 5t~amps, t h e m a i n p e a k s recorded were H~ +, H3 + a n d H + (Fig 3) T h e o t h e r p e a k s CO2 +, *Contribution No 1241

55

b_

Vacuum system for flash desorpt]on experiments

FIG 3

I

number

Spectrum of residual gases with ion gauge off.

C O +, a n d HeO + were smaller by at least 25 × The change in t h e relative intensities o f t h e p e a k s in Fig 2 a n d 3 ts d u e to t w o processes M o l e c u l a r h y d r o g e n is p u m p e d by t h e i o n g a u g e (Fig 4), t h e r e b y r e d u c i n g t h e intensity o f t h e He + p e a k with t h e ion g a u g e o n W h e n t h e ion g a u g e is t u r n e d off, t h e partial p r e s s u r e o f h y d r o g e n increases s u d d e n l y , followed by a slow decrease d u e to adsorption of hydrogen on the tungsten filament of the ion gauge T h i s a d s o r b e d h y d r o g e n is rap]dly d e s o r b e d a n d p u m p e d w h e n t h e i o n g a u g e is t u r n e d on. T h e c o n t a m i n a n t molecules C O , H 2 0 a n d CH4 are a t t r i b u t e d to t h e i n t e r a c t i o n of a t o m i c h y d r o g e n with glass, t h e atormc h y d r o g e n b e i n g p r o d u c e d f r o m m o l e c u l a r h y d r o g e n by t h e i n c a n d e s c e n t t u n g s t e n filament in t h e i o n gaugeS, 9 T h e partial p r e s s u r e of c a r b o n m o n o x i d e in t h e s y s t e m was 1 3 × 10 -v torr, w h i c h c o r r e s p o n d e d to a b o u t 10 per cent o f t h e total pressure, with t h e ]on g a u g e o p e r a t i n g at 10 m A e m i s s i o n current T h e partial p r e s s u r e o f c a r b o n m o n o x i d e decreased s h a r p l y w h e n t h e Ion g a u g e was t u r n e d off (Fig 5) T h i s is d u e partially to a d s o r p t w e processes a n d t h e a b s e n c e o f t h e h o t t u n g s t e n filament w h i c h p r o d u c e s atorruc h y d r o g e n The a d s o r p t i o n process a p p e a r s to be t h e p r e d o m i n a n t o n e f r o m t h e size o f t h e d e s o r p t i o n p e a k w h e n t h e Ion g a u g e is t u r n e d on. W h e n t h e 1on g a u g e ~s o p e r a t i n g at 1 m A e m i s s i o n current, t h e parttal p r e s s u r e o f c a r b o n m o n o x i d e is r e d u c e d by a factor of t w o to 6 5 × 10 _8 torr

Work was performed in the Ames Laboratory of the U S. Atomic Energy Commission. 229

230

Letters to the Edator

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Pumping action of the ]on gauge on molecular hydrogen

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The effect of the xon gauge on the parttal pressure of carbon monoxtde

T h e presence of mass n u m b e r 3(Ha +) in a high v a c u u m s y s t e m and an h y d r o g e n discharges has been reported by previous workers14, 15, 16 We have concluded f r o m o u r w o r k that the mass n u m b e r (3) peak was due entirely to H3 +, f o r m e d f r o m molecular hydrogen, and not f r o m a p r o d u c t of a d e c o m p o s i t i o n reaction o f CH4, or a h a r m o n i c of m a s s 6 or 9, as these were absent T h e r a t l o of the Ha + peak to the H2 + p e a k increases as the pressure ~s increased, which is to be expected f r o m a colhslon m e c h a m s m E y r m g et a117 attributed the f o r m a t i o n of Ha + to a secondary colllsaon

described by the e q u a t i o n H2 + Hz + = H3 + + H Clements and Muller 15 observed the a b u n d a n t occurrence of H3 + in field ion rmcroscopes Their observations s h o w H3 + only over single p r o t r u d i n g surface a t o m s B~,jeu and C o m s a present an experimentally checked relation between the H3 + ion current and the pressure of h y d r o g e n They also reporO ° that m s o m e cases the H3 + ion current m a y be greater than the H~ + 1on current This is due to specific features of the o m e g a t r o n ion source The relation p r o p o s e d by B~jeu a n d C o m s a , was used to calculate the expected value of the ion current due to H3 + , the result obtained agreed with the observed value within experimental error It is ewdent that surface p h e n o m e n a occurring at well charactermed surfaces o f small area are partially obscured due to c o n t a m i n a n t s produced by the glass walls of the v a c u u m system and the hot filament of the ion gauge The a m o u n t of contamanants can be reduced by using a low w o r k function filament. H l c k m o t t s has s h o w n that a l a n t h a n u m b o n d e coated t a n t a l u m filament reduced the p r m o p a l c o n t a m i n a n t s

References I A Klopfer, S Garbe and W Schmldt, Sixth Symp Vac Tech 1959 Trans, Pergamon Press, (1960) 2 y Margonlnskl, S P Wolskyand E J Zdanuk, Vacuum, 11, (1961), 287 3 j S Wagener and P T Marth, J Appl Phys, 28, (1957), 1027 4 S P Wolsky and E J Zdanuk, Vacuum, 10, (1960), 13 5 W Tretner, Vacuum, 10, (1960) 31 6 W K Huber and E A Trendelenburg. Etghth Symp Vac Tech , 1961, Trans Pergamon Press, (1962) 7 O L~chtman, J Appl Phys, 31, (1960), 1213 8 T W Hlckmott, J Appl Phys, 31, (1960), 128 0 T W Hlckmott and G Ehrhch, J Phys Chem Sohds, 5, (1958), 47 10 j R Arthur and R S Hansen, J Chem Phys, 36, (1962), 2062 it H Sommer, H A Thomasand J A Hlpple, Phys Rev, 82, (1957), 697 iz D Alpert and R S Buntz, J Appl Phys, 25, (1954), 202 13 A Klopfer and W Schmldt, Vacuum, 10, (1960), 363 14 F H Field and J L Frankhn, Electron Impact Phenomena, Academic Press, Inc. New York, (1957), 219 15 T C Clements and E W Muller, J Chem Phys, 37, (1962), 2684 16 G Bfijeu and G Comsa, Etghth Symp Vac Techn, 1961, Trans Pergamon Press, (1962) 17 H Eyrmg, J O Htrschfelder and J S Taylor, J Chem Phys, 4, (1936), 479 V J MIMEAULT,

R S HANSEN Department o f Chemistry, Instttute o f Atomw Research, Iowa State Umverstty, Ames, Iowa, U S A