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Pulsed field evaporation mass spectrometry
156
Internatlonal Journal o f ,~_ass Spectrometry and Ion Physic~
Elsevier Publishing Company, Amsterdam. Printed in the Netherlands
Pulsed field evaporation nl~ss spectrometry
In a recent s t u d y o f the three metals Fe, Co a n d Ni, Barofsl~- a n d Miillcr sought by mass spectroscopic m e a n s to detect the reaction p r o d u c t s at various temperatures between th,~e three metals a n d the gases N~_, 0 2 a n d C O t. T h e results o f this investigation were disappointing in that metal-gas complexes could be detect~I o n l y ar t e m p e r a t u r e s o f a b o u t 150 °K o r higher. Field ionization microscope studies, o n the o t h e r hand, indicate that such reactions d o occur at 78 ° K a n d even at 21 OK. _At temperatures below 150 °K only the d o u b l y charged ion species o f the p u r e metals have been observed. Because the emitter tip is constantly c o n s u m e d by the field e v a p o r a t i o n process, it was f o u n d necessary in o r d e r to inaintain the required evaporation field s t r e n N h to continuously increase the tip voltage at a c o n s t a n t rate while the mass analyzer scanned the mass range. It was p r o p o s e d in the a b o v e m e n t i o n e d s t u d y t h a t this technique was causing a relatively limited supply, perhaps only 1 o r 2 monolayers, o f ionic complexes to be depleted before the mass spectrometer s c a n n e d over the a p p r o p r i a t e masses. It was suKgested that some f o r m o f pulsed field e v a p o r a t i o n m i g h t alleviate this experimental difficulty. T h e initial results o f such a pulsed field e v a p o r a t i o n technique are reported in this c o m m u n i c a t i o n . T h e existing field e v a p o r a t i o n mass spectrometer ~ was modified to provide a pulsed m o d e o f o p e r a t i o n as s h o w n in Fig. 1. Voltage pulses, variable f r o m 0-700 V, were super/reposed o n the negati,,e D.C. voltage applied to the acceler-
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Fig. 1. Instrumentat/on diagram of the pulsed field evaporation mass spectrometer. Int. J. ..~faxs Spectrorn. Ion Phys., 3 0969) 156-158
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COMMUNICATIONS
!57
ating electrode. The magnet p o w e r circuit was modified to m a k e short sawtooth sweeps f r o m some preset level o f the magnetic field. T h e driving signals for these two operations were provided by the oscilloscope. "i-he square wave o u t p u t was used to drive the pulser, which in reality is a high vc.ltage amplifier employing an A m p e r e x A705 high voltage transistor, a n d the s a w t o o t h o u t p u t was used to provide the r a m p f o r th~ magnetic field drive. This instrumentation allows a n y portion o f the mass scale to be repeatably scanned with a relatively slow d u t y cycle o f 0.5 see. Si-_nultaneou~ly, a voltage pulse m a y be applied to the tip d u r i n g the sweep portion ot the d u t y cycle. ~'hus, evaporation is p e r f o r m e d only while the mass analyzer is scanning that portion o f the mass spectrum suspected to contain the sought after mass peaks. D u r i n g the off portion o f the cycle. 0.25 sec, the field strength is reduced back to a subevaporation value at which new reaction p r o d u c t s can be formed. The mass spectrum exhibited in Fig. 2a was recorded while evaporating Ni
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Fig. 2. 78 ~ K f i e l d e v a p o r a t i o n mass spectra o f (a) N i r e c o r d e d i n the presence o f 3 .'~ 10 - ~ T o r t N e ~ i t h 280 V p u l s e s ( e x p o s u r e 1 0 s e c ) a n d Co) C o r e c o r d e d i n t h e p r e s e n c e o f a 10 - 6 T o r t . l 0 : 1 m i x t u r e o f N_, a n d O_, r e s p e c t i v e l y b y m e a n s o f t h e t i m e i n t e g r a t i o n f e a t u r e .
at 78 °K a n d in the presence o f 3 × 10 -7 T o r r Ne. 280 V pulses were appl'.'ed to the emitter tip. Besides the two N e isotopes, which are used as reference peaks, the two d o u b l y c h a r g e d Ni isotopes, ~SNi÷- a n d 6 ° N i * - , a n d the doubly charged Ni oxides. SeNiO--- a n d 6°NiO-~-, are recorded in the spectrum. Neither this n o r a n y other field reaction complex o f nickel h a d previously been observed at 78 °K. In the absence o f the emitter pulses only the water spectrum remained. T h e presence o f some H a O - in the spectrum indicates that the field strength during the o f f p o r tion o f the evaporation pulses is low e n o u g h for the residual water v a p o r to reach the tip surface and begin to dissociate. Thus, residual water v a p o r in the v a c u u m system (a p o r t i o n o f the space-ionized H 2 0 + p e a k is observable at the far left in the Ni s p e c t r u m ) p r o b a b l y acts as the source o f oxygen for the f o r m a t i o n o f the ionic Ni-oxides. The sensitivity o f this pulse technique is also e n h a n c e d to some degree Int. J . .~l"~s S p e c t r o m . I o n P h y s . , 3 (1969) 156--158
158
SHORT
COMI~IU b l I C A T I O N S
b y the integration o v e r time o f the single scans. W i t h typical e x p o s u r e times o f 10 t o 20 see s o m e 20 t o 40 individual scans are s u p e r i m p o s e d o n the spectrogram. In this w a y v e r y l o w a b u n d a n c e mass p e a k s are allox~ ed t o develop. T h e s p e c t r u m in Fig. 2 b exhibits the effectiveness o f this integrating action alone. This mass s p e c t r u m is o f a C o tip e v a p o r a t i n g at 78 ° K in the presence o f a 1 0 : I ,mixture respectively o f N2 a n d Oz w h o s e total pressure is 10 - 6 T o r t . Immediately preceding the beginning o f the recording, the tip voltage was increased b y 500 V to a total tip voltage o f 10.S kV where it remained during the recording time o f 20 see. T h e mass l:¢aks o f N z +, C o "-+, Oz +, C o N z ++ a n d C o O z +~ are observable in the spectrogram. Again this is the first time that the metal 8as reaction p r o d u c t s have b e e n o b s e r v e d at 78 ° K. T h e meta_~gas complexes resulting f r o m nickel a n d cobalt at 78 °K are o b s e r v e d at higher t e m p e r a t u r e s as singly charged ions x. This transition f r o m a m o r e highly ionized t o a less highly ionized metal ion has b e e n observed for those p u r e metal spec,_-es studied, t o d a t e b y B a r o f s k y a n d MiJiler 1.z a n d has been explained qualitatively b y T s o n g 3 on the basis o f ionic tunneling. The a b o v e observations indicate this as a possibility for field reaction p r o d u c t s as well.
Acknowledgements T h e a u t h o r is i n d e b t e d to P r o f e s s o r E. W. Miiller, in w h o s e Field Emission L a b o r a t o r y this w o r k w a s c o n d u c t e d , for his valuable c o n t r i b u t i o n s a n d discussions. FfiaancSal s u p p o r t was supplied b y the N a t i o n a l Aeronautics a n d Space A d ministration_ the N a t i o n a l Science F o u n d a t i o n a n d the A m e r i c a n I r o n a n d Steel Institute.
Oregon Graduate Center, 9340 S. IV. Barnes Road, Portland, Oreg. 97225 (U.S.A.)
D O U G L A S f . BAROFSKY
I D . F . B~orsk-~r ~ E . W . Mr~n TI:R, 3". 3,1ras~ Sflectrorn. Ion Phys., 2 (1959) 125. 2 D . F . B . ~ o r s K Y A>,'D E. W . /~,ti3~.m% Surface Sci., 10 (1968) 177. 3 T . T . TZO.~G, Surface Sci., 10 (1968) I02.
Received M a y 30th. 1969 In.t.J..a, Cass Spectrom. Ion Phys., 3 (1969) 156-158
Internatlonal Journal o f ,~_ass Spectrometry and Ion Physic~
Elsevier Publishing Company, Amsterdam. Printed in the Netherlands
Pulsed field evaporation nl~ss spectrometry
In a recent s t u d y o f the three metals Fe, Co a n d Ni, Barofsl~- a n d Miillcr sought by mass spectroscopic m e a n s to detect the reaction p r o d u c t s at various temperatures between th,~e three metals a n d the gases N~_, 0 2 a n d C O t. T h e results o f this investigation were disappointing in that metal-gas complexes could be detect~I o n l y ar t e m p e r a t u r e s o f a b o u t 150 °K o r higher. Field ionization microscope studies, o n the o t h e r hand, indicate that such reactions d o occur at 78 ° K a n d even at 21 OK. _At temperatures below 150 °K only the d o u b l y charged ion species o f the p u r e metals have been observed. Because the emitter tip is constantly c o n s u m e d by the field e v a p o r a t i o n process, it was f o u n d necessary in o r d e r to inaintain the required evaporation field s t r e n N h to continuously increase the tip voltage at a c o n s t a n t rate while the mass analyzer scanned the mass range. It was p r o p o s e d in the a b o v e m e n t i o n e d s t u d y t h a t this technique was causing a relatively limited supply, perhaps only 1 o r 2 monolayers, o f ionic complexes to be depleted before the mass spectrometer s c a n n e d over the a p p r o p r i a t e masses. It was suKgested that some f o r m o f pulsed field e v a p o r a t i o n m i g h t alleviate this experimental difficulty. T h e initial results o f such a pulsed field e v a p o r a t i o n technique are reported in this c o m m u n i c a t i o n . T h e existing field e v a p o r a t i o n mass spectrometer ~ was modified to provide a pulsed m o d e o f o p e r a t i o n as s h o w n in Fig. 1. Voltage pulses, variable f r o m 0-700 V, were super/reposed o n the negati,,e D.C. voltage applied to the acceler-
LFI ©
I !
I o-15~v
I
I
Ne~-t,,~:-~---~I-O-TOOVl I I Powt-r
C,rcuit
l
I
Fig. 1. Instrumentat/on diagram of the pulsed field evaporation mass spectrometer. Int. J. ..~faxs Spectrorn. Ion Phys., 3 0969) 156-158
SHORT
COMMUNICATIONS
!57
ating electrode. The magnet p o w e r circuit was modified to m a k e short sawtooth sweeps f r o m some preset level o f the magnetic field. T h e driving signals for these two operations were provided by the oscilloscope. "i-he square wave o u t p u t was used to drive the pulser, which in reality is a high vc.ltage amplifier employing an A m p e r e x A705 high voltage transistor, a n d the s a w t o o t h o u t p u t was used to provide the r a m p f o r th~ magnetic field drive. This instrumentation allows a n y portion o f the mass scale to be repeatably scanned with a relatively slow d u t y cycle o f 0.5 see. Si-_nultaneou~ly, a voltage pulse m a y be applied to the tip d u r i n g the sweep portion ot the d u t y cycle. ~'hus, evaporation is p e r f o r m e d only while the mass analyzer is scanning that portion o f the mass spectrum suspected to contain the sought after mass peaks. D u r i n g the off portion o f the cycle. 0.25 sec, the field strength is reduced back to a subevaporation value at which new reaction p r o d u c t s can be formed. The mass spectrum exhibited in Fig. 2a was recorded while evaporating Ni
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In! i l l HNNNIni ll m n n l lllilll,alll !| ! R I D Ifllllni|llliltlll | glNllllilll I111!_!1!i 1i ! 1
i- ~ - ' - ' ~ -
Y."~ (a)
-"-"
I1fall Fill
,1 ~ i ' ~ a " " ' ~ a J ~ (b)
Fig. 2. 78 ~ K f i e l d e v a p o r a t i o n mass spectra o f (a) N i r e c o r d e d i n the presence o f 3 .'~ 10 - ~ T o r t N e ~ i t h 280 V p u l s e s ( e x p o s u r e 1 0 s e c ) a n d Co) C o r e c o r d e d i n t h e p r e s e n c e o f a 10 - 6 T o r t . l 0 : 1 m i x t u r e o f N_, a n d O_, r e s p e c t i v e l y b y m e a n s o f t h e t i m e i n t e g r a t i o n f e a t u r e .
at 78 °K a n d in the presence o f 3 × 10 -7 T o r r Ne. 280 V pulses were appl'.'ed to the emitter tip. Besides the two N e isotopes, which are used as reference peaks, the two d o u b l y c h a r g e d Ni isotopes, ~SNi÷- a n d 6 ° N i * - , a n d the doubly charged Ni oxides. SeNiO--- a n d 6°NiO-~-, are recorded in the spectrum. Neither this n o r a n y other field reaction complex o f nickel h a d previously been observed at 78 °K. In the absence o f the emitter pulses only the water spectrum remained. T h e presence o f some H a O - in the spectrum indicates that the field strength during the o f f p o r tion o f the evaporation pulses is low e n o u g h for the residual water v a p o r to reach the tip surface and begin to dissociate. Thus, residual water v a p o r in the v a c u u m system (a p o r t i o n o f the space-ionized H 2 0 + p e a k is observable at the far left in the Ni s p e c t r u m ) p r o b a b l y acts as the source o f oxygen for the f o r m a t i o n o f the ionic Ni-oxides. The sensitivity o f this pulse technique is also e n h a n c e d to some degree Int. J . .~l"~s S p e c t r o m . I o n P h y s . , 3 (1969) 156--158
158
SHORT
COMI~IU b l I C A T I O N S
b y the integration o v e r time o f the single scans. W i t h typical e x p o s u r e times o f 10 t o 20 see s o m e 20 t o 40 individual scans are s u p e r i m p o s e d o n the spectrogram. In this w a y v e r y l o w a b u n d a n c e mass p e a k s are allox~ ed t o develop. T h e s p e c t r u m in Fig. 2 b exhibits the effectiveness o f this integrating action alone. This mass s p e c t r u m is o f a C o tip e v a p o r a t i n g at 78 ° K in the presence o f a 1 0 : I ,mixture respectively o f N2 a n d Oz w h o s e total pressure is 10 - 6 T o r t . Immediately preceding the beginning o f the recording, the tip voltage was increased b y 500 V to a total tip voltage o f 10.S kV where it remained during the recording time o f 20 see. T h e mass l:¢aks o f N z +, C o "-+, Oz +, C o N z ++ a n d C o O z +~ are observable in the spectrogram. Again this is the first time that the metal 8as reaction p r o d u c t s have b e e n o b s e r v e d at 78 ° K. T h e meta_~gas complexes resulting f r o m nickel a n d cobalt at 78 °K are o b s e r v e d at higher t e m p e r a t u r e s as singly charged ions x. This transition f r o m a m o r e highly ionized t o a less highly ionized metal ion has b e e n observed for those p u r e metal spec,_-es studied, t o d a t e b y B a r o f s k y a n d MiJiler 1.z a n d has been explained qualitatively b y T s o n g 3 on the basis o f ionic tunneling. The a b o v e observations indicate this as a possibility for field reaction p r o d u c t s as well.
Acknowledgements T h e a u t h o r is i n d e b t e d to P r o f e s s o r E. W. Miiller, in w h o s e Field Emission L a b o r a t o r y this w o r k w a s c o n d u c t e d , for his valuable c o n t r i b u t i o n s a n d discussions. FfiaancSal s u p p o r t was supplied b y the N a t i o n a l Aeronautics a n d Space A d ministration_ the N a t i o n a l Science F o u n d a t i o n a n d the A m e r i c a n I r o n a n d Steel Institute.
Oregon Graduate Center, 9340 S. IV. Barnes Road, Portland, Oreg. 97225 (U.S.A.)
D O U G L A S f . BAROFSKY
I D . F . B~orsk-~r ~ E . W . Mr~n TI:R, 3". 3,1ras~ Sflectrorn. Ion Phys., 2 (1959) 125. 2 D . F . B . ~ o r s K Y A>,'D E. W . /~,ti3~.m% Surface Sci., 10 (1968) 177. 3 T . T . TZO.~G, Surface Sci., 10 (1968) I02.
Received M a y 30th. 1969 In.t.J..a, Cass Spectrom. Ion Phys., 3 (1969) 156-158