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DESORPTION OF SECONDARY NEGATIVE IONS INDUCED BY FAST HEAVY INCIDENT ATOMIC
AND MOLECULAR IONS
Mehran Salehpour, J. Hunt
To cite this version:
Mehran Salehpour, J. Hunt. DESORPTION OF SECONDARY NEGATIVE IONS INDUCED BY
FAST HEAVY INCIDENT ATOMIC AND MOLECULAR IONS. Journal de Physique Colloques,
1989, 50 (C2), pp.C2-155-C2-158. �10.1051/jphyscol:1989226�. �jpa-00229424�
DESORPTION OF SECONDARY NEGATIVE I O N S INDUCED BY F A S T HEAVY I N C I D E N T ATOMIC AND MOLECULAR IONS(')
M . S A L E H P O U R ( ~ ) and J . E . HUNT
200 CHM, A r g o m e National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, U.S.A.
RESUME
iics i o n s l o u r a s atov~iques e t r ~ o l e c u l a i r e s o n t e t e u t i l i s e s comme i o n s i n c i d e n t s dans des d u d e s de d e s o r p t i o n d ' i o n s secondaires. Des i o n s
c+,
Cz+, C3+, ~ r ' , CH', CH3+, C3H3+, i4H9+, C H +, CO*, 02+, LO2+, CF*, CF3+, C3F5+ and C F + o n t e t e p r o d u i t s e t acceleres dans7 15 4 7
i a gamme d'energie 600 keV
-
3.9 MeV. Les ions moleculaires o n t des rendements de desorption dleves. Ceci presente un i n t e r & p r a t i q u e en spectrometric de masse. Une i n t e r p r e t a t i o n de I'augmentation du rendement e s t presentee.ABSTRACT
F a s t heavy atomic and molecular ions have been used as i n c i d e n t i o n s i n secondary i o n desorption studies. A number o f i o n s such as
c',
CZ+¶ C3+. ~ r + , CH+, CH3+, C3H3+, C4H9+¶C H
+,
CO+, 02+¶ C02+, CF+, CF3+¶ C3F5+ and C F + have been produced and accelerated i n t h e7 15 4 7
energy range 600 keV
-
3.9 MeV. The molecular i n c i d e n t ions g i v e r i s e t o h i g h measured secondary i o n y i e l d s which i s o f p r a c t i c a l i n t e r e s t i n mass spectrometry. An i n t e r p r e t a t i o n 0.F enhancement i n t h e y i e l d w i t h molecular i n c i d e n t i o n s i s presented.(')work performed under the auspices of the Office of Basic Energy Sciences. U . S . Department of Energy under contract number W-31-109-ENG-38
( 2 ) ~ r e s e n t Address : Department of Radiation Sciences, Ion Physics Division. B o x 533. Uppsala University.
Uppsala. Sweden
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989226
C2-156 JOURNAL
DE
PHYSIQUEP a r t i c l e induced desorption o f polyatomic i o n s from surfaces i s o f general t h e o r e t i c a l i n t e r e s t , as w e l l as having p r a c t i c a l a p p l i c a t i o n s i n t h e f i e l d o f mass spectrometry. A l a r g e body o f experimental data e x i s t on t h e use o f f a s t ( v
>
vBohr) atomic i n c i d e n t ions i n d e s o r p t i o n s t u d i e s ( 1 ) . However, few r e p o r t s e x i s t on the use o f f a s t molecular ions as t h e primary ions i n desorption experiments. Brown e t a l . (2) and Thomas e t a l . (3) have used hydrogen c l u s t e r i o n s t o measure t h e y i e l d s o f H20 and C S + ions r e s p e c t i v e l y ( y i e l d i s d e f i n e d as t h e number o f t h e species o f i n t e r e s t desorbed per i n c i d e n t i o n ) . We r e c e n t l y r e p o r t e d on desorption experiments w i t h the use o f f a s t heavy molecular i o n s such asc+,
c2+, C3+, ~ r + , CH', cH3+, C H +, C4Hg+, C7HI5+, CO+, 02+, C02+, CF+, CF3+, c3F5+ and C F +,
3 3 4 7
i n the energy range 600 keV
-
3.9 MeV (4,5,6,7).The experimental s e t up i s explained i n d e t a i l elsewhere ( 7 ) and w i l l n o t be discussed here i n any d e t a i l . The ions a r e produced i n an e l e c t r o n - i o n i s a t i o n i o n source l o c a t e d a t t h e h i g h voltage t e r m i n a l o f a 4.5 MV Dynamitron a c c e l e r a t o r . The accelerated molecular ions a r e unambigously i d e n t i f i e d and reach t h e t i m e - o f - f l i g h t (TOF) mass spectrometer. The secondary i o n y i e l d s a r e measured u s i n g conventional TOF techniques. The standard sample has been t h e amino a c i d val i n e ( I I W = 117) electrosprayed on a t h i c k s t a i n l e s s s t e e l p l a t e . The r e s u l t s presented here a r e l i m i t e d t o negative secondary ions since t h e secondary e l e c t r o n s have been used t o t r i g g e r the s t a r t pulse f o r t h e TOF measurements.
Figure 1 shows t h e negative secondary molecular i o n y i e l d o f v a l i n e , Y , as. a f u n c t i o n of the v e l o c i t y o f t h e primary
c',
C2+, C3+ i n c i d e n t ions. Fron t h e r e l a t i v e secondary i o n y i e l d s of v a l i n e , Y, as a r e s u l t o f i ~ , ~ g a c t w i t hc+,
C2+ and C3+, i t i s apparent t h a t non l i n e a r i t i e s e x i s t ; th e secondary i o n y i e l d o f v a l i n e molecular ions f o r i n c i d e n t C2+, Y ( c ~ + ) i s much more t h a t t w i c e t h a t f o r atomic carbon, i.e., Y(C2+)>>
~ . Y ( c + ) . S i m i l a r l y Y(c,+)>>
3.Y(CC).Lie have e m p i r i c a l l y determined t h a t Y(c+) f o r v a l i n e n e g a t i v e molecular i o n s s c a l e w i t h t h e square of t h e e l e c t r o n i c stopping power, dE (6,7). Such a r e l a t i o n s h i p would then p r e d i c t f o r molecular ions ; Y(C2+)
2
~ . Y ( c + ) and Y(C3+)2
~ . Y ( c ' ) . However, the experimental p o i n t show much h i g h e r r a t i o s over a range o f primary i o n v e l o c i t i e s , i'.e., approximatelyY(C2+)
%
~ . Y ( c + ) and Y(C3+)2
I ~ . Y ( c + ) . S i m i l a r r e s u l t s have been obtained w i t h o t h e r i n c i d e n t ions such as C+ and 0' i n comparison w i t h CO+ and 02+ ( 7 ) . I t thus seems t h a t o t h e r processes nay be present.I t has been shown e x p e r i m e n t a l l y t h a t t h e e l e c t r o n i c stopping power o f an atom i n a f a s t moving c l u s t e r i n s o l i d s i s h i g h e r i n magnitude than t h e sum of t h e stopping powers o f t h e atomic c o n s t i t u e n t s i n the saae medium. The enhancement i n dE per atom X i n a c l u s t e r
dE
Xn was d e f i n e d by Brandt (8) t o be, -
5
I X n ) Brandt e t a1 r e p o r t e d R values i n t h e d Ez
(Xi)range 1-1.5 f o r Hz+ and H3+ i n t h i n carbon f i l m s . Tape e t a l . ( 9 ) have obtained s i m i l a r d E
r e s u l t s f o r Hz+ and O2- i n c i d e n t i o n s . The enhancement i n was i n t e r p r e t e d as being a
" c o l l e c t i v e " e f f e c t due t o t h e temporal and s p a t i a l c o r r e l a t i o n o f t h e f d s t moving charges g i v i n g r i s e t o an i n t e r f e r e n c e term i n t h e t o t a l e l e c t r o n i c stopping power o f t h e c l u s t e r ( 8 ) .
As discussed i n d e t a i l i n references ( 7 ) , t h e d e s o r p t i o n depth i s comparable t o t h e depth r e q u i r e d f o r t h e i n c i d e n t molecular ions t o separate completely and thus, any enhan- cemnts i n
-
dE would be r e f l e c t e d i n desorption y i e l d s . The desorption data can be describeddx
d E
by assuming a q u a d r a t i c dependence on and u s i n g R values i n t h e range 1.1-1.4 f o r a1 1 o f t h e experimental p o i n t s . This i s comparable t o t h e R values r e p o r t e d by Tape e t a l . (9) f o r 02- i n c i d e n t i o n s i n t h e same v e l o c i t y range as our experiment.
P r e l i m i n a r y r e s u l t s w i t h another sample w i t h d i f f e r e n t chemical p r o p e r t i e s , c h l o r o p h y l l
a
(Mw = 893) showed the same enhancements as v a l i n e ions. Hedin e t a1.
( 1 0 ) r e c e n t l y have r e p o r t e d d e s o r p t i o n experiments w i t h C2+ i n c i d e n t ions a t a v e l o c i t y o f 0.49 cm/ns. T h e i r v a l i n e negative i o n data a r e i n q u a l i t a t i v e agreement w i t h ours ; they obtained R = 1.2+0.1.However, f o r p o s i t i v e v a l i n e molecular ions they found R = 1 ( t h e conclusions a r e however,
dE dE
r a t h e r s e n s i t i v e t o the choice o f Since any enhancements i n
-
a r e expected t o be due dxt o t h e d i s t a n t c o l l i s i o n s (8) (and n o t t h e c l o s e c o l l i s i o n s ) , such d i f f e r e n c e s between the
C2-158 JOURNAL
DE
PHYSIQUEp o s i t i v e and n e g a t i v e secondary ions might present a c l u e as t o t h e t h e i r d i f f e r e n t o r i g i n s . More experimental r e s u l t s are needed t o c l a r i f y t h i s .
The i m p l i c a t i o n s o f t h i s work a r e two-fold. F i r s t , w i t h t h e l a r g e i n c i d e n t molecular i o n s such as C H + and C4F7+, h i g h secondary i o n y i e l d s have been measured (up t o 60 times
7 15
t h a t w i t h atomic carbon) which c o u l d be o f p r a c t i c a l i n t e r e s t i n mass .spectrometry. Second, n e g a t i v e i o n d e s o r p t i o n s t u d i e s might be u s e f u l i n t h e f u t u r e f o r studying t h e i n t e r a c t i o n o f c l u s t e r i n c i d e n t ions w i t h s o l i d s .
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4
(1985) 421 ( 2 ) 11.L. Brown,W.M.
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Salehpour, D.E. F i s h e l and J.E. Hunt, I n t . J. Mass Spectrom. I o n Proc. - 84 (1988) R7.(6) M. Salehpour, D.E. F i s h e l and J.E. Hunt, J. Appl. Phys.
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(1988) 831.( 7 )
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Salehpour, D.E. F i s h e l and J.E. Hunt, To appear i n Phys. Rev. B. (1988).( 8 )
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Brandt, A. Ratkowski and R.H. R i t c h i e , Phys. Rev. L e t t . 33 (1974) 1325.(9) J.W. Tape, 11.11. Gibson, J. Remillieux, R. Laubert and H.E. Wegner, Nucl. I n s t r . Meth.
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(10) A. Hedin, R. Hakansson and B. Sundqvist, these proceedings.
