Notizen 1111 Cross Section Ratios for the Electron Impact
Production of Singly and Multiply Ionized Rare Gas Ions
F . Egger and T. D. Mark
In stitu t für A tom p hysik der U n iversität Innsbruck, Österreich
Z. Naturforsch. 33a, 1 1 1 1 - 1 1 1 3 (1978);
received Ju ly 3, 1978
E lectron im pact ionization o f H e, N e, Ar, K r and X e has been studied w ith a double focussing m ass spectrom eter Varian MAT CH 5. R atios o f various m ultiple ionization cross sections w ith respect to single ionization cross sections for H e, Ne, Ar, K r and X e a t electron energies o f 50, 100 and 150eV are given . These cross section ratios are com pared with previous determ inations.
E xperim ental d a ta on electron im p act ionization of gaseous species are of g reat in te re st for predicting a n d understanding th e properties of nonequilibrium plasm as, cf. e.g. ref. [1], H ow ever, even for th e rare gases discrepancies betw een published p a rtia l ion
ization cross section ratios am o u n t to m ore th a n a factor of 2. The present p ap er is d ev o ted to precise m easurem ents of p a rtia l cross section ratios in H e, Ne, Ar, K r and Xe.
Experimental
The experim ental arrangem ent was identical w ith th a t previously described [2, 3]. In short, it con
sists of a m olecular ty p e electron im p act source V arian MAT In te n sitro n M, a high resolution double focussing mass spectrom eter V arian MAT CH5, an d a gas handling system . The w orking conditions of th e ion source have been im proved, i.e.: The elec
tro n tra p collector p otential was raised to 24 V to ensure sa tu ra tio n of electron currents a t all electron energies. The range of th e continuously selectable electron accelerating voltage was expanded u p to 185 eV and th e voltage of th e electron beam focus
sing W ehnelt cylinder was m ain tain ed proportional to th e electron accelerating voltage in order to im prove electron cu rren t collim ation over th e whole electron energy range. Thus s tra y electron currents could be reduced to < 2 0 % of th e electron tra p current and were allowed for in th e calibration.
Consistency checks necessary in electron im pact studies and th e energy scale calibration have been carried out a n d discussed previously [2, 3].
In order to o b tain th e relev an t inform ation for th e present stu d y , ionization efficiency curves have
been m easured as a function of applied ex tractio n a n d focussing potentials [4], I t has been found th a t these curves depend on th e ex tractio n p o ten tial applied, an d only a relatively small range of e x tra c tio n potentials can be used [4], The m easured cross section ratios had a m axim um deviation from each o th er of ± 1 0 % in th is ex tractio n voltage range.
T he reported cross section ratios are averages over rep eated m easurem ents un d er various ex tractio n p otentials in th is range.
In all norm alization procedures th e ion currents have been m easured w ith a F a ra d a y collector cup, a n d electron currents of 50 fxA have been used.
T he gas tem p eratu re in th e collision cham ber has been stabilized a t 400 K during m easurem ents. The pusher electrode, which for m easurem ents of m ass spectra is usually operated positive w ith respect to th e collision cham ber p otential, was p u t a t th e sam e p o ten tial as th e collision cham ber. The rare gases used were obtained from Air R eduction Com
p a n y an d Fa. Linde w ith a p u rity of b e tte r th a n 99.995% . The reproducibility of m easured ion c u r
ren ts was in general b e tte r th a n ± 2 % . However, for very low ion currents, e.g. as in case of H e++, A r+++, Kr+++, and X e++++, th e statistical error could be as large as 5 to 10%. The estim ated m ax i
m um possible error is for cross section ratios > 0.01 a b o u t 10 to 20% an d for cross section ratios < 0.01 ab o u t 20 to 40% .
In order to dem onstrate th e reliability of th e presently m easured cross section ratios it is interesting to m ake a com parison w ith tw o p re vious studies [5, 6], in wich eith er b y v irtu e of th e ap p a ra tu s used [5] or by virtu e of th e m ethod a p plied [6] reliable cross section ratios have been o b tain ed . A dam czyk et al. [5] have used a cycloidal m ass spectrom eter, which does n o t suffer th e d raw back of charge dependent collection efficiency, b e
cause w ith o u t slits it is possible to a tta in com plete collection of all ions produced in th e source. Com
parison of g'(He++/He)/gf(H e+/H e) a n d g,(Ne++/N e)/
<7(Ne+/Ne) reported by those au th o rs w ith th e p re sent values (see Table I) shows fairly good agree
m en t w ithin th e experim ental error bars. D rew itz [6] has used a m agnetic sector field mass sp ectro m eter elim inating initial energy discrim ination by m eans of a deflection m ethod. Com parison of q ( Ar ++1 At) I q ( Ar+1 At) a t 100 eV rep o rted by D re
w itz w ith th e present value also shows agreem ent w ithin th e experim ental error bars. Thus it is con-
1112 Notizen
Table I. Ion ization cross section ratios o f the electron im pact production o f m u ltip ly to singly ionized rare gas ions at three different electron energies.
E nergy in eV 50 100 150 Author Apparatus
<7(He++/H e) — — 0,0025 B leakney et al. 1936 [7] not m entioned
q( H e+/H e) — 0,0007 0,0031 Harrison 1956 [8] thesis n o t available
— 0,00037 0,0020 A dam czyk et al. 1966 [5] cycloidal m ass spectrom eter
— 0,0025 0,0040 Gaudin et al. 1967 [10] 90° m agnetic sector field m .s ., N ier ty p e ion source
— 0,0003 0,0023 present stu d y —
<7(Ne++/N e) — 0,017 0.041 B leakney 1930 [11] B leak ney ty p e m .s.
g,(N e+/N e) — 0,010 0,034 A dam czyk e t al. 1966 [5] see above
— 0,022 0,039 Gaudin et al. 1967 [10] see above
— 0,013 0,038 present stu d y —
q( Ar++/Ar) 0,014 0,101 0,113 Bleakney 1930 [11] see above
q( Ar+/Ar) 0,007 0,102 0,104 Stevenson et al. 1942 [12] 180° m agnetic sector field m .s.
0,009 0,101 0,087 F ox 1960 [13] 90° m agnetic sector field m .s.
— 0,079 — M elton et al. 1967 [14] 60° m agnetic field m. s., dual
electron beam radiolysis source
— 0,050 0,066 Gaudin et al. 1967 [10] see above
0,004 0,050 — Morrison et al. 1970 [15] quadrupole mass filter w ith
electron m ultiplier
0,005 0,070 0,073 Crowe et al. 1972 [16] quadrupole m ass filter w ith chaneltron multiplier pulse coun
tin g, field free ion source
— 0,070 — D rew itz 1975 [6] 60° m agnetic field m .s. in con
nection w ith deflection m ethod
0,008 0,084 0,080 present stu d y —
q(Ar+++/Ar) — 0,0003 0,00144 F ox 1960 [13] see above
q(A i+JAr) — 0,0035 — Melton e t al. 1967 [14] see above
— 0,0009 0,0022 Gaudin et al. 1967 [10] see above
— 0,00028 — D rew itz 1975 [6] see above
— 0,00059 0,0037 present stu d y —
9(Kr++/Kr) q(K r+/iss)
0,03 0,125 0,125 T ate et al. 1934 [17] 180° m agnetic sector field m .s., B leak ney ty p e ion source
0,04 0,151 0,136 Fox 1960 [13] see above
0,03 0,130 0,120 Ziesel 1967 [18] not m entioned
0,030 0,118 0,114 present stu d y —
<7(Kr+++/Kr) — 0,0025 0,0102 Tate et al. 1934 [17] see above
?(Kr+/Kr) — 0,0037 0,0132 F ox 1960 [13] see above
— 0,0025 0,0102 Ziesel 1967 [18] see above
— 0,0027 0,0103 present stu d y —
q(Xe++/X e) 0,102 0,172 0,138 T a t e e t al. 1934 [17] see above
q(Xe+/X e) 0,076 0,146 0,155 present stu d y —
q(X e+++/X e) — 0,026 0,063 T ate et al. 1934 [17] see above
q(X e+/X e) — 0,025 0,069 present stu d y —
q(Xe++++/Xe) — — 0,004 T ate et al. 1934 [17] see above
q(Xe+/X e) — — 0,005 present stu d y —
eluded th a t all of th e presently reported cross sec
tio n ratios can be regarded as reliable d eterm in a
tions w ithin th e experim ental error range discussed above.
Results
Table I gives th e ratio of th e m easured p artial cross section values of m ultiply to singly ionized
rare gas ions a t th re e different electron energies, 50, 100 a n d 150 eV. Also show n in Table I are all available lite ra tu re values as given in Ref. [4]. It can be seen th a t th e agreem ent betw een th e dif
feren t au th o rs is generally q u ite poor. The big dif
ferences for instance in case o f g,(H e++/H e)/g,(H e+/
He) a t 100 eV or g(Ar++ + /Ar)/g,(Ar+/Ar) a t 50 eV are pro b ab ly due to u n certain ties in th e electron
Notizen 1113 energy scales of th e different au th o rs. R esults of
S tuber [19] are n o t included in Table I, because a secondary electron m ultiplier was used to m easure th e ion signal.
A detailed appraisal of th e previous d a ta is d if
ficult, because of lack of experim ental detail given by some of th e authors. However, in general it can be sta te d th a t all d a ta reported from m easurem ents w ith a m agnetic sector field will suffer in accordance w ith Drewitz [6] from considerable initial-energy discrim ination even for th erm al ions leading to an overestim ation of m ultiply ionized species. As has
been found in th e present study, ex tractio n p o te n tials in th e ion source m ay also strongly influence m easured cross section ratios, i.e. leading to sm aller fractions of m ultiply ionized particles a t too low or too high ex tractio n potentials [20],
Acknowledgement
The au th o rs are grateful to th e Ö sterreichischer Fonds zur F örderung der w issenschaftlichen F o r
schung for financial assistance un d er P ro ject N r.
1490, 1727 an d 2781.
[1] F. Howorka, J. Chem. P hys. 67, 2919 (1977).
[2] T. D. Mark, J . Chem. P hys. 63, 3731 (1975).
[3] T. D. Mark and F. Egger, In t. J . Mass Spectrom . Ion Phys. 20, 89 (1976).
[4] F. Egger, Thesis, U n iversität Innsbruck, 1977.
[5] B. Adam czyk, A. J. H . Boerboom , B. L. Schram, and J . Kistem aker, J . Chem. P h ys. 44, 4640 (1966).
[6] H. J. Drew itz, In t. J . Mass Spectrom . Ion P h ys. 19, 313 (1975); and In t. J. Mass Spectrom . Ion P hys. 21, 212 (1976).
[7] W. Bleakney and L. G. Sm ith, P h ys. R ev. 49, 402 (1936).
[8] A. Harrison, Thesis, The Catholic U n iversity o f A m e
rica, W ashington DC, 1956, data for Table I taken from reference [9].
[9] L. J . Kieffer, J IL A Inform ation Center R eport N o. 6, Boulder 1968.
[10] A. Gaudin and R. Hagem ann, J. Chem. P h ys. 64, 1209 (1967).
[11] W . B leakney, P hys. R ev. 36, 1303 (1930).
r 12] D. P. Stevenson and J. A. H ippie, P hys. R ev. 62, 237 (1942).
[13] R . E. F ox, J. Chem. P hys. 33, 200 (1960).
[14] C. E. M elton and P. S. R udolph, J. Chem. P hys. 47, 1771 (1967).
[15] J . D. Morrison and J. C. Traeger, J . Chem. P hvs. 53, 4053 (1970).
[16] A. Crowe, J. A. Preston, and J. W . M cConkey, J.
Chem. P hys. 57, 1620 (1972).
[17] J . T. Tate and P. T. Sm ith, P hys. R ev. 46, 773 (1934).
[18] J. P. Ziesel, J. Chim. P hys. 64, 695 (1967).
[19] F. A. Stuber. J. Chem. P hys. 42, 2639 (1965).
[20] K . Stephan, T. D. Märk, and H. H elm Proc. 1st. SA SP , Tirol 1978, p. 77.
