T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY
Registered in U.S. Patent Ofice
@! Copyright, 1969, by the American Chemical Society
VOLUME 73, NUMBER 12 DECEMBER, 1969
Ionization and Dissociation of Pentafluorosulfur Chloride by Electron Impact by P. Harland and J. C. J. Thynne Chemistry Department, Edinburgh University, Edinburgh, Scotland
(Received July 31, 1960)
Positive and negative ion formation as a result of the electron bombardment of pentafluorosulfur chloride has been studied. Various ionbation processes have been suggested to account for the formation of the ions, and for several negative ions the dependence of their formation upon electron energy has been studied.
As part of a continuing study of positive and negative ion formation by molecules as a result of electron impact,1r2 we have examined pentafluorosulfur chloride, SF6Cl. This molecule is of particular interest from a negative ion standpoint since the closely related molecule sulfur hexafluoride readily forms the ion SFe- at low energiesj3s4and it is of interest to observe if the SF6C1- ion is formed similarly. In electron impact studies, when the electron source is a heated filament, uncertainties arise in the evaluation of experimental ionization data because of the energy spread of the thermionically emitted electron beam; this is largely due to the ionization thresholds becoming smeared-out as a result of the high energy tail of the electron energy distribution. Analytical methods have been developed to reduce this problem for positive6 and negative2 ions, and we have applied this technique to the negative ions formed by SFbC1 a t low electron energies.
In both the positive and negative ion studies, the electron current was maintained constant by automatic regulation over the whole energy range investigated. Ionization curves were usually measured three to five times, the appearance potentials for negative ions being reproducible to *O.l eV. The appearance potential of the 0- ion from SO2 was used as the reference for energy scale ~alibration,~ both the onset at 4.2 eV and the maximum of the resonance peak at 5.0 eV being taken as the calibration For the positive ion studies argon was used to calibrate the energy scale, the method used for determining the appearance potentials being the semilogarithmic plot technique. The electron energy distribution, which was required to be known for the deconvolution procedurej2was measured using the SF6- ion formed by sulfur hexafluoIt was found that performing 15 smoothing and 20 unfolding iterations on the basic experimental data enabled satisfactory evaluation of appearance po-
Experimental Section The data were obtained using a Bendix time-offlight mass spectrometer, Model 3015. The pressure in the ion was usually maintained below 5 x 10-0 mm in order to reduce the possibility of ion formation due to ion.-molecule reactions. The energy of the ionizing electrons was read on a Solatron digital voltmeter, LM 1619!and the spectra recorded on two l-mV Kent potentiometric recorders.
(1) K. A. G. MacNeil and J. C. J. Thynne, Int. J . Mass Spec., 2, l (1969)(2) K. A. G. MaoNeil and J. C. J. Thynne, ibid., 2, 35 (1969). (3) W.M. Hickman and R. E. Fox, J . Chem. Phys., 25,642 (1956). (4) G.J. Schulz, J. Appl. Phys., 31, 1134 (1960). (6) J. D. Morrison, J . Chem. phy3., 39, 2oo (1963). (6) K, Kraus, z. Naturforsch,, 16a, 1378 (1961). (7) J. G.Dillard and J. L. Franklin, J . Chem. Phys., 48,2349 (1968). (8) F.H. Dorman, ibid., 44,3856 (1966).
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P. HARLAND AND J. C. J. THYNNE
tentials, resonance peak maxima, and peak widths (at half-height) to be made. Results and Discussion
(a) SF5+. We have measured a value of 13.2 f 0.2 eV for the appearance potential of the SFb+ ion. SF5Cl
+ e -+ SF5+ + C1 + 2e
( I ) Positive Ion Formation by Pentafluorosulfur Chloride. In Table I we show the positive ion mass spectra for SF&1 and SF6 measured a t 70 eV. For both molecules, doubly charged fragments containing sulfur and fluorine are common, but it is noteworthy that for SF6C1few ions containing sulfur, fluorine, and chlorine are formed and these are generally of low abundance.
(3) Dibeler and Mohlerg have reported that, for SF6, A(SFb+) = 15.9 f 0.2 eV. These results suggest that, neglecting possible excess energy contributions, D(SF6-F) - D(SF6-Cl) = 2.5 f 0.4 eV, a result which is in reasonable accord with the difference deduced above. ( b ) Clf. Our data yield a value of 20.8 f 0.3 eV for the appearance potential for the C1+ ion (Table 11);
Table I : Positive Ion Mass Spectrum of SF6Cl and SFe for 70-V Electrons
Table I1 : Appearance Potentials of Various Ions in the Mass Spectrum of SFaC1
SFe
SFsCl 7-
S2+
16 19 32 35
F+
S+
c1+
SFz" c1+ SF32+ SF+ SF4" SFj'+ SC1+ SFz SFCl+ SFs SFzCl+ SF4 SF3Cl+ SFj+ SF4C1+ SFjCl + +
+
+
m/e
0.14 0.44 3.10 2.02 1.53 0.67 0.04 6.13 0.44
