Investigations of low energy electron attachment to ground state group 6B hexafluorides (SF 6, SeF 6, and TeF 6) using an electron-swarm mass spectrometric technique

Abstract

Studies of low energy electron attachment to SF 6, SeF 6, and TeF 6 have been carried out in an atmospheric pressure nitrogen buffer gas (number density N) at 300 K. The experiments are conducted under nonthermal electron-swarm conditions, using an instrument that combines an atmospheric pressure drift tube, with a quadrupole mass spectrometer. Details of the design, construction and operation of the drift tube and the associated fast electron gate are presented. Electron drift times can be measured, and mean electron drift velocities in N 2 as a function of the density reduced electric field strength E/ N are reported. Density normalised electron attachment coefficients, α, and electron attachment rate constants, k a , together with product anion branching ratios (for SeF 6 and TeF 6) are determined as a function of E/ N. The studies presented here cover the range E/ N = (0.4–17) × 10 −18 V cm 2, corresponding to mean electron energies of 0.04–0.6 eV. For all three molecules, k a decreases as E/ N increases. SF 6 attaches electrons much more rapidly than either SeF 6 or TeF 6. The ratios k a (SF 6): k a (SeF 6): k a (TeF 6) ≈ 3000:10:1 are found not to vary with E/ N. The estimated thermal (300 K) electron attachment rate constants are k th(SF 6) ≈ (2.5 ± 0.3) × 10 −7 cm 3 s −1, k th(SeF 6) ≈ (8.0 ± 1.2) × 10 −10 cm 3 s −1, and k th(TeF 6) ≈ (8.2 ± 1.1) × 10 −11 cm 3 s −1. For all three molecules, attachment is dominated by the capture of near-zero-energy electrons. In each case the dominant anion product is XF 6 − (X = S, Se, Te), accompanied by XF 5 −. No other anion products directly arising from electron attachment to XF 6 are observed. Extrapolation of the relative product anion intensities to zero attaching gas concentration yields the following branching ratios for attachment under swarm conditions: SeF 6–SeF 5 − (20%), SeF 6 − (80%); and TeF 6–TeF 5 − (3%), TeF 6 − (97%). These ratios are found to be independent of E/ N. The observation of SeF 6 − and TeF 6 − as the dominant anions from SeF 6 and TeF 6 is ascribed to stabilisation of the initial anion formed by electron capture through collisions with the nitrogen buffer gas. For SF 6, the observed proportion of SF 5 − decreases from 8% to 1% over the E/ N range of this study, whereas an increase in the SF 5 − branching ratio with E/ N is anticipated from previous low-pressure, electron beam investigations.