Investigations of low energy electron attachment to ground state group 6B hexafluorides (SF₆, SeF₆ and TeF₆) using an electron-swarm mass spectrometric technique

Studies of low energy electron attachment to SF6, SeF6, and TeF6 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, alpha, and electron attachment rate constants, k(a), together with product anion branching ratios (for SeF6 and TeF6) are determined as a function of E/N. The studies presented here cover the range E/N = (0.4-17) X 10(-18) V cm(2), corresponding to mean electron energies of 0.03-0.6 eV. For all three molecules, k(a) decreases as E/N increases. SF6 attaches electrons much more rapidly than either SeF6 or TeF6. The ratios k(th)(SF6):k(a)(SeF6):k(a)(TeF6) approximate to 3000:10:1 are found not to vary with E/N. The estimated thermal (300 K) electron attachment rate constants are k(th)(SF6) approximate to (2.5 +/- 0.3) x 10(-7) cm(3) s(-1), k(th)(SeF6) approximate to (8.0 +/- 1.2) X 10(-10) cm(3) s(-1), and k(th)(TeF6) approximate to (8.2 +/- 1.1) x 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 XF6- (X = S, Se, Te), accompanied by XF5-. No other anion products directly arising from electron attachment to XF6 are observed. Extrapolation of the relative product anion intensities to zero attaching gas concentration yields the following branching ratios for attachment under swarm conditions: SeF6-SeF5- (20%), SeF6- (80%); and TeF6-TeF5- (3%), TeF6- (97%). These ratios are found to be independent of E/N. The observation of SeF6- and Te-6(-) as the dominant anions from SeF6 and TeF6 is ascribed to stabilisation of the initial anion formed by electron capture through collisions with the nitrogen buffer gas. For SF6, the observed proportion of SF; decreases from 8% to 1% over the E/N range of this study, whereas an increase in the SF; branching ratio with E/N is anticipated from previous low-pressure, electron beam investigations. (Int J Mass Spectrom 205 (2001) 253-270) (C) 2001 Elsevier Science B.V.