THE VARYING INFLUENCES OF GAS AND ELECTRON TEMPERATURES ON THE RATES OF ELECTRON-ATTACHMENT TO SOME SELECTED MOLECULES

The rate coefficients, beta, for the attachment reactions of electrons with CCl4, CHCl3, SF6 and C7F14 have been measured in a Rowing afterglow/Langmuir probe (FALP) apparatus within the range of electron temperature, T-c, from 300 to about 4000 K at a carrier gas temperature, T-g, of 300 K and for the SF6 reaction additionally at a T-g of 540 K. The above gases were chosen in the light of the results obtained from previous FALP studies under truly thermalized conditions, i.e. for T-c = T-g = T. These previous experiments showed that the beta for the CCl4 reaction (product Cl-) and SF6 reaction (products SF6-, direct attachment, and SF5-, dissociative attachment, becoming increasingly important with increasing T) were close to the maximum value beta(max), and that the beta for both the CHCl3 dissociative attachment reaction and the C7F14 direct attachment reaction increased with increasing T indicating the presence of activation energy barriers in these reactions. Now the present studies indicate that (i) the beta for both CCl4 and the SF6 reactions decrease with increasing T-c at a rate predicted by s-wave capture theory, and that the SF5- product of the SF6 reaction becomes increasingly important with increasing T-c, (ii) the beta for both the CHCl3 and the C7F14 reactions increase with increasing T-c, but a much greater increase of T-c than T is required to produce a given increase in beta. To explain these experimental observations, a simple model of these attachment reactions has been constructed which is consistent with the experimental data, based on the notion that the energy of the incoming electron is distributed amongst the accessible vibrational states of the molecules, and that this vibrational excitation promotes these particular attachment reactions. A careful analysis of our data indicates that the endothermicity of the dissociative attachment reaction of SF6 producing SF5- is close to 0.12 eV which is consistent with the available thermochemical data.