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 CCl 4 , CHCl 3 , SF 6 and C 7 F 14 have been measured in a flowing 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 SF 6 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 CCl 4 reaction (product Cl - ) and SF 6 reaction (products SF 6 - , direct attachment, and SF 5 - , dissociative attachment, becoming increasingly important with increasing T) were close to the maximum value beta max , and that the beta for both the CHCl 3 dissociative attachment reaction and the C 7 F 14 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 CCl 4 and the SF 6 reactions decrease with increasing T c at a rate predicted by s-wave capture theory, and that the SF 5 - product of the SF 6 reaction becomes increasingly important with increasing T c , (ii) the beta for both the CHCl 3 and the C 7 F 14 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 SF 6 producing SF 5 - is close to 0.12 eV which is consistent with the available thermochemical data.