Re-investigation of the kinetic model or thermal electron attachment by the pulse sampling technique



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The kinetic model for the study of thermal electron attachment to molecules by the pulse sampling technique has been re-examined in detail. The dependence of the electron concentration on flow rate, pulse width, pulse frequency and temperature have been measured for Ar-10% methane carrier gas in addition to the temperature dependence for ethylene as a carrier gas. The experimental results are consistent with the following facts about the kinetic models (1) the rate of production of electrons through ionization by p particles is independent of temperature; (2) the loss of e- without an electron capturing species present is only slightly temperature dependent to the extent that the change in electron concentration is less than 20%; (3) electron attachment to radicals is important only at low flow rates 100 ml/min; (4) at high flow rates (F>100 ml/min) the loss of electrons is due to neutralization with positive ions; (5) at low flow rates the positive ions are lost through reaction of R- which in turn are formed by electron attachment to radicals; (6) at high flow rates the positive ions are lost not only by neutralization with electrons but also by collection at the cathode under the influence of the applied negative potential. As a result of these studies, under normal operating conditions (flow rate = 150 ml/min), the change in rate constant for loss of electrons (k[lowered D]) with ill pulse interval can be properly accounted for in terms of the change in positive ion concentration. Since electron loss is principally due to reaction with positive ions, the rate constant for neutralization with molecular negative ions should be proportional to that for electrons and the ratio should be constant, independent of the pulse interval. Furthermore, the assumption that the positive ion concentration is approximately constant has been justified. Mathematical analysis of the kinetic model for thermal electron attachment suggests that the electron capture coefficient dependence on pulse interval can be used to differentiate between different mechanisms of electron attachment. This technique has been applied to oxygen at 260[degrees]C and acetophenone at 130[degrees]C. The mechanisms seem to be consistent with those interpreted previously from the temperature dependence of the capture coefficient. However, there are some discrepancies which must be resolved before the technique can be generally acceptable.