The influence of excited states on the kinetics of excitation and dissociation in gas mixtures containing methane

Abstract

In this paper, we extend the calculations for rare gas discharges, which aim to establish the influence of excited states on the kinetics of electron-induced excitation, to rare gas-methane mixtures and pure methane which are often used in diamond-like film deposition. In particular, we address the effect of non-thermal vibrational populations on the rate coefficients in methane-containing gas discharges using the procedure applied previously for pure silane. Furthermore, we investigate the kinetics of electronically excited levels of rare gases and methane in the presence of a significant population of excited states. These states may contribute to the overall ionization, excitation and dissociation rates through stepwise processes, superelastic collisions and energy transfer processes. The influence of superelastic processes on the development of the negative differential conductivity (NDC) is discussed on the basis of the momentum transfer theory, and it is shown that the NDC is reduced when significant populations of excited states are present. This is of importance for calculations of the transport coefficients for a.c. electric fields where NDC leads to a complex temporal dependence of the drift velocity and thus directly affects the power deposition in the discharge. Finally, we present the rate and transport coefficients calculated for methane in r.f. fields based on the Monte Carlo simulation for time-dependent fields. A good agreement with the effective field approximation and earlier Boltzmann calculations is found.