Measurements of swarm parameters and derived electron collision cross sections in methane

Abstract

A pulsed drift tube has been used to measure the electron drift velocity in methane over the range of E/N from 10 to 1000 Td. In addition, measurements of the positive ion mobility and ionization coefficient have been made over the range of E/N from 80 to 1000 Td. Within the experimental sensitivity, no evidence of attachment has been observed in this range. A set of electron collision cross sections has been assembled and used in Monte Carlo simulations to predict values of swarm parameters. The cross-section set includes a momentum transfer cross section which is based primarily on the present and previous drift velocity measurements, cross sections for vibrational excitation and ionization based on published experimental cross-section measurements, and a cross section for dissociation into neutral products obtained by subtracting a measured dissociative ionization cross section from a measured total dissociation cross section. Isotropic scattering is assumed for all types of collisions in the Monte Carlo simulations. Good agreement between the predicted and measured values of swarm parameters is obtained without making any adjustments to these cross sections. A two-term Boltzmann equation method has also been used to predict swarm parameters using the same cross sections as input. The two-term results are in poor agreement with experiment and confirm the well-known inadequacy of two-term methods in the case of methane.

Ramsauer-Townsend minimum in methane — modified effective range analysis

“…The use of data derived from swarm experiments [29,49,50] would not bring new information; furthermore, different sets disagree significantly in magnitude. Practically, in the R-T minimum region the only theoretical data other than the Born approximation are several models by Gianturco et al [45,51,52].…”

Predicting the amount of carbon in carbon nanotubes grown by CH4 rf plasmas

“…10 The input parameter of this model, the electron drift velocity W e , and the rate coefficients k of electron-CH 4 reactions were obtained from a Boltzmann equation analysis using a set of the electron collision cross sections of CH 4 . 22 The reaction rate coefficients k for H 2 , CH 2 , CH 3 , C 2 H 2 , C 2 H 4 , C 2 H 5 , C 2 H 6 were estimated from their reaction cross sections. 10,23,24 The mobilities and diffusion coefficients of charged species, except electron mobility, the secondary electron emission coefficient ͑␥ ion = 0.01͒, the initial energy for secondary electrons ͑=1.0 eV͒, the detachment coefficient ͑=3.69ϫ 10 −11 cm 3 /s͒ and the recombination coefficient ͑=0.0 cm 3 /s͒ are cited from Gogolides et al 20 D j of neutral species in the CH 4 plasma were calculated as referred to in Herrebout et al 25 An overview of the species ͑nonradi-cal neutrals, ions, radical neutrals͒ taken into account in the present model is given in Table II.…”

Chemical Kinetics in Cold Plasmas

“…In order to estimate the average electron density in the spark channel, we have neglected the contribution of ions to the measured discharge current. Under such approximation the current is determined by the size of the discharge section and the electron mobility as a function of the electric field, which was taken from literature for pure methane (Davies, 1989). This should be approximately right also for argon/methane mixtures, since mobility at the same electric field increases by a merely 10% even in pure argon.…”