Neutral dissociation of methane by electron impact and a complete and consistent cross section set

TL;DR

This paper develops a complete, consistent set of electron collision cross sections for methane, including neutral dissociation, validated by swarm parameters, and highlights the impact on hydrogen production rate predictions.

Contribution

It extends previous cross section data for methane with analytical approximations, creating a comprehensive set that does not rely on data fitting for validation.

Findings

The new cross section set accurately predicts experimental swarm parameters.

Different hydrogen production rates are predicted by the new and swarm-fitted data sets.

Swarm-fitting procedures may reduce the accuracy of chemical production rate estimates.

Abstract

We present cross sections for the neutral dissociation of methane, in a large part obtained through analytical approximations. With these cross sections the work of Song $\textit{et al.}$ [J. Phys. Chem. Ref. Data, $\textbf{44}$, 023101, (2015)] can be extended which results in a complete and consistent set of cross sections for the collision of electrons with up to 100 eV energy with methane molecules. Notably, the resulting cross section set does not require any data fitting to produce bulk swarm parameters that match with experiments. Therefore consistency can be considered an inherent trait of the set, since swarm parameters are used exclusively for validation of the cross sections. Neutral dissociation of methane is essential to include (1) because it is a crucial electron energy sink in methane plasma, and (2) because it largely contributes to the production of hydrogen radicals that can be vital for plasma-chemical processes. Finally, we compare the production rates of hydrogen species for a swarm-fitted data set with ours. The two consistent cross section sets predict different production rates, with differences of $45\%$ (at $100$ Td) and $125\%$ (at $50$ Td) for production of H$_2$ and a similar trend for production of H. With this comparison we underline that the swarm-fitting procedure, used to ensure consistency of the electron swarm parameters, can possibly deteriorate the accuracy with which chemical production rates are estimated. This is of particular importance for applications with an emphasis on plasma-chemical activation of the gas.