Electron thermalization in methane and argon–methane mixtures is studied by using the Boltzmann equation. The presence of low‐lying vibrational excited states in methane significantly changes electron energy distribution functions and relaxation times. We found that (i) the mean electron energy just below the first vibrational excited state is reached faster by 1000 times when the vibrational states are taken into account, and (ii) electron energy distribution functions have distinct peaks at energy intervals equal to the vibrational threshold energies. Both these effects are due to large vibrational stopping cross section. The thermalization time in mixtures of argon–methane (without vibrational states) smoothly changes as the mixture composition varies, and no significant difference in the electron energy distribution function is observed. When the vibrational excited states are taken into account, thermalization is almost completely defined by CH4, even at very low fractional concentrations of CH4. The sensitivity of the electron energy distribution functions on the momentum transfer cross sections used in calculation on the thermalization is discussed.

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