The effect of low-lying vibrational excited states on electron thermalization in methane

The electron slowing-down proceeds through elastic and inelastic collisions of electrons with gas atoms or molecules. In the subexcitation energy region, where electron energy is lower than the first electronic excitation threshold (∼10 eV), electron thermalization proceeds in vibrational and rotational collisions, and through the momentum transfer in elastic collisions. Understanding of time-dependence and temperature-dependence of thermalization of low-energy electrons in various gases is an important fundamental problem in chemical physics and radiation chemistry, and has numerous practical applications. Electron thermalization in some atomic gases (H, Ar, Cs), in which only the momentum transfer in elastic collisions is possible, has been already studied. Molecular gases have rotational and vibrational excitation processes with thresholds below 1 eV. These processes may substantially change the electron energy distribution function and influence the electron thermalization time. Here we present a study of electron thermalization in methane by solutions of the Boltzmann equation. Energy relaxation. and time-dependent electron energy distribution functions are calculated for several pairs of gas temperature (T) and pressure (p), that lead to three different gas densities