Isotope Effects of H₂ and D₂ on Electron Thermalization in Gases

Abstract

The isotope effects of H₂ and D₂ on the thermalization of epithermal electrons in gases are studied with the Monte Carlo simulation using the experimental cross sections. The influence of the gas temperature T on the isotope effects is examined at T=300 K, 77 K [∼θ_r(H₂)], and 40 K [∼θ_r(D₂)], because the characteristic rotational temperature θ_r corresponds to the rotational threshold. The electron velocity distribution deviates significantly from the local Maxwell distribution (MD) in both H₂ and D₂; consequently the degradation of the effective electron temperature T_e is slower than that obtained with the MD assumption. The electron thermalization time τ_th when T_e⁄T=1.1 is larger than that for the MD to the extent of 290% in D₂ at T=40 K. The isotope effect is most prominent for the pressure normalized thermalization time τ_thp at T=77 K [τ_thp(D₂)⁄τ_thp(H₂)=0.35] because θ_r(H₂)\gtrsimT>θ_r(D₂). T_e and τ_thp are compared with those calculated from available experimental energy exchange rate coefficients.