Nonequilibrium electron velocity distribution and temperature in thermalization of low-energy electrons in molecular hydrogen

Abstract

The thermalization of low-energy electrons (ε≲0.1 eV) due to the rotational and elastic collisions in normal H2 at the gas temperature T=300 or 77 K is studied by the Monte Carlo simulation, where electrons are so diluted in molecules that the electron–electron collision is neglected as compared with the electron–molecule collision. The accuracy of the approximate theory based on the assumption of the local Maxwell velocity distribution (MD) is examined using, for simplicity, the rotational cross section of the Gerjuoy–Stein formula and the elastic cross section of the hard-sphere model, which are a little larger than the experimental cross sections at low electron energy (ε≲0.1 eV); the initial electron velocity distribution is taken to be the MD. The electron velocity distribution significantly deviates from the MD especially at low gas temperature (T=77 K); consequently the degradation of the electron temperature Te is slower than that for the MD and the thermalization time τth when Te/T=1.1 is larger than that for the MD to the extent of 20% at T=300 K, where τth is dominated by the rotational collision, and 140% at T=77 K, where τth is dominated by the elastic collision. τthp≂1.9 μs Torr at T=300 K, where p is the gas pressure, is about 27% larger than the experimental value of 1.5 μs Torr at T=296 K for the higher initial electron energy (ε∼1 eV).