Relaxation of Velocity Distribution of Electrons Cooled (Heated) By Rotational Excitation (De-Excitation) Of N₂

Abstract

It is well known¹ that low-energy electrons in molecules lose or gain the energy through the rotational excitation or de-excitation of molecules. When the electrons are so diluted in molecules that the effect of the electron-electron collision is negligible as compared with that of the electron-molecule collision the behavior of the electron velocity distribution in the electron cooling or heating process is not well understood. It is not obvious that the electrons cooled or heated by the molecular rotational excitation or de-excitation obey the Maxwell velocity distribution, although the Maxwell distribution is often assumed for the theoretical interpretation of the measured results in the cross-modulation experiments^2,3 and the D region of the ionosphere⁴. In fact, it is shown⁵ that the stationary electron velocity distribution, which depends on the initial velocity distribution, in molecules with two energy levels is not the Maxwell distribution and reveals the sawtooth pattern for the initial Maxwell distribution, where the electron-electron and electron-molecule elastic collisions are ignored as compared with the electron-molecule inelastic collisions.