Electron Collisions with Molecules

Abstract

In most gas discharges which are used for ion sources, for laser media, in plasma chemistry and for the generation of thin films, the average electron energy ranges from a few eV up to some 10-30 eV. The energy distribution is not quite a Maxwellian distribution, often it has an extended high energy tail and it may have some deviations due to individual collision processes with very large cross sections. This is for example the case in the earth lower ionosphere because of the process

$$ {{e}^{ - }} + {{N}_{2}} \to {{N}_{2}}^{ - }(\nu ) \to {{N}^{2}}(\nu ) + {{e}^{ - }} $$

i.e. the formation of short living negative ion states of the nitrogen molecules, which decay by autoionisation but the molecules remain excited in high vibrational states. Since the majority of the electrons have energies of only a few eV, vibrational and rotational excitations by electrons are of great importance. On the other hand, most electronic excitation and ionisation processes need impact energies of more than 10 eV, i.e. their thresholds are already in the range of the exponentially decreasing intensity of the electron distributions. This means that it is necessary to know both, the electron energy distributions and the cross sections close to the relevant thresholds.