Low Energy Electron Interaction with Molecules at Surfaces

Abstract

Electron initiated processes play a key role in any kind of laboratory plasma. It is primarily the electron-molecule interaction from which the feed gas molecules receive energy and which maintains the plasma. These primary interactions generate molecules in various excited states, in ionized forms (cations and anions) and finally as fragmentation products, also in excited and ionized forms1. All these particles mutually interact, incuding photons from emission processes. It is hence a vast variety of different interactions between primary and secondary particles which characterize a plasma. In principle, knowledge about the relative density of the components in their different states and the respective cross sections would be necessary to model and eventually control the plasma. In actual pactice, however, it is often sufficient to restrict on two body interactions between the most abundant components which, in the case of laboratory plasmas, are usually electrons and neutral gas molecules. The plasmas used in materials processing are often so called cold or anisothermic plasmas.2 Although they contain a variety of high energy species (neutrals, radicals and ions in excited states) the plasma does not considerably heat its container, i. e., the excited species are far from equilibrium. In particular, the electron energy distribution in such a cold plasma peaks at a few eV and is hence much higher than the average energy of the heavy particles (kT (300K) = 0.026 eV). The weak coupling between the electrons and heavy particles is a consequence of the large difference in masses. From energy and momentum conservation it follows that in a collision, an electron can only transfer an energy amount of the order m/M (m: electron mass, M: mass of the heavy particle) onto the heavy target. Exceptions are low energy electron collisions with polar molecules, collisions when resonances are involved (see below) but also collisions at higher energies when electronic excitation becomes accessible.