Ionization by Low and Intermediate Energy Ion and Neutral Beams

One of the most important basic processes that take place in collisions of atomic systems is ionization resulting in the emission of electrons. A complete understanding of this process is complicated by the fact that electron emission can take place via a number of mechanisms, some of which are related. Simple measurements of the cross section for electron ejection as a function of projectile velocity does not usually provide enough information to unravel the various mechanisms. But cross sections which are differential in the angle and energy of the ejected electrons, which are now available for protons on a variety of target gases, can be used to elucidate these mechanisms. While most of the experiments of this type have been done at high energies (above, say, 100 keV) there is now data available at energies as low as 5 keV for protons on various gases and to 15 keV for neutral hydrogen atoms on helium gas. At these low energies the angular distributions tend to become less anisotropic than at higher energies and do not yield as much information, but from the energy distributions integrated over angle we have been able to construct a mechanism which describes the results quite well. Also we have found a simple empirical equation for the cross sections differential in ejected electron energy which is of great potential usefulness to workers in various applied areas in which secondary electrons from collisions are involved