Theory of Auger Ejection of Electrons from Metals by Ions

Electrons ejected from atomically clean metals by slow ions of the noble gases arise in Auger transitions which involve either the direct neutralization of the ion or the de-excitation of an excited atom. A theory of these processes is presented in which the form of the distribution in energy and relative total yield, ${\gamma }_i$, of ejected electrons are derived. Matrix elements are not evaluated from first principles, but specific use of experimental results at two points in the theory leads to a determination of the dependence of the matrix element on distance between the atomic particle and the metal surface and the angle between the excited electron's velocity and the surface normal. Inclusion of the effects of variation of atomic energy levels near the metal surface and the Heisenberg uncertainty principle makes it possible to account in some detail for the experimentally observed energy distributions as well as the variation of these and of ${\gamma }_i$ with ion kinetic energy. The effect upon the resonance ionization and neutralization processes of the variation of atomic energy levels near the metal surface has also been investigated. The theory predicts a critical distance from the metal surface outside which resonance neutralization and inside which resonance ionization are possible. It has also been possible for the specific case of noble gas ions on tungsten, used as an illustrative example, to determine the relative proportion of electrons ejected by each of the possible Auger processes, to estimate ${\gamma }_i$ values for ions incident upon a metal with thermal energies, and to fix limits on the width of the filled portion of the conduction band in the metal. The role of the state density function in the metal and the effect of possible variation of the matrix element with electron energy in the band are also investigated.