Charge state of low energy reflected particles

Abstract

Low energetic noble gas particles, scattered from a metal surface, have only a small probability to leave this surface in the charged state. Even particles scattered from target atoms in the outermost surface layer are nearly all neutralised. Consequently analysis of the charged fraction of the scattered beam guarantees that information is obtained of only this layer. To quantify these low energy ion scattering (LEIS) measurements data on the ion fractions must be known, especially because these fractions have such small values. Besides this practical aspect there is the fundamental question: how does it work, this neutralization mechanism, does the transition rate for electron capture of the scattered particle depend upon its distance from the surface, or are it distant binary collisions with individual metal atoms along its trajectory which give rise to interatomic Auger transitions. Already for many years Hagstrum's theory has been applied to correct for the neutralization effect. This theory is developed to study the ejection of Auger electrons as a result of the interaction of low energetic noble gas ions with a metal surface. Due to the low ion energies used in those experiments, namely the tens eV-region, Hagstrum assumed a structureless metal surface. This means that the normal velocity of the scattered particle, v 1, takes part in determining the ion fraction and not v, the velocity of the scattered particle in its trajectory. It is a question, however, whether this theory may be applied for keV-ions. The distance between scattered particle and metal surface is for these energies namely much smaller than for tens eV's and the surface can no longer be regarded as a plane; this may result in dominating interatomic Auger transitions. But then the trajectory of the scattered particle with its velocity v plays a part in determining the occurrence of neutralization. Another mechanism that may determine the charge state of the scattered particle occurs in the violent collision; neutralization as well as (re)ionization of this particle may take place and influence its charge state. These models are treated in this review and discussed with the help of recent measurements and results of computer simulations of reflected particles.