FEMTOSECOND ELECTRON DYNAMICS ON SOLID SURFACES PROBED BY ION SCATTERING AND STIMULATED DESORPTION OF SECONDARY IONS

In this article, the mechanism of electronic transitions during scattering and stimulated desorption of ions from solid surfaces is discussed. Reactive ions such as H + and O + experience transient chemisorption during scattering from solid surfaces. These ions are neutralized almost completely on metal and semiconductor surfaces due to the band effect on resonance neutralization. The neutralization probability of H + is suppressed considerably on highly ionic compound surfaces and is dependent on the target species due to the formation of the bound state (on cations) or the surface molecule (on anions). Because of this, the H - ion is formed preferentially on the cationic site rather than on the anionic site. The noble-gas ions are neutralized via the Auger process so that the neutralization probability is basically independent of the band effect. The stimulated desorption of secondary O + and F + ions does not exhibit the band effect. This is because the desorption is initiated by the core hole state, which is followed by ionization via the intra-atomic Auger decay after breakage of the chemisorptive bond. The stimulated desorption of H + might occur from the valence holes but is more likely to be caused by the core-excited OH species via the interatomic Auger decay. The core hole is created not only by the electron and photon irradiation but also by the energetic ion bombardment due to the nonadiabatic transition of the primary ion/s core hole. Also presented are some applications of ion scattering and ion stimulated desorption for the analysis of the diffusion/segregation dynamics of oxygen and hydrogen on solid surfaces.