AN ISS STUDY OF THIN METALLIC LAYERS EVAPORATED ON GOLD SUBSTRATE - EXPERIMENTATION VERSUS SIMULATION

ISS measurements have been performed on very thin (1-50 angstrom) Al and Cu layers deposited on Au substrates. The He-3(+) (2 keV) scattering spectra are very different for the two cases. For Al-Au, in addition to the single collision surface peaks, an important broad background distribution is apparent. Its shape and its maximum change radically with the Al deposited thickness. Above 50 angstrom, the spectrum is similar to that obtained on pure Al. For Cu-Au, no background is observed. The results are interpreted as the consequence of He-3 re-ionization taking place in the Al layer for particles backscattered by the Au substrate atoms. This re-ionization is produced by an electron promotion mechanism. It depends on the electronic structure and is negligible for Cu atoms. MARLOWE simulations, including the charge exchange processes (Auger neutralization and re-ionization) agree well with the experimental spectra obtained for the various Al thicknesses deposited on Au. The analysis of background should allow to extract not only information on the composition and structure of the surface layer but also on its electronic structure.