HREELS STUDY OF SUBSURFACE OXYGEN ON THE NI(100) SURFACE

New high frequency vibrational loss features have been measured on O/N i(100) for low oxygen coverages, using high-resolution electron-energy -loss spectroscopy (HREELS). These high frequency losses only appear a fter thermal annealing. Oxygen atoms are migrating underneath the surf ace with increasing temperature, giving rise to the high frequency mod es. The modes are shown to result from subsurface oxygen atoms, squeez ed in interstitial sites. We have detected five different high frequen cy loss signals, while five different interstitial sites are precisely available below the Ni(100) surface. Model calculations have been per formed in order to relate frequencies to interstitial sites. The match ing technique approach has been applied to analyze the case of periodi c structures, whereas a cluster approach has been used to obtain the l ocalized vibrational states associated with interstitial oxygen atoms.