Anomalously high yield of doubly charged Si ions sputtered from cleaned Sisurface by keV neutral Ar impact

The energy spectra of positively charged and neutral species ejected from t he Si(1 1 1) surfaces by keV Ar impact have been measured by means of a com bined technique of the time-of-flight (TOF) analysis with the multi-photon resonance ionization spectroscopy (MPRIS). It is shown that positively char ged species of Si+, Si+ and SiO+ are ejected from the as-cleaned 7 x 7 surf ace by 11 keV Ar impact, It is also shown that Ar sputter cleaning of the a s-cleaned 7 x 7 surface for 14 min at the flux of 2 x 10(13)/cm(2)s removes completely the oxygen impurity and the yields of Si2+ is comparable to tha t of Si+. Moreover, the ionization probability of Si atoms sputtered is sho wn to be expressed as an exponential function of the inverse of their veloc ity. The production mechanism for the doubly charged Si ion is discussed ba sed on the L-shell ionization of Si atoms due to quasi-molecule formation i n the collisions of the surface atoms with energetic recoils and subsequent Auger decay of the L-shell vacancy to doubly ionized Si ions. (C) 2001 Els evier Science B.V, All rights reserved.