ISOTOPE EFFECTS IN THE FORMATION OF MCS(+) MOLECULAR SECONDARY IONS

Bombardment of solid surfaces by Cs+ ions results in the emission of molecular secondary ions of the type MCs(+), where M stands for an element contained in the sample. Generally, these molecular species exhibit emission characteristics distinctly different from those of the atomic ions. Previous experiments ascribed this observation to the possible formation mechanism of MCs(+) ions: the association of a neutral M atom with a Cs+ ion in the sputtering event. To further elucidate these formation processes, the possible influence of isotopic mass differences was investigated: the yields of MCs(+) ions sputtered from various elements (M = B, Si, Ge, and Mo) under 5.5 keV Cs+ bombardment were measured as a function of the emission energy for two (or more) isotopes of any of those elements. While the atomic ions Mf exhibit a pronounced (up to similar to 10%) light-isotope enrichment in the flux which decreases with increasing energy, for MCs(+) species the isotope effects are much smaller and show a different emission-energy dependence. Above similar to 10 eV the light/heavy ratio of the MCs(+) nux tends to increase with increasing emission energy; with MCs(+) ions probing the emission of neutral M atoms, this finding agrees with the predictions of computer simulations which indicate a pronounced enrichment of the lighter isotope in the flux of sputtered neutral atoms at higher energies. At very low energies (< 5 eV), MCs(+) ions from Si, Ge, and Mo show an isotope fractionation similar to that of atomic ions, while the flux of BCs+ is enriched in the heavier B isotope.