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

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