ISOTOPIC MASS EFFECTS IN LOW-ENERGY SPUTTERING OF COPPER AND MOLYBDENUM

Spherical and planar targets of copper and molybdenum were sputtered with Ar+ ions in the energy range from 40 to 440 eV. The isotopic composition of the flux of neutral atoms under steady-state conditions was determined by means of a novel secondary-neutral mass spectrometer with electron-gas post-ionization. In combination with a retarding-field energy analyzer, the ejected particle flux was investigated with respect to an emission-energy dependence of the natural isotopic abundances. It was found that the isotopic composition in the near-threshold regime of sputtering changes drastically with the Ar+ impact energy. Mass-dependent variations of the angular distributions lead to an enrichment of the lighter isotopes of several percent. Most prominent is the influence of the different emission energy distributions. The lighter isotopes are preferably ejected with higher kinetic energies; this effect exhibits a pronounced enhancement at very low bombarding energies. Depending on the retardation potential of the energy analyzer and thus the energy of the transmitted atoms, deviations from the natural isotopic composition of several 10% were observed.