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

Spherical and planar targets of copper and molybdenum were sputtered w ith Ar+ ions in the energy range from 40 to 440 eV. The isotopic compo sition of the flux of neutral atoms under steady-state conditions was determined by means of a novel secondary-neutral mass spectrometer wit h electron-gas post-ionization. In combination with a retarding-field energy analyzer, the ejected particle flux was investigated with respe ct to an emission-energy dependence of the natural isotopic abundances . It was found that the isotopic composition in the near-threshold reg ime of sputtering changes drastically with the Ar+ impact energy. Mass -dependent variations of the angular distributions lead to an enrichme nt of the lighter isotopes of several percent. Most prominent is the i nfluence of the different emission energy distributions. The lighter i sotopes are preferably ejected with higher kinetic energies; this effe ct 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 isot opic composition of several 10% were observed.