DOUBLY CHARGED SPUTTERED IONS OF 4TH-ROW ELEMENTS

We have measured the initial-kinetic-energy distributions of sputtered doubly charged ions of the fourth-row elements (Ca-As) over a range 0-1000 eV and energy distributions of singly charged ions 0-500 eV. The energy distributions of the doubly charged ions lack the characteristic low-energy peak exhibited by most sputtered ions, and some actually have peak intensities at high energy. Oxygen enhances the doubly charged ion yields of Sc2+-Ni2+ and alters the energy distributions of these ions, increasing the proportion of low-energy ions, but either does not affect or suppresses ion yields from Ca2+ and Cu2+-As2+. These results can be explained in terms of the kinetic emission model, in which an atom sputtered with a core-level vacancy resulting from an energetic collision has a finite probability, dependent on the atom velocity and the vacancy lifetime, of undergoing Auger deexcitation far enough from the surface for the resulting multiply charged ion to survive. The effect of oxygen is interpreted as a lengthening of the vacancy lifetime due to electron withdrawal from the cations in an oxygenated lattice.