Ion-induced secondary electron and negative ion emission from Mo/O

Abstract

Absolute yields of secondary electrons and negative ions resulting from collisions of Na + with Mo(100) and a polycrystalline molybdenum surface have been measured as a function of the oxygen coverage of the surface for impact energies below 500 eV. The sputtered negative ions have been identified with mass spectroscopy, and O − is found to be the dominant sputtered negative ion for the surfaces at all oxygen coverages and impact energies. Both the electron and O − yields have an impact energy threshold at about 50 eV and exhibit a strong dependence on oxygen coverage. The kinetic energy distributions of the secondary electrons and sputtered O − were determined as functions of the oxygen coverage and impact energy. The distributions for O − are characterized by a narrow low-energy peak (at ∼1–2 eV) followed by a low-level high-energy tail. The secondary electrons have a narrow (FWHM ∼1–2 eV) kinetic energy distribution, centered approximately at 1–2 eV. The shapes of the distributions and their most probable energies are essentially invariant with impact energy, oxygen coverage and the nature of the Mo surface. The emission is explained and analyzed in terms of a simple model which involves a collision-induced electronic excitation of the MoO − surface state. The decay of this excited state leads to the production of both secondary electrons and O − with energy distributions and yields comparable to those observed.