Sputtering of ionic clusters from KCl, NaCl and LiF

Abstract

Clusters invariably originate from the topmost layers of a solid and thus provide important information concerning the state of the surface. We have studied the mass and energy spectra of the cluster component of the positive ion sputtering yield during 45 keV argon ion irradiation of (100) cleavage faces of KCl, NaCl, and LiF. The mass spectra, with 1 amu resolution reveal the relative positive ion yield to be predominantly composed of the alkali ion M + with the second most abundant species (by an order of magnitude) being the dimer M 2 +. The spectra also show a variety of lower yield metal and alkali halide ion clusters of the form M n + and M n X −1, where M is tha cation and X the anion of the alkali halide. During the course of irradiation of insulators a positive surface charge develops on the surface and the resultant surface potential distorts the energy spectra. We propose a two component surface potential model based on the heating effect on the surface by the irradiation beam. The model is used to deconvolute the observed energy spectra. The corrected results of the monomer M + energy distribution show an E −2 dependence at high energies in agreement with simple collision cascade theory. The cluster energy profiles on the high energy side give an E − m dependence with the index m increasing with increasing cluster size. The results suggest a collisional mechanism responsible for their ejection.