Ionic Clusters.

The effects of irradiation of 45KeV argon ions on (100) cleavage faces of KCl, NaCl and LiF has been studied. The sputtered positive ions have been analysed in mass and energy with the specific aim of studying their polyatomic or cluster components. The mass spectra, of 1 a.m.u. 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(,n -1)('+), where M is the cation and X the anion of the alkali halide. Traces of metal oxide, halide cations and doubly charged potassium halide species are also present. During the course of irradiation of insulators a positive surface charge develops on the surface and the resultant surface potential distorts the energy spectra. A two component surface potential model based on the surface heating effect of the irradiation beam enables the observed energy spectra to be deconvoluted and the true energy distribution to be estimated. The corrected results of the monomer M('+) distribution show an E('-2) dependence at high energies in agreement with simple collision cascade theory. The cluster energy profiles also give an E('-m) dependence with the index m increasing with increasing cluster size. The high energy tail of the M(,2)('+), MX('+) and M(,2)X('+) species suggest a collisional mechanism responsible for their ejection. A computer simulation of a simple surface correlated collision sequence model is shown to be able to generate the M(,2)('+) species and give predictions of their energy distributions in good agreement with experiments.