Molecular versus atomic secondary ion emission from solids

Abstract

Mass-selected ion currents resulting from 6 keV Ar+ bombardment of specially prepared samples of PbF2, PbCl2, PbBr2, PbI2, PbO, NaCl, and Bi2O3 containing small specified amounts of metallic elements have been measured using either 0–20 or 40–60 eV secondary ions. With either energy range the yield of M+ ions from a given matrix is inversely proportional to the exponential of the ionization potential EM of the element M. Also, the ratio of the monohalide or monoxide MX+ current to the corresponding M+ current is directly proportional to the exponential of the bond strength D (M–X) of the molecular ion; this result is utilized to obtain hitherto unknown bond strengths, thus allowing some estimates of monoxide ionization potentials to be given. MX+ emission is therefore substantial in those instances when (a) the bonding between M and X is strong and (b) the ionization potential of MX is low. These remarks seem to apply to molecular ion emission in general. Substantial MX+ emission leads to a reduction in M+ emission, suggesting competition between these two processes. A simple model for the production of these molecular and atomic ions is proposed.