Bombardment Induced Electron-Capture Processes at Sodium Halide Surfaces

Discrete features observed in the energy distribution of electrons emitted from ion-bombarded sodium halide surfaces can be attributed to a new type of collisional deexcitation mechanism. Such a mechanism involves sodium atoms in bombardment-excited autoionizing states that are the result of cascade collisions within the crystal lattice. This deexcitation process, in contrast to that for a metal, is not simply a consequence of the inner-shell lifetime of the initial collisionally excited sodium Na(+)* ion. Rather, the deexcitation consists of a sequence of lattice collisions during which the excited Na(+)* ion captures an electron to form the inner-shell-excited Na(0)* states responsible for the observed transitions. The formation of such autoionizing Na(0)* states is described within the framework of a new model in which excitation processes and localized collisional electron-transfer mechanisms are taken into account. These localized electron-transfer processes make possible new channels for electronic deexcitation, chemical dissociation, and defect production; they are critical for understanding inelastic ion-surface collisions in solids.