Electron-stimulated desorption from alkali halide surfaces: Yield and kinetic-energy distributions of positive alkali ions

Low electron-current densities (average values below 100 pA/mm2) and a time-of-flight technique were used to receive electron stimulated desorption spectra from LiF/Si(1 0 0) and from NaCl/Si(1 1 1) samples kept at room temperature. Such an experimental approach allows to obtain the results which are not distorted by the charging-up effects, are free of ions originated in post-ionization of atoms, and for which specific oscillatory structures can be distinguished in the kinetic-energy distributions of 6Li+, 7Li+ and 23Na+ ions, respectively. These oscillatory structures reveal that during an ion desorption sequence the wave packet squeezing effects are present and that, moreover, they are strong enough to be detected experimentally. Taking into account that three different potentials (ground-state, excited-state, final-state) are involved in the ion desorption scenario, it is shown that the obtained results can directly be used to determine the final-state potential from which the ion desorption occurs. Moreover, when the reduction in the ion desorption probability, related to the two-hole hopping process, is applied, not only the ion kinetic-energy distributions can be very well reproduced but also the calculated desorption yields are in agreement with experiments (including the isotope effect). The analysis performed indicates that most of the desorbed ions come from adatom sites and that only their small fraction is originated from normal lattice positions on planar parts of the sample surface.