ABSENCE OF A THRESHOLD EFFECT IN THE ENERGY-LOSS OF SLOW PROTONS TRAVERSING LARGE-BAND-GAP INSULATORS

The electronic stopping cross section epsilon of slow hydrogen project iles in large-band-gap insulators has been measured at energies of a f ew keV. Even at velocities as low as upsilon(0)/3 (upsilon(0) = c/137) , we find no influence of the band gap on the velocity dependence of e psilon, contrary to the case of gaseous targets with similar minimum e xcitation energy. The magnitude of epsilon and its essentially linear velocity dependence allow us to arrive at the following conclusion: El ectron promotion processes contribute substantially to stopping due to formation of molecular orbitals. This points towards the existence of a bound electron state at a proton that moves slowly in an insulator. A simple model based on the calculation of molecular orbital correlat ion diagrams for the H/LiF collision system supports the idea of local reduction of the band gap of an insulating target.