Absence of a ``Threshold Effect'' in the Energy Loss of Slow Protons Traversing Large-Band-Gap Insulators

The electronic stopping cross section $\varepsilon$ of slow hydrogen projectiles in large-band-gap insulators has been measured at energies of a few keV. Even at velocities as low as $v_0/3\mathrm{}$ $(v_0=c/137\mathrm{})$, we find no influence of the band gap on the velocity dependence of $\varepsilon$, contrary to the case of gaseous targets with similar minimum excitation energy. The magnitude of $\varepsilon$ and its essentially linear velocity dependence allow us to arrive at the following conclusion: Electron 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 correlation diagrams for the H/LiF collision system supports the idea of local reduction of the band gap of an insulating target.