Threshold in the Stopping of Slow Protons Scattered from the Surface of a Wide-Band-Gap Insulator

We have measured the energy loss of slow protons scattered with energies from 300 eV to 28 keV from a clean and flat LiF(001) surface under a grazing angle of incidence. Our data reveal a threshold behavior of stopping at low projectile energies. The effect on the outgoing charge state indicates that electron capture and loss are dominant mechanisms for the stopping of slow protons interacting with a wide-band-gap insulator. Our data allow one also to deduce information on charge transfer in front of the surface of an insulator.