ELECTRON-IRRADIATION EFFECT ON CAF2(111) STUDIED WITH HE ATOM SCATTERING

Elastic He atom scattering was used to investigate nonintrusively the effect of electron irradiation on a high-symmetry cleaved CaF2(111) su rface. Diffraction was observed in the [<(1)over bar 01>] and [<(2)ove r bar 11>] directions for this surface using a He supersonic beam of 6 4 meV. The diffraction patterns for the cleavage plane correspond to t ransitional symmetry of an ideal, unreconstructed CaF2(111) surface wi th a lattice vector length of 3.86 Angstrom. Layer-by-layer modificati on of the CaF2 surface by the 1-12 keV e(-) beam was investigated by m onitoring the intensity of the specular He reflection. Formation of su rface F-centers is proposed to account for an increase in the specular intensity.