Surface Losses of Alkali Halide Single Crystals in Electron Energy Loss Spectra

Angle resolved electron energy loss spectra of alkali halide single crystals have been measured with primary energies from 50 to 950 eV, and incident and scattering angles from 20 to 80°. The loss spectra show not only the loss peaks which have been observed in the transmission measurements at higher primary energies but also the surface losses at 7.4 eV for KCl, 6.6 eV for KBr, 9.3 eV for NaF and 10.5 eV for LiF. The intensities of these surface loss peaks decrease both with the increase in primary energies and with the decrease in incident angles. These peaks are slightly below the first exciton excitation energy and attributed to the excitation of the surface exciton. Moreover, it is found from the angular dependence of the intensities of the inelastically scattered electrons that the inelastic scattering is not the dipole but the impact scattering.