Plasma Oscillation of Valence Electrons and Exciton in Alkali Halides

Abstract

The characteristic energy losses of electrons passed through a thin foil of alkali halides, LiF, NaCl, KCl and KBr have been studied using the transmission technique. The differential cross section, the dispersion relation and the cutoff scattering angle of the strongest loss peak in a spectrum were measured. It is pointed out that the loss peak comes from the plasma oscillation of valence electrons. For the first loss peak due to the exciton, and for the 2nd (3rd for LiF) loss peak, the same sort of measurements are carried out. These measured values for the plasma oscillation were supported roughly by the free electron theory with 6 electrons per molecule taking part in the oscillation, and especially the value of the plasma energy agrees well with that by Horie's theory. The loss peak located between the exciton and the plasma peak may be due to the tangential (surface) plasma oscillation. The ratio of the total cross section of these loss peaks to that of the loss peak due to the plasma oscillation were measured.