Optical Emission Studies of the Characteristic Energy Losses in Cd, In, and Zn

Recent investigations of light emission from electron-bombarded Al and Mg foils yielded results in good agreement with the theory of plasma oscillations in metals and the electron energy-loss experiments. Since the optical method is a more accurate means of measuring the plasma frequency and studying the characteristic energy losses than the electron energy loss method, optical emission studies were continued by bombarding Cd, In, and Zn foils 600 {} to 1000 {} in thickness with 60- to 100-keV electrons, and analyzing the light emitted at 30^∘{} from the foil normal with a Seya-Namioka vacuum ultraviolet spectrometer. A sharp, intense peak with a half-width of 0.9 eV was observed in Cd at 9.1 eV, and a less intense peak was observed in In at 11.8 eV, whereas in Zn only an edge at 8.6 eV beyond which the intensity dropped sharply was observed. These values correspond quite well with soft x-ray energy levels. Thus the emissions are due either to interband transitions or to plasma decay. Using known optical constants, the theoretical transition radiation spectrum was calculated for In and showed good agreement with the experimental line shape.