Energy Distribution of Secondary Electrons

The energy distribution of secondary electrons ejected by fast charged particles from free atoms and molecules should show certain qualitative features according to theory. Recent electron-impact data by Opal, Beaty, and Peterson indeed exhibit these features clearly. The spectrum of fast secondaries tends to agree better with the Mott cross section rather than with the binary-encounter theory with the corrections for the initial momentum distribution of bound electrons. A method is proposed for extrapolating the secondaryelectron spectra to ranges where experiment becomes difficult (e.g., for very slow and very fast secondary electrons). With this method, secondary-electron spectra can be put easily on an absolute scale, provided that reliable total ionization cross sections are available. The method applies also to the analysis of experimental data on electrons ejected by fast heavy particles, such as protons and alpha particles.