Energy distributions of secondary electrons. III. Projectile energy dependence for ionization of He, Ne, and Ar by protons

A theoretical analysis of secondary electron spectra, which has previously been applied to electron-impact ionization of atoms and molecules, is applied to the experimental data on ionization of He, Ne, and Ar by protons in the 0.1-1.5-MeV energy range. The data are also compared with ab initio Born-approximation calculations. Our theoretical analysis clearly brings out expected features in the secondary-electron spectra for incident proton energies of 1 MeV and up. Inner-shell contributions and the charge transfer to the continuum process tend to obscure the appearance of a simple asymptotic behavior at lower proton energies; for a given incident energy, the analysis is simpler for atoms and molecules with lower nuclear charge.