$Z_1$ dependence of ion-induced electron emission from aluminum

We have measured the electron emission yields $\gamma$ of clean aluminum under bombardment with ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, ${\mathrm{D}}^{+}$, ${\mathrm{D}}_2^{+}$, ${\mathrm{He}}^{+}$, ${\mathrm{B}}^{+}$, ${\mathrm{C}}^{+}$, ${\mathrm{N}}^{+}$, ${\mathrm{N}}_2^{+}$, ${\mathrm{O}}^{+}$, ${\mathrm{O}}_2^{+}$, ${\mathrm{F}}^{+}$, ${\mathrm{Ne}}^{+}$, ${\mathrm{S}}^{+}$, ${\mathrm{Cl}}^{+}$, ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Xe}}^{+}$ in the energy range 1.2-50 keV. The clean surfaces were prepared by in situ evaporation of high-purity Al under ultra-high-vacuum conditions. It is found that kinetic electron emission yields ${\gamma }_k$, obtained after subtracting from the measured $\gamma$ a contribution due to potential emission, are roughly proportional to the electronic stopping powers, for projectiles lighter than Al. For heavier projectiles there is a sizable contribution to electron emission from collisions involving rapidly recoiling target atoms, which increases with the mass of the projectile, and which dominates the threshold and near-threshold behavior of kinetic emission. The results, together with recently reported data on Auger electron emission from ion-bombarded Al show that the mechanism proposed by Parilis and Kishinevskii of inner-shell excitation and subsequent Auger decay is negligible for light ions and probably small for heavy ions on Al and in our energy range. We thus conclude that kinetic electron emission under bombardment by low-energy ions results mainly from the escape of excited valence electrons.