Electron emission from clean metal surfaces induced by low-energy light ions

We have measured the electron emission yields of clean Li, Al, Cr, Cu, Ag, and Au surfaces under bombardment with ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, ${\mathrm{D}}^{+}$, ${\mathrm{D}}_2^{+}$, and ${\mathrm{He}}^{+}$ ions in the energy range 2-50 keV. The clean surfaces were produced by in-situ evaporation of high-purity metals under ultrahigh-vacuum conditions. It is found that the $Z_2$ dependence of the yields for hydrogen and helium projectiles are very similar, that the yields for ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ show the same energy dependence as that of the electronic stopping powers, that molecular ions give lower yields per atom than atomic ions, and that isotope effects are negligible in our energy range. It is proposed that kinetic-electron emission under low-energy-light-ion bombardment results mainly from the escape of excited electrons produced by direct binary collisions between the projectile and the valence electrons of the target.