Dynamical resonant neutralization of low-energy Na+ ions scattered from Au(111), Pd(111), Cu(111), and Cu(110) surfaces

The nonmonotonic energy dependence of the $\mathrm{L}{\mathrm{i}}^{0}$ fraction on high-work-function surfaces has not been understood so far. To further study this phenomenon, in this work, efficient neutralization of $\mathrm{N}{\mathrm{a}}^{+}$ ions has been reported on various surfaces, instead of $\mathrm{L}{\mathrm{i}}^{+}$ ions. The nonmonotonic energy dependence of the $\mathrm{N}{\mathrm{a}}^{0}$ fraction becomes more and more obvious with the increase of the surface work function. For Cu(111) and Cu(110) at the scattering angle of 7\ifmmode^\circ\else\textdegree\fi{}, the energy dependence of the neutral fraction is still nonmonotonic as compared to the scattering angles of 135\ifmmode^\circ\else\textdegree\fi{} and 53\ifmmode^\circ\else\textdegree\fi{}. For the scattering angle of 53\ifmmode^\circ\else\textdegree\fi{}, the nonmonotonic angle dependence has been observed for these surfaces. The essences of the nonmonotonic angle and energy dependences are the same. The quantum-mechanical calculations reveal that the width and position of the atomic level below the Fermi level at short ion-surface distances are responsible for high neutral fractions at large exit energies, and that the neutral fraction at much smaller exit energies is probably related to the neutralization at large distances. For the exit angle dependence, the competition between neutralization at short and large distances strongly depends on the surface work function. In particular, the neutralization is enhanced by the relatively large parallel velocity.