Self-image excitation mechanism for fast ions scattered by metal surfaces at grazing incidence

Energy-loss spectra and electronic-transition probabilities of atomic and molecular ions scattered by surface plasmons are evaluated for experiments in which fast ions are specularly reflected from a metal surface at grazing incidence. The coupling between plasmons and the ion monopole charge governs the scattered-ion loss spectrum. For ion velocities not small compared to plasmon phase velocities and for small grazing angles, large numbers of surface plasmons may be excited, resulting in large average energy losses and broad loss spectra, as observed experimentally. Further, the interaction between the ion multipole moments with the surface plasmons induces transitions between ground and excited states of the ions. For molecular ions this may result in their dissociation when the state produced is unstable. The final electronic states populated by the plasmon scattering mechanism are shown to exhibit a high degree of orientation, as required by the observation of elliptically polarized fluorescence emitted by the scattered beam. As applications, we calculate explicitly the dissociative scattering of ${\mathrm{H}}_2^{+}$ and the production of coherently oriented $2P$ states in ${\mathrm{He}}^{+}$ and compare with available experimental results.