Charge state of 2-MeV carbon ions in the vicinity of a SnTe(001) surface.

Abstract

Energy losses and charge-state distributions for 2-MeV ${\mathrm{C}}^{3+}$ and ${\mathrm{C}}^{4+}$ ions incident under glancing angles on flat and step-rich SnTe(001) single crystal surfaces are measured by using a magnetic spectrometer with high-energy resolution. The charge-state distributions and the mean charge number 〈Q〉=3.74\ifmmode\pm\else\textpm\fi{}0.07, measured for specular reflection from flat surfaces, are almost independent of incident charge state and angle. By means of a freezing distance model, the freezing distance ${\mathit{D}}_{\mathit{f}}$ and the effective charge number ${\mathit{Q}}_{\mathit{f}}$ within the distance ${\mathit{D}}_{\mathit{f}}$ from the surface are derived as ${\mathit{D}}_{\mathit{f}}$=1.5\ifmmode\pm\else\textpm\fi{}0.1 \AA{} and ${\mathit{Q}}_{\mathit{f}}$=3.22\ifmmode\pm\else\textpm\fi{}0.06 from the observed energy losses for specular reflection. Reflection from stepped surfaces allows a direct measurement of the charge-state distribution in the surface vicinity as a function of the distance from the surface. The obtained charge-state distribution does not depend on the distance, which is in harmony with the freezing distance model. However, the mean charge number in the surface vicinity 〈${\mathit{Q}}_{\mathit{H}}$〉=3.53\ifmmode\pm\else\textpm\fi{}0.05, calculated from the charge-state distributions, is slightly larger than the effective charge number ${\mathit{Q}}_{\mathit{f}}$ derived from the energy loss data. This difference between ${\mathit{Q}}_{\mathit{f}}$ and 〈${\mathit{Q}}_{\mathit{H}}$〉 may be attributed to autoionization processes. The general trend that the mean charge in the surface vicinity is smaller than that for specularly reflected ions can be explained in terms of velocity matching of the 2-MeV ${\mathrm{C}}^{\mathit{i}+}$ ions and the N-shell electrons of SnTe. \textcopyright{} 1996 The American Physical Society.