Reflection of Noble Gas Ions at Solid Surfaces

Abstract

A method has been developed for distinguishing the reflection of ions at solid surfaces as ions or as metastable atoms. Results are given for ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{Ar}}^{+}$ ions incident on clean W, Mo, and Si(100) and on contaminated W, Hf, and Ge(111) surfaces. The reflection coefficient of ions to ions ($R_{\mathrm{ii}}$) is found to be small (0.0004 to 0.002) and essentially independent of incident ion energy. The reflection coefficient of ions to metastable atoms ($R_{\mathrm{im}}$) is found to increase with ion energy from values comparable to $R_{\mathrm{ii}}$ at 10 ev to values as high as 0.04 at 1000 ev. Discussion of these results is given in terms of the known resonance and Auger transitions which can occur near a solid surface for ions of sufficiently large ionization energy. It is shown that the results can be accounted for only if ions are transformed to metastable atoms very close to the surface, and a possible mechanism for this process is proposed.