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Charge-state distribution of MeV He ions specularly reflected from a SnTe(001) surface.

The charge exchange process of fast He ions at solid surfaces has been investigated by the specular reflection of MeV He ions from a clean (001) surface of a SnTe single crystal. A large difference is observed between the charge-state distribution of specularly reflected ions and that of ions transmitted through a self-supporting foil. The observed results indicate that the charge-state distribution of the specularly reflected ions is determined by a charge exchange process with the valence electrons in the tail of the electron distribution at the solid surface, whereas that of the foil-transmitted ions is mainly determined inside the foil. The electron-capture cross section of He/sup 2+/ ions in collisions with valence electrons near the surface of SnTe(001) is estimated from the observed charge-state distributions. It is shown that the surface plays an important role in the charge exchange process of MeV He ions for takeoff angles of the order of 10 mrad.