Charge exchange for medium energy He and Ne ions in a large-angle collision at solid surfaces

Charge exchange in a large-angle single collision was studied for medium energy He+ impact on NiSi2(1 1 1), MgO(0 0 1), KI(0 0 1) and Si(1 1 1)-3×3-Sb and for Ne+ impact on NiSi2(1 1 1) using a toroidal electrostatic analyzer with an excellent energy resolution (ΔE/E) of 9×10−4. We measured charge state distributions (CSDs) for He ions backscattered from the top-layer atoms of single crystals under channeling and blocking geometries. It was found that the CSDs were not equilibrated for He ions scattered from low Z atoms and the lower the target Z number, the larger the average charge. The path-length to attain a charge state equilibrium is very short, in a few mono-layer range and thus it yields surface peaks in backscattering spectra even from amorphous solids which were measured with an electrostatic or magnetic spectrometer. In the case of Ne+ impact on Ni atoms of NiSi2(1 1 1), the CSDs are also not equilibrated and the average charge is gradually decreased with increase in Ne velocity in the velocity range from 0.4 to 0.8×108 cm/s. The CSDs are strongly dependent on exit angle with respect to surface plane. We propose a simple model to explain qualitatively the charge exchange process in a large-angle collision at solid surfaces.