Z2- and energy-dependent charge fractions for medium energy He ions after a large-angle single collision at solid surfaces

We measured the absolute He+ and He2+ fractions for medium energy He ions after a large-angle single collision with the top-layer atoms of NiSi2(1 1 1), KI(0 0 1) and MgO(0 0 1) surfaces. The scattering components from the deeper layers atoms were suppressed by setting double alignment geometries. Thus, velocity and target Z number (Z2) dependent He+ and He2+ fractions were derived directly from the scattering yields from the top layer atoms. The present results showed that the lower the target Z2 number, the larger the He+ and He2+ fractions. We propose a simple model that immediately after a large-angle collision, He+ state is dominant and a small fraction of He2+ state still exists and then the He+ (He2+) ion captures a bound electron of the target atom before emerging the surface. The Z2- and energy-dependence of the He+ and He2+ fractions is explained quantitatively based on this simple model assuming that the electron capture probability at a position r(t) is proportional to the target electron density ρe(r(t)) and to the time lapse Δt from r(t) to r(t)+Δr. It is found that the average charge of He ions as a function of the integration ∫0∞ρe(r)(dr/v) is scaled by an appropriate universal curve in the low and medium energy ranges (v : ion velocity).