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

Abstract

We measured the absolute He + and He 2+ fractions for medium energy He ions after a large-angle single collision with the top-layer atoms of NiSi 2(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 ( Z 2) dependent He + and He 2+ fractions were derived directly from the scattering yields from the top layer atoms. The present results showed that the lower the target Z 2 number, the larger the He + and He 2+ fractions. We propose a simple model that immediately after a large-angle collision, He + state is dominant and a small fraction of He 2+ state still exists and then the He + (He 2+) ion captures a bound electron of the target atom before emerging the surface. The Z 2- and energy-dependence of the He + and He 2+ 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 ρ e( 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 ∞ ρ e( r)(d r/ v) is scaled by an appropriate universal curve in the low and medium energy ranges ( v : ion velocity).