Charge exchange of medium energy H and He ions emerging from solid surfaces

Charge exchange of medium energy H and He ions emerging from clean solid surfaces is studied extensively using a toroidal electrostatic analyzer with an excellent energy resolution. The charge distributions of He ions scattered from sub-monolayers near a surface are non-equilibrated, resulting in a surface peak even for poly-crystal solids. By solving simultaneous rate equations numerically, we derive electron capture and loss cross sections for Ni and Au surfaces. Based on a free electron gas model, non-equilibrated He⁺ fractions dependent on emerging angle reveals uniform electronic surfaces for metals and corrugated surfaces for Si and graphite with covalent bonds. It is also found that equilibrium charge fractions of H⁺ are independent of surface materials (Z₂) and in contrast equilibrium He⁺ fractions depend pronouncedly on Z₂. The data obtained are compared with semi-empirical formulas.