Angle resolved scattering and electron ejection by helium metastable atoms incident on a tungsten (110) surface

Abstract

Angle resolved measurements have been made of the metastable scattering and electron ejection consequent upon the interaction of a thermal energy, helium metastable beam (angular divergence 24') with a clean and oxygen covered tungsten (110) surface at 300 K. The azimuthal angle used was 6° off the (111) direction. Reflection of metastable atoms from a clean W(110) surface occurs at angles of incidence greater than 35° with the fraction reflected increasing, up to 85° incidence. The reflection is apparently non-specular and is accompanied by a broadening of the scattered beam to ~25° at half height. For the oxygen covered surface reflection is found to occur at angles of incidence less than 35° and again it is apparently non-specular. The reflection process can be modelled crudely by allowing the incident atoms to experience a repulsive Born-Mayer potential. The apparently non-specular effects are attributed to rainbow scattering from the periodic potential at the surface. Polar plots of the electron ejection process show marked angular effects for both the clean and oxygen covered surface, which vary with angle of incidence. Electron yield measurements show that the yield of ejected electrons is greater for the oxygen covered surface and both the clean and oxygen covered surface give a yield which increases with increasing angle of incidence. The ejected electron spectrum for both the clean and oxygen covered surface is unlike that which can be expected from a two-electron resonance ionization/Auger neutralization process. The oxygen covered surface shows a peak in the energy spectrum which corresponds with UPS measurements. It is concluded that, at normal incidence, for an oxygen covered surface, the electron ejection mechanism may be Auger de-excitation.