Angular distribution of reemitted positrons from W(100) with over layers of oxygen and LiF

The angular distribution of reemitted positrons has been measured from clean and oxygen exposed W(100), as well as from W(100) with LiF deposited on the surface, at an incident positron energy of 600 eV. Both the deposition of LiF and oxygen adsorption on W(100) caused a drastic reduction in the reemitted positron intensity and broadening of the angular distribution of the remitted positrons from W(100). The angular distribution of reemitted positrons in each case could be best fitted to the sum of two Gaussians, a narrow Gaussian whose width is in the range expected from a one-dimensional step model of positron reemission and an additional broad Gaussian. The area under the narrow Gaussian is more sensitive to surface over layers than the broad Gaussian.