Estimation of adsorbed hydrogen on Ni(1Â 1Â 1) surface by slow-positron beam

We have measured reemitted slow-positrons from clean Ni(1 1 1) surface and hydrogen saturated one, and have observed the reduction of reemitted slow-positron by adsorbed hydrogen atoms in the case of saturated coverage. The origin of the reduction has been discussed, taking into account of electrostatic repulsion between hydrogen adatoms.