Branching ratios for electron-volt positrons at a Cu(110) surface

The branching ratios for free-positron emission, positronium formation and surface annihilation have been measured for positrons of energies in the range 0.40 eV incident on a clean Cu(110) surface at room temperature, 773 K and 1073 K, on the same surface after bombardment by Ar+ ions, and after oxidation. For the clean surfaces a pronounced decrease in positron emission is observed for incident energies less than 20 eV, with corresponding increases in the other two channels. This effect, which has not been directly observed before, is much less pronounced for the sputtered surface and essentially non-existent for the oxide. A model is proposed in which the probability of surface state trapping or positronium formation is enhanced at low energies as a consequence of the presence of the positron surface potential well. The possible relevance of these results to earlier experimental and theoretical work on the slow-positron emission process is discussed. (author)