Theory of positronium formation at metal surfaces

A general theory of the direct formation of positronium at metal surfaces is presented using a resonant charge-transfer theory which is similar to the theory of slow ion-beam neutralization. In contrast to the previous theory [A. Ishii, Surf. Sci. 147, 277 (1984); 147, 295 (1984)] a three-dimensional momentum distribution is considered. Since the momentum parallel to the surface is conserved in the process of positronium formation, the momentum distribution of electrons on the topmost layer of the surface can be obtained from the angle-resolved positronium energy distribution.