Using a high-intensity beam of 200-eV positrons we have measured the two-dimensional angular correlation of the $2\gamma$ annihilation radiation from a clean Al(100) surface. The momentum distribution identified with the positron surface state has a nearly isotropic conical shape and a (7.1 ± 0.5)-mrad full width at half maximum. The data are not consistent with a simple interpretation based on either the usual model of a positron bound in a surface state by its "image-correlation potential" or a positronium atom weakly bound to the surface.

• Received 28 December 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.1702