The distribution function of the distance between a mobile and an immobile atom on a single-crystal surface has been measured for various temperatures, resulting in the first experimental demonstration of nonmonotonic long-range interactions. With W as the immobile atom and Pd as the mobile atom, at least two well-defined bond distances, 3.2 and 11 Å, are observed; with Re replacing W, at least one well-defined bond distance, 5.0 Å, and a strong anisotropy are seen. Additional bound states are indicated at larger distances.

• Received 3 December 1979

DOI:https://doi.org/10.1103/PhysRevLett.44.1008