The combined effect of single-particle and collective surface excitations in the decay of image-potential states on Ag surfaces is investigated, and the origin of the long-standing discrepancy between experimental measurements and previous theoretical predictions for the lifetime of these states is elucidated. Although surface-plasmon excitation had been expected to reduce the image-state lifetime, we demonstrate that the subtle combination of the spatial variation of $s\mathrm{\text{−}}d$ polarization in Ag and the characteristic nonlocality of many-electron interactions near the surface yields surprisingly long image-state lifetimes, in agreement with experiment.

• Received 12 March 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.096401