Scattering losses and inhomogeneous broadening are reduced by an optimized cocondensation of laser evaporated Ag atoms with Xe gas allowing the determination of absolute quantum efficiencies in light-induced processes. A jump probability per absorbed photon increasing from 10% to about unity for higher temperatures is observed for the migration of Ag atoms originating from electronic excitation. A model based on a concerted host-guest motion is proposed. A well. defined metastable site with a strongly red shifted absorption band and a generation probability of 1.5% which decreases with temperature is attributed to an intermediate position in the migration process.