The effect of the adparticle motion on the time-resolved two-photon photoemission spectra of metal surfaces on which the adparticle is adsorbed is investigated theoretically. In the two-photon photoemission process, first a pump photon excites an electron from the bulk into the anti-bonding orbital formed between the adparticle and the metal, then the adparticle begins to move, and finally a probe photon excites the electron into a free electron state above the vacuum level. It is demonstrated by the Green function method that the peak position in the energy spectrum depends on the pump-probe delay time due to the adparticle motion and it represents the potential energy for the adparticle motion. (C) 2001 Elsevier Science B.V. All rights reserved.