Ultrafast dynamics induced by femtosecond laser pulse irradiation of atom-adsorbed metal surfaces

We present a theory of time-resolved two-photon photoemission (time-resolve d 2PPE, TR2PPE) spectroscopy on metal surfaces on which an atom (adparticle ) is adsorbed, and study the effect of the adparticle motion induced by fem tosecond laser pulse irradiation on the TR2PPE spectra. The TR2PPE spectrum is calculated by nonequilibrium perturbation theory treating the adparticl e motion within the trajectory approximation. First the laser pulse irradia tion causes excitation of an electron from a bulk state into a localized st ate on the adparticle, which results in the change of the potential energy curve for the adparticle motion, then the adparticle begins to move. Numeri cal results show that the effect of the adparticle motion is manifested in the motion of the peak position in the 2PPE energy spectrum as a function o f the pump-probe delay time. The motion of the peak position in the long de lay time region mainly provides information on the potential energy (kineti c energy of the adparticle) whereas the dynamical change of the energy leve l of the localized electron state due to the adparticle motion can modify t he peak structure. (C) 2001 Elsevier Science B.V. All rights reserved.