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

We present a theory of time-resolved two-photon photoemission (time-resolved 2PPE, TR2PPE) spectroscopy on metal surfaces on which an atom (adparticle) is adsorbed, and study the effect of the adparticle motion induced by femtosecond laser pulse irradiation on the TR2PPE spectra. The TR2PPE spectrum is calculated by nonequilibrium perturbation theory treating the adparticle motion within the trajectory approximation. First the laser pulse irradiation causes excitation of an electron from a bulk state into a localized state on the adparticle, which results in the change of the potential energy curve for the adparticle motion, then the adparticle begins to move. Numerical 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 of the pump–probe delay time. The motion of the peak position in the long delay time region mainly provides information on the potential energy (kinetic energy of the adparticle) whereas the dynamical change of the energy level of the localized electron state due to the adparticle motion can modify the peak structure.