Coherent surface and bulk vibrations induced by femtosecond laser excitation of the Gd(0001) surface state

Excitation of the Gd(0001) surface state by 800 nm femtosecond laser pulses initiates transient coherent surface and bulk vibrations. These are investigated by time-resolved second harmonic and linear reflectivity probing simultaneously the motion of the surface layer and its coupling to bulk modes localized near the surface. Initially, the surface vibration frequency of 2.8 THz lies similar to18% below the frequency detected in linear reflectivity. Within the first 3 ps the surface vibration frequency is found to increase while the bulk one decreases. Both types of vibrations persist only during nonequilibrium between electron and lattice temperature and are damped within the electron-phonon relaxation time. We conclude that the observed frequency shifts cannot be accounted for by anharmonic effects but originate from time-dependent changes of the potential energy at and near the surface due to nonequilibrium electron distributions.