Dynamics of the image surface state in two-photon photoemission spectroscopy

The study of the dynamical evolution underlying the two-photon photoemission process has been quite active recently. It is now well understood that the cross-correlation curve, corresponding to the resonance n=1 of the two-photon photoemission spectrum, results from a simple sequential process where the laser pump pulse populates the image potential state, which is ultimately tested by the probe pulse. However, the case of the cross-correlation curve associated with the second resonance n=0, observed at a different energy, is still the subject of controversy. In the present work, we demonstrate that the dynamical process underlying this photoemission corresponds to a purely coherent process where the electrons are photoemitted as a consequence of the coherence induced between the intrinsic surface state and the final photoelectron state through the intermediate image potential state. Both the spectral distribution of the photoemitted electrons and the analytical structure of the cross-correlation curve n=0 confirm this model. Also, the influence of the relaxation and dephasing constants on the cross-correlation curves is clarified.