Description of the coherent and incoherent processes in two-photon photoemission on Cu(111) surface

Two-photon photoemission spectroscopy is now widely used, to determine the ultrafast dynamics of the electrons pertaining to the surface states. In the particular case of the Cu(1 1 1) surface, the energy spectrum exhibits two main resonances termed n = 1 and n = 0. The dependence of the two-photon photoemission signal as a function of the delay time between the pump and the probe laser pulses is well understood in the case n = 1 and can be explained in terms of two consecutive population processes. However, the corresponding dependence for the second resonance n = 0, slightly shifted from the first one by an amount corresponding to the frequency difference between the pump field frequency and the image potential to intrinsic state transition frequency, has frequently been interpreted in terms of a non-resonant excitation from the intrinsic surface state. Here, we show that the process underlying this resonance is a purely coherent process resulting from the overlapping of the laser pulses. The influence of the surface state lifetime and its corresponding electronic dephasing constants enable us to recover the purely symmetric shape of the cross-correlation function. From the fit, lifetime and dephasing constants of the image potential state are evaluated. (C) 2000 Elsevier Science B.V. All rights reserved.