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 ˆ 1 and n ˆ 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 ˆ 1 and can be explained in terms of two consecutive population processes. However, the corresponding dependence for the second resonance n ˆ 0, slightly shifted from the ®rst one by an amount corresponding to the frequency difference between the pump ®eld 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 in¯uence 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 ®t, lifetime and dephasing constants of the image potential state are evaluated. # 2000 Elsevier Science B.V. All rights reserved.