THE EFFECT OF INTERBAND EXCITATIONS ON TIME-RESOLVED 2-PHOTON PHOTOEMISSION VIA A LOCALIZED STATE AT A METAL-SURFACE

The effect of interband excitations on time-resolved two-photon photoe mission from an adsorbate-surface system is investigated. The system i s assumed to be composed of two sub-bands in the bulk and a localized state of the adsorbate, which is in resonance with the upper energy su bband. rn the two-photon photoemission process, first, an electron in the lower energy subband is excited into the localized state by the pu mp photon, and subsequently excited above the vacuum level by the prob e photon. The first excitation can be divided into two processes: i.e. , the direct and the indirect one involving the interband excitation. The excitation probability due to the latter process reaches a maximum at a certain photon energy because of momentum conservation. A two-ph oton photoemission spectrum shows a peak due to the indirect process a t an energy position where the initial state is fixed, as well as one due to the direct process where the intermediate state is fixed. The m aximum of the cross-correlation trace of the pump and probe pulses (th e pump-probe delay dependence of the photoelectron intensity) at the f ormer peak shows little shift compared to that at the latter one. The intensity of the peak due to the indirect process increases as the pul se duration is decreased.