Time-Resolved Two-Photon Photoemission
Combined with ultrafast laser techniques, time-resolved two-photon photoemission is one of only a few techniques that can monitor the state of an excited electron during the course of its transformation by laser-induced surface reactions. This is possible because it is a two-photon process which is sensitive to the density and lifetime of the intermediate (unoccupied) state. The dynamic information can be obtained by means of a pump-probe experiment in which the first laser pulse excites electrons into the intermediate state, followed by a second pulse which subsequently photoemits the excited electrons (see above figure). By varying the temporal delay between the two pulses, the decay of the intermediate (unoccupied) states can be determined at an accuracy of a few femtoseconds (10-15 seconds).