8 Time-resolved two-photon photoemission

In this chapter time resolution will be introduced to photoemission experiments. This means not just a series of regular photoemission measurements to monitor the changes of a sample with time. In two-photon photoemission the time delay between the two photons is an addi-tional experimental parameter which can be controlled with femtosecond resolution, i.e., on a time scale relevant for electronic processes. At the same time, the introduction of the second photon leads to the participation of an additional electronic state in the energy diagram for photoemission. 8.1.1 Energy diagram In Figure 8.1 the transition from initial state |1 〉 to an intermediate state |2 〉 after the absorption of the photon with energy hνa is shown. The second photon of energy hνb excites the electron out of the intermediate state into the final state |3〉. If the final state energy E3 is above the vacuum energy Evac, the electron might leave the surface and its kinetic energy Ekin is measured with an electron energy analyzer. Obviously, the initial state has to be occupied, i.e., its energy E1 should be below the Fermi energy EF. The intermediate state on the other hand has to be empty at first. In most cases it is desirable to keep the photon energy hνa belo