It is demonstrated that the dependence of the two-photon photoemission (2PPE) yield on the polarization of the exciting laser light provides detailed information about the excitation mechanism and the orientation of transition dipole moments in the 2PPE process. In particular, it is possible to distinguish between a direct two-photon excitation process, where both electronic transitions are induced by the electric fields at the surface, and an indirect mechanism, where the first excitation step occurs in the substrate. In the latter process the intermediate state in 2PPE is populated by scattering of photoexcited hot electrons from the substrate which are subsequently photoemitted by the second laser pulse. The analysis is applied to 2PPE from clean and CO covered Cu(111). Furthermore we have derived analytical expressions for the 2PPE signal based on the optical Bloch equations for a three level system excited with continuos light beams. They allow to calculate 2PPE spectra of surface states for a variety of cases.
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