Watching the life of electrons at surfaces with two-photon photoemission

An electron put on a metal surface is attracted by the image force. A Rydberg-like series of image-potential states is formed which describes loosely bound electrons which can move freely parallel to the surface. With time-resolved two-photon photoemission spectroscopy the electronic band structure and the femtosecond dynamics of these states are studied in the energy- and time-domain. The decay of the image-potential-state population is attributed to electron–electron interaction with metal electrons, which constitutes an inelastic scattering process. Defects (or phonons) may lead to elastic scattering which changes the phase of the electronic wave function without changing the population. ime-resolved two-photon photoemission inelastic and elastic scattering processes can be separated. From these experiments we can extract information on the lifetime of excited electrons at surfaces in excellent agreement with theoretical calculations by Echenique and co-workers. The interaction of image-potential states with adsorbates or steps gives insight into the inelastic and elastic scattering by the various surface defects which are not accessible by any other technique.