Image potential states seen via two-photon photoemission and second harmonic generation

Two new techniques for the observation of image potential states are discussed, i.e., two-photon photoemission with the image potential state as intermediate and resonant second harmonic generation with the image state as final state. Presently, two-photon photoemission provides the best energy and momentum resolution for probing image states. It is possible to achieve high enough population densities to study electron-electron interaction. An overview of experimental results is given and the prospects for observing two-dimensional electron gas phenomena are discussed.