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Observation of image-potential-induced resonances on Cu(110) using the two-photon photoemission technique.

Using the technique of two-photon photoemission, we have observed image-potential resonances on Cu(110). The image-potential resonances have a short, bulklike lifetime, resulting in a weak, but observable, photoemission signal that is broad in energy; the peak of the density of states for the n=1 resonance is 0.68±0.06 eV below the vacuum level, with a width of 0.66±0.06 eV. This investigation demonstrates the versatility of using relatively long-pulse, two-photon photoemission for observing short-lived surface resonances, which are excited from bulk initial states