Identification of Image-Potential Surface States on Metals

Rydberg-like surface states are identified at Au(100) and Cu(100) surfaces by use of inverse photoemission with tunable photon energy. The binding energy (0.6 eV below vacuum) and width (≤ 0.3 eV full width at half maximum) of the $n=1\mathrm{}$ state agree well with previous calculations. Theory yields the expectation that these states represent a widespread phenomenon at surfaces.