Two-photon photoemission via image-potential states

Strong two-photon photoemission is observed from Ag(111) and Cu(111) surfaces with image-potential states as intermediates. Three processes are found: (a) resonant excitation, (b) relaxation into the intermediate state, and (c) energy pooling via collisions between excited electrons. For Ag(111) we find two image-potential states with binding energies of 0.77±{}0.03 and 0.23±{}0.03 eV, close to the hydrogenic values, and an unidentified state 2.57±{}0.05 eV below the vacuum level. The work function is 4.49±{}0.02 eV.