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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. 770. 03 and 0. 230. 03 eV, close to the hydrogenic values, and an unidentified state 2. 570. 05 eV below the vacuum level. The work function is 4. 490. 02 eV.
Giesen et al. (1985) studied this question.