Hydrogenic image-potential states: A critical examination.

Accurate binding energies ${E}_{B}$ of image-potential states are determined with two-photon photoemission. The observed values of ${E}_{B}$ [0.83 eV for Cu(111), 0.77 eV for Ag(111), and 0.80 eV for Ni(111)] lie close to the hydrogenic n=1 value of 1 Ry/16=0.85 eV and do not depend significantly on the effective mass. This result rules out a recent assignment of the observed image state to an n=2 state that is strongly perturbed by surface corrugation. The high accuracy of \ifmmode\pm\else\textpm\fi{}0.03 eV is used to test various models.