Effective mass of image-potential states.

Effective masses ${m}^{\mathrm{*}}$ are determined for the E${(\mathrm{k}}_{?}$) dispersion of the n=1 image-potential state with use of two-photon photoemission. We obtain ${m}^{\mathrm{*}}$/m=1.3\ifmmode\pm\else\textpm\fi{}0.15 for Ag(111), ${m}^{\mathrm{*}}$/m=1.15\ifmmode\pm\else\textpm\fi{}0.1 for Ag(100), and ${m}^{\mathrm{*}}$/m=0.9\ifmmode\pm\else\textpm\fi{}0.1 for Cu(100). The results are explained by a phase-shift analysis which predicts an effective mass larger than unity if the image state is located near the top of the bulk band gap and smaller than unity near the bottom of the gap.