Effective mass of image potential states.

Effective masses m(*) are determined for the epsilon(mean)k|) dispersion of the n=1 image potential state using two-photon- photoemission. We obtain m(*)/m = 1.35 +- 0.1 for Ag(111), m(*)/m = 1.2 +- 0.1 for Ag(100), and m(*)/m = 0.9 +- 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.