Effective mass and momentum-resolved intrinsic linewidth of image-potential states on Ag(100)

The intrinsic linewidth of Ag(100) electronic image-potential states versus the momentum parallel to the surface has been obtained from line-shape analysis of high-resolution angle-resolved two-photon photoemission spectra. For the $n=1$ state, the measured intrinsic linewidths have been observed to increase from 14 meV at normal emission (k_=0) to 70 meV at k_=0.18^-1. This finding, when compared to the significantly lower inverse lifetime broadening measured on Cu(100), opens the question of the interplay between collective particle effects and nonlocal mechanisms associated with the image-potential electrons decay dynamics. In addition, the effective electron masses for the $n=1$ and $n=2$ states, observed in our experiments along the ΓM̄ and ΓX̄ directions of the surface Brillouin zone, improve the agreement between the measured values and the theoretical predictions.