Trapping of image-potential resonances on a free-electron-like surface

Image-potential states have been studied by two-photon photoemission for the surface of Al(100) where the whole series is energetically degenerate with free-electron-like bulk states. In contrast with expectations, the series of resonances is not smeared out to one broad structure as a result of a strong coupling to the bulk continuum. Instead, the first resonance (n=1) is found to be suppressed, and the resonances with quantum numbers n=2,3,4,5 are resolved as individual peaks in the time-resolved spectra. Both effects are suggested to be a consequence of resonance trapping.