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Momentum-resolved photoemission spectra from Cu(100) exhibit structure at 4 eV above the Fermi energy in a bulk band gap. This emission is pinned to the vacuum level and shows a free-electron-like dispersion. It is polarized with electric vector normal to the surface. The emission intensity is independent of the sample temperature unlike that of a simultaneously observed bulk direct transition. These properties identify the observed phenomenon as transitions into image-potential surface states.
Dose et al. (Mon,) studied this question.
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