Spike structure in the very low energy photoelectron spectra of Ag(0 0 1)

We have measured the photoelectron spectra of the Ag(0 0 1) surface excited by tunable-laser photons of very low energy (4.42-4.74 eV). A spike structure, which arises from the energy loss of photoelectrons by interaction with the image charge, was observed just above the vacuum level in the spectra. Since no unoccupied state exists near the vacuum level at the Γ̄ point of the Ag(0 0 1) surface, the photoelectrons cannot return to the solid after the energy loss process and are detected as the spike structure. This result supports our previous proposal that this spike structure appears when the vacuum level lies in the energy gap at the Γ̄ point.