Energy loss of photoelectrons by interaction with image charge

The photoelectron spectra of the Cu(001) and Cu(110) surfaces excited by tunable-laser photons of very low energy were measured. It was found that the photoelectrons can lose energy through interaction with its image charge. The energy loss occurred outside the solid surface and appears as a spike structure at the vacuum edge in the photoemission spectra. The energy loss structure can be observed in the absence of unoccupied states at the vacuum level.