Spectroscopic evidence for energy loss of photoelectrons interacting with image charge

We have measured the spectra of very low-energy photoelectrons emitted from the Cu single crystal surface excited by tunable laser light. The difference between the photon energy and the work function of the sample surface (i.e., the highest kinetic energy of the photoelectron) was less than 300 meV. A spike structure appeared just above the vacuum level in the photoelectron spectra of the Cu(001) surface. In contrast the spectra from the Cu(110) and Cu(111) surfaces contained no such structure. We have concluded that the spike structure in the Cu(001) spectra arises from the electron energy loss in vacuum by interaction with its image charge. Finally we propose the mechanism through which the spike structure is produced. The spike structure appears when there is no electronic state at the vacuum level (at the Γ point) to which zero kinetic energy photoelectrons can return.