Final-state interactions in photoemission: Energy loss by the exiting electron

In photoemission, the excited electron may lose energy when it is in the vacuum, on its outgoing trajectory. It may couple to excitations in the substrate (surface plasmons, for example) via the long-ranged Coulomb interaction, as is well known from electron energy-loss spectroscopy. Joynt has argued that such losses can be severe for poor conductors, with the consequence that their inclusion is essential for the interpretation of their photoemission spectra. We show that the losses considered by Joynt have their origin simply in the work done by the image force experienced by the outgoing electron. This then just shifts the kinetic energy of all photoemitted electrons downward by roughly the same amount, save for a correction with origin in the velocity dependence of the effective image force.