A quantum-mechanical formulation of the image-force problem is presented. It has been applied to the problems of internal photoemission and photon-assisted tunneling and is found to give a unified theory of the two processes. The theory is found to be in good agreement with experiment for the aluminum-silicon dioxide interface. Barrier heights derived from internal-photoemission measurements are shown to be larger than the actual barrier heights by approximately 0.2 eV. © 1979 The American Physical Society.