Mechanism of Exciton-Enhanced Photoelectric Emission in Alkali Halides

A theoretical interpretation is made of the photoelectric yield from RbI as observed by Apker and Taft. The mechanism used is the following: (1) In the neighborhood of the first peak in the optical absorption, excitons produced in the crystal diffuse to $F$-centers and eject electrons from them. (2) The electrons diffuse to the surface and appear as external photo-electrons. Competing with (1) are other processes of destruction of excitons. It is found necessary to suppose that these are localized near the surface to form a dead layer either because of preferential destruction there or a sparsity of $F$-centers needed for (1). In (2) electrons gradually lose energy to the lattice so that probability of escape through the surface decreases with increasing depth of origin. Combination of these two effects gives photoelectric yield first rising then falling with increasing absorption coefficient in agreement with observation. Energy distribution of emitted electrons is calculated and compared with experiment.