The spectral response of photocathodes is severely degraded toward the ir portion of the optical spectrum. This paper compares the quantum efficiency of the S-11 and the S-20 photocathodes, based on an absorption diffusion model, with their quantum efficiency as reported by the ITT spectral response curves. This comparison (ratio) shows that the irdegradation is due primarily to the poor escape probability of a low energy photoelectron over the surface potential barrier rather than degradation in absorption. This ratio is used to estimate an escape probability that is plotted as a function of photon energy.
John A. Love III and John R. Sizelove, "Escape Probability of Photoelectrons into Vacuum," Appl. Opt. 7, 1559-1563 (1968)