Photoelectric Emission fromF-Centers in KI

$F$-centers were produced in thin evaporated layers of KI by ultraviolet irradiation or by electron bombardment. Photoelectric emission became measurable for $h\nu =2.5\mathrm{}$ ev and increased to a plateau value near ${10}^{-4\mathrm{}}$ electron/quantum at 4 ev. Yields rose further to a sharply peaked value about 20 times higher at 5.66 ev. This point is practically coincident with the first optical absorption maximum for pure KI as measured by Fesefeldt. The peak shifted with temperature in the same way as the optical absorption. The yield was roughly proportional to the absorption constant. Below 5 ev, the emission was attributed to direct ionization of $F$-centers. The enhanced yields at the peak were tentatively ascribed to ionization of centers by excitons produced in the optical absorption.