Abstract

The work required to move an electron from a metal into an insulator has been investigated photoelectrically. The work function thus found was about 4.3 ev for Ag evaporated in vacuum on KBr (Fowler plot) as well as on NaCl (threshold wavelength). The photoelectrons become trapped in these crystals at vacant anion sites and may be released by $F$-band illumination. For Ag on AgCl a work function of 1.1 ev is measured at the low temperatures used to freeze out ionic conductivity.In AgCl seven new bands of photosensitivity were found; the corresponding optical absorption was too small to be measured directly. These bands have a half-width of $\mathrm{ca}$ 0.2 ev at 90\ifmmode^\circ\else\textdegree\fi{}K and are bleached by irradiation. The 4400, 5000, 5700, 6600, and 7500A bands increase in intensity with an increasing supply of electrons provided by photoelectric emission. These bands are identified by their location and behavior, with the $F$, $E$, $D$, $C$, and $G$ bands (Petroff's notation) in the alkali halides. The 4800A ($A$ band) and 9200A bands are favored by a scarcity of electrons and may correspond to trapped holes. Photosensitivity in all these bands can be created by irradiation in the intrinsic optical-absorption edge of AgCl at low temperature. They may not be characteristic of a perfect crystal.The intrinsic band-like photoconductive response of AgCl and the absorption edge on which it occurs shift toward shorter wavelengths with decreasing temperature. The maximum of response which appears with decreasing wavelength may arise from a bimolecular recombination of charge carriers released by light. The absorption edge steepens as it shifts toward shorter wavelength with decreasing temperature.