Additively colored alkali halide crystals represent, in a first approximation, a transparent solid with frozen-in electrons which can be mobilized by light absorption. When moving towards the anode, these electrons leave a positive space charge behind, and an adjustable cathode fall results which can be steepened until electrons are released by field emission. The steady-state and transient solutions for the charging and discharging cycle are calculated, the effects of light intensity, wavelength, and field emission discussed, and KBr crystals investigated experimentally as function of time, voltage, light intensity, and color-center density. Field emission has been produced with voltages as low as 1 volt, and a field-emission photocell has been realized.

• Received 18 March 1953

DOI:https://doi.org/10.1103/PhysRev.91.568