XIV. The fluorescence of silver halides at low temperatures

Absteact Fluorescence emission spectra of AgCl, AgBr and Agl, believed to be characteristic of the pure solids, have been recorded at liquid nitrogen and liquid hydrogen temperatures. The spectra are all in the visible and lie just to the long wavelength side of the absorption bands of the respective halides. No resolution of the spectra was observed down to the lowest temperatures; no pure halide specimen emitted any phosphorescence or thermal afterglow. The two different structures of Agl capable of existence at ordinary and low temperatures (wurtzite and zincblende) gave slightly different emission spectra. The temperature at which the fluorescence just became visible was about 180°K. for AgCl and AgI, but only 77°K. for AgBr. Excitation of the fluorescence could be effected by any wavelength which the halide was capable of absorbing. All three halides emitted an extra band in the red after being exposed to light action at ordinary temperature, or slightly decomposed by heat treatment, so as to give a dispersion of free silver in the halide. Large quantities of impurity (∼1-3 M. per cent) were required in order to alter appreciably the characteristic emission of the basic silver halide. The low-temperature fluorescence of the pure halides is believed to be due to the direct recombination of excited electrons with positive holes with the emission of visible radiation, instead of the emission of thermal quanta as at higher temperatures.