Vacuum Photolysis of Silver Bromide and Silver Chloride

Abstract

The quantum yields of halogen production in the vacuum photolysis of silver bromide and silver chloride have been measured at various temperatures, wavelengths, and light intensities. Silver-bromide crystals containing small amounts of copper and cadmium bromide have also been studied. At room temperature, the yields of thick crystals follow the equation, φ=Q(1−e−ka),where Q is the surface efficiency, k is the absorption coefficient, and a is a parameter which is defined below. At room temperature, in the halogen evolution range studied, the value of a for silver bromide is about 0.4 micron and that for silver chloride is about 0.24 micron. The values of Q are usually between 0.5 and 1.0. The quantum yield of halogen production at room temperature is similar to the yield of a fast wall reaction. Both depend on the diffusion of reactants to the surface. The parameter a is related to the thickness of the diffusion layer. A numerical solution of the diffusion equation by C. A. Duboc has shown that a=1.154D12k14k114Io14,where D is the diffusion coefficient of positive holes and electrons, k1 is the recombination coefficient of positive holes and electrons in the silver halide, and Io is the incident intensity of light. Above a certain temperature, the quantum yields decrease with increasing temperature, the long-wavelength yields increase relative to those at short wavelength and the yields increase with increasing light intensity. These effects are inhibited by the addition of copper bromide to silver-bromide crystals. The decrease in yield at high temperatures is probably caused by the decreased effectiveness of traps. The cadmium results indicate that this decrease is also affected by the ionic conductivity of the crystals. The addition of copper or cadmium ion does not have a large effect on the quantum yields at room temperature. This indicates that positive-ion vacancies in the silver halides are not good positive hole-traps at room temperature. The data and the solution of the diffusion equation indicate that the quantum yield of photoconductivity and photolysis are closely related. Additional measurements of photoconductivity and light-absorption coefficients are needed before the results can be completely interpreted.