Enhanced yield of photoinduced electrons in doped silver halide crystals

The conventional photographic process(1-3) involves several steps: the photogeneration of electron-hole pairs in crystals of a silver halide; the reduction of silver cations to atoms by some fraction of these electrons; the subsequent build up of atoms to give clusters (the 'latent image'); and the complete reduction by a developer of crystallites having more than a critical number of silver atoms per cluster. The effective quantum yield, Phi(eff), of photoinduced electron-hole pairs produced per photon absorbed is less than the theoretical limit (Phi(theory) = 1), because of the fast recombination of some fraction of the pairs(1-6). Here we describe an approach for enhancing the yield of useful photogenerated electrons, in which the silver halide is doped with formate ions, HCO2-. The dopant ions act as hole scavengers, thus enhancing the escape of electrons from pair recombination. Moreover, the resulting CO2.- radical can itself transfer an electron to another silver cation, so raising the theoretical yield to two silver atoms per photon absorbed. This photoinduced bielectronic transfer mechanism is strictly proportional to the light quanta absorbed-the dopant ions do not induce spontaneous reduction of silver cations in the dark-and appears to be close to the theoretical limit of efficiency. The efficiency is constant at all illumination levels and applies to both dye-sensitized and unsensitized crystals. We suggest that this approach is a promising route for improving the performance of photographic emulsions(7).