Photography: enhancing sensitivity by silver-halide crystal doping

The physical chemistry of the silver photography processes, exposure, development and fixing, is briefly summarized. The mechanism of the autocatalytic development by the developer of the clusters produced in silver bromide crystals during the exposure which is controlled by the critical nuclearity of these clusters was understood from pulse radiolysis studies. The effective quantum yield Feff of photoinduced silver cluster formation in silver halide microcrystals is usually much lower than the photoionization theoretical limit Ftheor 1 electron–hole pair per photon absorbed, owing to a subsequent very fast intra-crystal recombination of a part of the electron–hole pairs. In order to inhibit this recombination and favor the silver reduction by photo-electrons, the AgX crystals were doped with the formate HCO2 as a specific hole scavenger. First, the dopant scavenges the photoinduced hole, thus enhancing the electron escape from the pair recombination. Second, the CO2 d radical so formed transfers an electron to another silver cation, so that the Feff limit may be of 2Ag 0 per photon. This Photoinduced Bielectronic Transfer mechanism is strictly proportional to the light quanta absorbed and induces an exceptional efficiency for enhancing the radio- or photographic sensitivity insofar as it totally suppresses the electron–hole recombination