Silver clusters of photographic interest Part 5: formation of R centres and P centres by reduction sensitization and light absorption of silver bromide grains

摘要

A model of silver clusters with R centres as traps for positiveholes and P centres as traps for photoelectrons in silver halidegrains was proposed in this series of papers, and provided severalsubjects for future studies after discussions at severalinternational meetings. Considerations and some experimentalexaminations on those subjects by use of fine silver bromide grainswere made in this study. With the amount of dimethylamine borane(DMAB) used for reduction sensitization, the sensitivity of thesegrains increased through two steps, as reported in this series ofpapers. At the beginning of the first step, the radiowavephotoconductivity with positive holes as carriers at room temperaturebegan to decrease, while the microwave photoconductivity withphotoelectrons as carriers at room temperature began to decrease atthe beginning of the second step. Thus, these experiments couldclearly confirm the formation of R centres and P centres by reductionsensitization. It was observed by applying electron microscopy togelatinate shells of the grains that reduction sensitization led tothe formation of many silver clusters per grain, being highlycontrasted with latent image formation, which led to the formation ofonly one silver cluster per grain regardless of exposure, owing tothe concentration principle. The above-stated silver clusterformation by reduction was therefore not based on the concentrationprinciple, but on the proposed mechanism without contri- butions froma free electron or a single silver atom to the cluster formation.