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Abstract
Octahedral core–shell AgBr emulsions sensitized by sulphur-plus-gold with gold added at 40°C either before or after sulphur sensitization were prepared and studied. The core emulsion was sensitized with a low level of dimethylamineborane (DMAB), and it enhanced the intrinsic sensitivity and long-wavelength sensitivity of the emulsion with an unsensitized shell. Oversensitization was observed for a high gold level in sulphur-plus-gold sensitization for both types of gold addition. The electronic effect of gold is small in these emulsions. Sulphur-plus-gold sensitization increases the long-wavelength sensitivity over sulphur-only sensitization, but generally more so for gold added before sulphur sensitization. To explain the enhancement of the long-wavelength sensitivity of the sulphur-plus-gold sensitization over sulphur sensitization, it is proposed that the energy level of the trapped electron in silver–gold–sulphide centres is slightly higher than in silver–sulphide centres. The dependence of long-wavelength sensitivity on the type of gold addition is explained by proposing that gold ions have to overcome an energy barrier to incorporate into the silver–sulphide centres. Gold ions decrease the long-wavelength sensitivity in gold-only sensitization, which is suggested to be due to a reaction with surface impurities or gelatin components that lead to the long-wavelength response in the unsensitized emulsion.