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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. [ABSTRACT FROM AUTHOR]

A simulation programme based on the nucleation-and-growth model of latent-image formation was used to study how trap depth and trap density at various tabular grain thicknesses affected quantum sensitivity and reciprocity failure. Using a 1.2 x 0.2 µm model 'grain', the unsensitized case was simulated with 0.05 eV traps located on both the face and the core of the grain, the latter to simulate the effect of twin planes on latent-image location. The trap densities were adjusted to achieve a higher internal speed than surface speed, as seen experimentally with the emulsion used to validate the simulation. To simulate the effects of chemical sensitization, these parameters were held fixed while edge traps of depths 0.2 0.6 eV were added at various trap densities for grain thicknesses of 50. 100 and 200 nm. All but the lowest trap densities at 0.2 eV changed the situation to complete or almost complete edge domination for latent-image location. Maximum efficiencies for latent-image formation were six to eight absorbed photons/grain for a 0.01 s exposure, although the trap density had to be decreased as the trap depth increased to achieve these maximum values. A decrease in efficiency with decreasing thickness as well as with decreasing diameter was seen for the lower trap depth values. These grain diameter and thickness effects disappeared for simulations using a 10-6 s exposure, indicating that the decreasing efficiency at 0.01 s was due to differences in the onset of low-irradiance reciprocity failure. Reciprocity failure was simulated for chosen trap depth/density combinations. These data were compared with experimental reciprocity failure data to help validate the model. Reasonable agreement was obtained for trap depths in the range of those deduced from the experimental phase of the project. However, uncertainties regarding other parameters that affect the position of the reciprocity failure curve with respect to exposure time must be reduced... [ABSTRACT FROM AUTHOR]

Sensitometric and spectroscopic techniques are used to study the effect of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (TAI) during the sulphur sensitization of AgBr octahedra. TAI increased the speed of an oversensitized emulsion, causing it to have a speed comparable with the optimally sulphur-sensitized emulsion at high irradiance. TAI decreased the absorption by sulphide centres at 1.5 mmole concentration but not at 1 mmole. The long wavelength sensitivity (> 500 nm) increased at 1 mmole TAI concentration, but decreased at 1.5 mmole TAI concentration, whereas the activation energy for this response was reduced at both TAI levels. These results are interpreted using a model which assumes that TAI interacts with multiple-sulphide centres to lower their ground-state energy level, but not their electron-trapping energy level. This interaction reduces the degree of tree-hole/trapped-electron recombination that occurs at these centres. The enhancement of this recombination channel by high levels of sensitizer is what causes the oversensitization effect. [ABSTRACT FROM AUTHOR]

Cubic AgIBr emulsions sensitized by sulphus-plus-gold with gold added at 40 °C either before or after sulphur sensitization at 70 °C were prepared and studied. Gold ions incorporate into the silver-sulphide centres and decrease the diffuse reflectance spectroscopy (DRS) response. The emulsions with gold added before the heat cycle through 70 °C showed a dependence on the amount of gold for a speed increase at 0.01 s and their reciprocity failure property, whereas the emulsion with gold added at 40 °C after the heat cycle did not. Gold present at the elevated sensitization temperature enhanced long-wavelength sensitivity in a wider wavelength region than gold added after the heat cycle. The different sensitometric behaviour between gold added before or after the head cycle could be explained by the enhanced thio-sulphate conversion effect of gold at 70 °C. The electron trapping energy level of the sensitization centre is speculated to be slightly higher for sulphur-plus-gold than sulphur sensitization. Opposite trends were observed in DRS and long-wavelength sensitivity when the gold level increased. Whether the electronic effect of gold during exposure on top of its latensification effect in sulphur-plus-gold sensitization could be observed seems to be dependent on the sulphur level. A mechanism is proposed. [ABSTRACT FROM AUTHOR]

Sensitometric and spectroscopic techniques are used to study the effect of sodium thiocyanate addition to sulphur-sensitized AgBr octahedra. Thiocyanate increased the speed of an oversensitized emulsion, causing it to have a speed comparable to the optimally sulphur-sensitized emulsion at high irradiance and greater speed at low irradiance. Thiocyanate decreased the absorption by sulphide centres at shorter wavelengths (500-650nm), but increased the absorption at longer wavelengths. The long-wavelengths sensitivity (>500 nm) was also increased by thiocyanate,whereas the activation energy for this response was reduced. These results are interpreted with a model that assumes that thiocyanate interacts with multiple-sulphide centres to lower their ground state energy level. This interaction reduces the degree of free-hole/trapped-electron recombination which occurs at these centres and which is responsible for the over-sensitization effect. A computer simulation based on this model have simulated reciprocity failure data which were qualitatively consistent with the experimental trends. The effects of thiocyanate on absorption are interpreted as a extinction coefficient. [ABSTRACT FROM AUTHOR]

Sensitometric and spectroscopic techniques are used to characterize sensitizer centres produced by sulphur sensitization of AgBr octahedra. Activation energies for long-wavelength sensitivity were independent of thiosulphate concentration for the single-sulphide centres (550nm), but showed a concentration dependence for the multiple-sulphide centres (700nm). The achievement of maximum photographic speed is associated with the production of the multiple-sulphide centres. Using activation energies for long-wavelength sensitivity, with the lowest one for the multiple-sulphide centres being taken as closest to reality, an energy level scheme was constructed for the two sulphide centres. The single-sulphide centres are estimated to have an electron trap depth between 0 and 0.1 eV, whereas the multiple-sulphide centres have trap depths between 0.25 and 0.45 eV. Deconvolution of thiosulphate-induced absorption spectra suggested that only 65 per cent of the converted thiosulphate is photographically active and that the single-sulphide centres are the predominant sulphide species. Computer simulated sensitometry based on these ideas was consistent with experimental sensitometry. Computer simulation of the sensitometry for the optimum thiosulphate concentration required a small concentration of the multiple-sulphide centres. Attempts to simulate the sensitometry of the oversensitized emulsion were less successful. Computer simulation based on the single-sulphide centres acting as hole traps were inconsistent with the experimental data. [ABSTRACT FROM AUTHOR]