The sulfur sensitization of photographic emulsions leads to the formation of silver sulfide clusters on the surface of emulsion microcrystals (MC). The size of the clusters is between 3 and 10 nm. The clusters of small size (Ag2S)n are hole traps, the mixed lusters (Ag2S)pAgk+ (k < m = 4) are the sensitivity centers, but larger clusters (Ag2S)pAgm+ (m ≥ 4) are the fog or latent image centers. In addition, emulsion dye can be adsorbed not only by AgBr MC but also by the silver sulfide clusters. We have shown that light absorption by dye can give rise to the cluster luminescence and, vice versa, the light absorption by the clusters can excite the luminescence of adsorbed dye. We also have shown that when a TAI layer is formed between clusters and adsorbed dye, the dye luminescence disappears if light is absorbed by the cluster. These results prove that in the presence of the TAI layer the relocalization of charge carriers from the excited levels of clusters to dye is impossible. Thus, at room temperature, the TAI layer can inhibit the process of desensitization of Type I, and therefore, TAI works as supersensitizer. We have also established, using luminescence studies, the important role of the surface anions of the emulsion MC in the mechanism of spectral sensitization.
The photoluminescent emission of AgCl dispersions doped with small concentrations of γAgI and Imn− clusters has been investigated. Heterogeneous γAgI phases exist within the cubic AgCl host lattice to provide enhanced photographic properties. The spatial correlation diagram involving recombination energy at peak photoluminescence wavelength for βAgI (wurtzite) and γ (sphalerite) AgI versus the silver iodide cluster diameter allowed the calculation of the αAgI incorporated cluster diameter within a cubic AgCl dispersion. The competition for the conduction band electron by the bound hole on the iodide sites with the dopant is noted by the quenching of the photoluminescence. The (LUMO) lowest unoccupied level of the iodide species is positioned very close to the bottom of the conduction band. The incorporation of 0.30 M% soluble KI or 4.30 M/V% at 93% of the crystal growth provides a maximum speed position and significant change in the spectral band profile correlating with the structures of γAgI. The Imn− clusters observed at longer wavelengths were assigned to 2 Imn− dimers that relate only indirectly to the photographic process.
Silver clusters adsorbed to different sites on cubic AgBr surfaces have been treated by classical and quantum mechanical methods. The properties computed include structure, bond energies, and ionization energies. The optimized geometry of the adsorbed silver cluster tends to be planar up to and including four atoms in size. Distinct odd–even oscillations in the ionization potential and electron affinity, similar to those known in the gas phase, are found for the adsorbed silver clusters. The site of adsorption exerts a strong effect on the calculated energy levels consistent with Coulombic reasoning based upon formal partial site charges. Structural relaxation of the clusters plays an important role in their electron accepting properties. The Ag2 and Ag4 clusters have a large ionization potential, which correlates with high stability. The Ag3 cluster is less stable, due to midgap levels capable of accepting electrons or holes. Overall, the calculations are consistent with the nucleation and growth model of latent image formation.
Many interdependent variables are involved in the processes of manufacture, exposure, and processing of silver halide photosensitive materials. Understanding of the conditions and mechanisms for emulsion grain growth, chemical sensitization, spectral sensitization, latent image formation, and development is facilitated by discussion based upon a number of physico-chemical concepts which have evolved from the earliest days of photographic experience. There has been much controversy over the relevance and validity of alternative hypotheses and theories which have been proposed, adopted, and then vigorously defended, some after they have outlived their usefulness. The purpose of this paper is to trace the historical development of basic concepts of photographic sensitivity with emphasis on principles of physics and chemistry and relevance to emulsion technology.
In this work, the chemical states of sulfur in two typical photographic gelatins, both of which were undoped and doped with ferric ions, respectively, were studied by x-ray photoelectron spectrometry. Sulfur contained in gelatin I may be in three chemical states: the O = S∣∣ = O group of methionine sulfone, the O = S∣∣ group of methionine sulfoxide, and the –S– group of methionine. The proportions of these three chemical states reflect the relative amount of methionine sulfone, methionine sulfoxide, and methionine in elatin to a certain extent. Under the experimental conditions of this study the addition of ferric ions could induce a part of the methionine sulfone to be reduced to methionine sulfoxide. At the same time some of the –CH2– groups of gelatin were oxidized to – CH∣ – OH groups.
Behavior of the delayed formation of latent image specks in a vacuum on sulfur+gold-sensitized emulsions with cubic and octahedral grains was studied. The fraction of developed grains P increased by storing in a vacuum after exposure. In the cubic grain emulsion P exhibited an exponential one-step increase. On the octahedral grain emulsion P indicated a two-step increase. The first step was exponential and the second step was S-shaped. The delayed formation diminished with the increase of sensitization level and disappeared completely at the highest level. Latent subimage specks in the sulfur+gold-sensitized emulsion were developable by absorbing one photon, but not by adding one silver atom in the delayed formation process. The mechanism of the two-step increase in developability is also discussed.
Dependence of fog on chemical sensitization and iodide amount in cubic AgBr emulsions has been studied. Similar fog curve shapes were obtained from the iodide-covered AgBr emulsions by varying the iodide amount and the sulfur-sensitized AgBr emulsions by varying the sulfur amount. Subsequent gold sensitization of the two kinds of emulsions produces two fog peaks in the curves with varying the iodide or sulfur amount. The surface properties of the emulsion grains have been discussed with reference to fog formation. The origin of fog is attributed to the reactivity difference between the chemical site and the bulk crystal surface. The reactivity sequences of different chemical sites were derived: Ag2S > AgI > AgBr > AgCl and Aun > Agn, etc. Direct electron transfer development (DETD) and indirect electron transfer development (IETD) were supposed to be the two processes in development. Based on the proposed mechanisms, the main experimental results were successfully explained with a distribution model of chemical impurities at the cubic AgBr grain surface.
It is crucial to take into account the shape and structure of the AgX grains when considering light scattering and absorption in emulsion layers. Most models of light propagation treat photons as corpuscles and grains as spheroids. However, for modern T-grain emulsions this is not a satisfactory approach. Light wave propagation in the vicinity of a grain of arbitrary shape can be calculated from the Maxwell equations of classical electrodynamics, but up until now the computing power required was too great. A new application of multiple-grid algorithms has now been developed that drastically cuts the required computation time from 6 months to less than 10 h for 1 million grid points when solving the equations for a volume element of (2 μm)3. This wave model replaces the MIE model when calculating the cross section of extinction, absorption probability, and angular distribution of light scattering for silver halide grains. Light distribution and internal exposure can be simulated by introducing the layer structure and grain distribution of the real film into the model. Area exposures of up to 0.1 mm2 are possible with an edge exposure to simulate MTF measurements. A method has been developed to remove granularity noise from simulated and measured MTF test scans.
Phenomena essential for practical photography were investigated by means of electron microscopy with the emphasis on the transformation of the size of colloidal silver particles and change of their spectral properties. Ways to change this size during photographic processing of the layers were studied and their mechanisms were worked out in detail in order to optimize them. It was established that in the process of spectrozonal intensification massive image silver with particle size of 0.1 to 0.2 μm was transformed into colloidal particles with the size of 3 to 6 nm. This process was accompanied by increase of spectral absorption intensity with its peak at 450 nm. Roles of formation of soluble silver complexes, influence of halogen ion, and multicyclicity of the process were investigated. Study of oxidizing dispersion properties of silver in a chromium halide bleacher with successive intensification of the silver image showed that effective dispersion of silver halide microcrystals could be achieved for the case of oxidation in chromium(6+) chloride or chromium(6+) bromide bleachs with high hromium(6+): chloride (bromide) molar ratio, as well as in the case of strong repeated reduction in the stannous chloride or borohydride salt solutions. It was noted that the chromium compounds played only a small part in the processes connected with silver particle transformation, and the intensification ratio depended on factors influencing morphology of silver halides (formed in the process of bleaching of initial colloidal silver particles) as well as the morphology of secondary silver obtained after redeposition of AgCl.