Silver halides chemical sensitization

Photography, Silver Halide, Chemical Sensitization, Spectral Sensitization, Latent Image Formation (James). 

CH rCHCH NHCSNH. Colourless crystalline solid with a faint garlic-like odour m.p. 74 C. Manufactured by treating propenyl isothiocyanate with a solution of ammonia in alcohol. It has been given by injection in the treatment of conditions associated with the formation of excessive fibrous tissue. Toxic side reactions may occur. Propenyl thiourea is a chemical sensitizer for photographic silver halide emulsions. 

In order to circumvent this sensitivity limitation, the San Jose researchers sought to design resist materials that incorporate chemical amplification of the sort that characterizes the silver halide photographic emulsion system. In these systems a single photo event initiates a cascade of subsequent chemical reactions that ultimately result in the intended function. 

There are two basic methods by which a developed silver image can be obtained. The one commonly employed in practice is termed chemical or, better, direct development. The exposed sensitive layer is placed directly in a suitable reducing solution, and the silver image is derived from the reduction of the silver halide grains. This is the type already considered to some extent in the preceding section. 

Photography based on silver halides as the light-sensitive material depends on physical and chemical properties that, although occurring individually in other compounds, form a combination in the silver salts that makes them unique in their... 

In summary, there is evidence that the multitalented sulfur sensitization product can trap electrons, can trap holes to reduce recombination, can stabilize photolytic silver atoms, and can accelerate reduction sensitization. Conceivably, each of these properties could be of importance for latent image formation under at least some conditions. The silver sulfide centers are not of uniform size, they probably are not uniformly related energetically to the silver halide matrix, and they may differ in chemical consititution. 

The formation of the latent image should be considered as a photo-induced transformation of the dielectric molecular silver phase into the metallic one." (230). During chemical sensitization the silver halide grains pass into a nonequilibrium... 

The pronounced dependence of the crossover potential on chemical sensitization, and particularly on the gaseous environment, shows that the crossover does not represent a division between dyes that can cause the appearance of conduction electrons in the silver halide and dyes that cannot. Instead, it represents an energy level determined by a kinetic balance between the formation and loss of photoelectrons and/or silver in the silver halide, a kinetic balance between sensitization and desensitization (259,265). One cancels the other and the net formation of latent image is zero. The actions of oxy-gen/moisture and of mobile holes are important sources of desensitization. 

The discovery of the sensitizing properties of dyes that formerly had been considered as strictly desensitizing dyes, and the high efficiency with which some of these dyes can spectrally sensitize under proper conditions, requires reconsideration or extension of earlier theories of spectral sensitization. Unless previous estimates of the position of the singlet excitation level of these dyes relative to the conduction band of the silver halide are in serious error, these dyes should not be able to photoinject electrons directly into the conduction band from the level. The clue to their sensitizing action, I think, is contained in an explanation offered by Sturmer, Gaugh, and Bruschi for the "abnormal" spectral sensitization obtained with one of the desensitizing dyes used in their study of the effect of chemical sensitization on the efficiency of spectral sensitization (273). They wrote that... 

Chemical Sensitization, Spectral Sensitization, and Latent Image Formation in Silver Halide Photography By T. H. James, 1600 Ala Moana Boulevard,... 

Coordination compounds and coordination chemistry are involved in a number of these steps. Chemical sensitizing agents, antifoggants and stabilizers are discussed separately. This discussion concerns silver halide emulsion precipitation and physical ripening methods. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

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