Mechanism, 183 Silver halide

Many patents and studies are still published in the field of thiazolo dyes because the photographic industrx is always looking for new sensitizing dyes with improved efficiency and eager to know more about the mechanisms of their action on silver halide. 

The use of sensitizing dyes in photography has been the subject of many studies and constitutes. still now. one of the most studied areas in specialized periodic publications (125, 126) or in textbooks (88. 127). It can be ascertained that one hundred years after Vogel s discovery of spectral sensitization, the basic mechanisms of action of dyes on their silver halide support still remain not fully understood. However, the theoretical reasons explaining why among many other dye families practically only cyanine methine dyes appear to be spectral sensitizers (128) are better known. 

Direct observations of the decompositions of a wide range of inorganic compounds [231—246], which are unstable in the electron beam, particularly azides and silver halides, have provided information concerning the mechanisms of radiolysis these are often closely related to the processes which operate during thermal decomposition. Sample temperatures estimated [234] to occur at low beam intensity are up to 470 K while, at higher intensity, 670 K may be attained. 

Shalem S., German A., Barkay N., Moser F., Katzir A., Mechanical and optical properties of silver-halide infrared transmitting fibres, Fiber and integrated optics 1997 16 27-54. 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

Despite the fact that not all details of the photographic process are completely understood, the overall mechanism for the production of the latent image is well known. Silver chloride, AgBr, crystallizes with the sodium chloride structure. While Schottky defects are the major structural point defect type present in most crystals with this structure, it is found that the silver halides, including AgBr, favor Frenkel defects (Fig. 2.5). 

Among other applications of electrolyte solution theory to defect problems should be mentioned the application of the Debye-Hiickel activity coefficients by Harvey32 to impurity ionization problems in elemental semiconductors. Recent reviews by Anderson7 and by Lawson45 emphasizing the importance of Debye-Hiickel effects in oxide semiconductors and in doped silver halides, respectively, and the book by Kroger41 contain accounts of other applications to defect problems. However, additional quantum-mechanical problems arise in the treatment of semiconductor systems and we shall not mention them further, although the studies described below are relevant to them in certain aspects. 

Lattice defects and latent image formation in silver halides. Fundamental mechanisms of photographic sensitivity, p. 242. London Butterworth s Sci. Publ. 1951. 

Optical sensitizing of silver halides by dyes. II. The mechanism of... 

Silver halides have the character of solid electrolytes, where the silver ion acts as the charge carrier (see [125, 204, 266] for AgCl) which moves according to the Frenkel mechanism in the crystal. This type of transport is depicted schematically in fig. 6.1. As the halide ions are located in fixed sites, no diffusion potential is formed within the membrane and (3.4.9) to (3.4.13) are valid for the membrane potential As mentioned in chapter 3, they can be used for determining either halide ions or silver. 

Sheppard s mechanism was formulated primarily in reference to direct development (Sheppard, 3 Sheppard and Meyer, 3a) and he assumed that the complex was formed in the act of or as a result of adsorption of the developing agent by the silver halide. Since he did not specifically suggest application of the basic mechanism to physical development, consideration of his mechanism will be deferred until direct development is treated in detail. 

Several writers have expressed the view that the silver nucleus acts as an electrode which is charged by the developing agent. A cardinal postulate in such mechanisms is that the electrons can be transferred from the developing agent to the silver nucleus at any point where its surface is in contact with the developer solution. Silver ions are reduced primarily at the silver/silver halide interface. 

The suggested mechanisms differ in detail (Mott, 66, 67 Berg, 68 Anastasevich, 69 Frank-Kamenetskii, 70 Bagdasar yan, 17, 71) but all involve the idea that electrons can be transferred to silver much more readily than to a silver halide crystal. Each mechanism can be criticized on some detail (cf. Sheppard, 15 James, 72). As a general criticism, however, none of the mechanisms has explained the fact that the rate of development under simplified conditions varies with the square root of the hydroquinone and catechol concentrations, whereas the rate of reduction of silver ions from solution by the same agents varies as the first or somewhat higher power of the concentration. 

The mechanisms of the other methods of Intensification are more in doubt (cf. review by Sheppard el ah, 76). These methods include bathing the photographic material in a solution of silver salt, in a solution of hydrogen peroxide or sodium perborate (Vanselow et ah, 77), in a solution of aurous thiocyanate (James et ah, 31) or by fuming the material in the vapor of certain organic acids (Mueller and Bates, 78) or of ammonia. Such treatment may result in an increase in the effective size of the sub-nuclei, or simply in bringing about more favorable conditions for development at the silver/silver halide interface. 

Under the usual conditions of commercial practice, the development reaction does not occur entirely at the silver/silver halide interface. Some reduction of silver ions from solution takes place. Such reduction presumably can occur at any point on the silver/solution interface, and the mechanism should be the same as that for post-fixation physical development. The relative extent of the physical development in comparison with that at the silver/silver halide interface will depend upon the silver halide solvent action of the developing solution and upon the rate of the direct development. 

The reactions which will be discussed here are basic in the application of dyes as sensitizer for photographic materials like silver halides, zinc oxide and others. Model experiments can be performed at electrodes of such materials which help to understand the mechanism of spectral sensitization in photography. 

Dye sensitization plays an important role in photography. The sensitization mechanism for ZnO-materials as used in electro-photography is obviously in complete correspondence with these electrochemical experiments as shown for single crystals under high vacuum conditions by Heiland 56> and for imbedded ZnO-particles by Hauffe 57). Even for silver halides where electron injection as sensitization mechanism has been questioned by the energy transfer mechanism 58> electrochemical experiments have shown that the electron injection mechanism is at least energetically possible in contact with electrolytes 59>. Silver halides behave as mixed conductors with predominance of ionic conductivity at room temperature. These results will therefore not be discussed here in any detail since such electrodes are quite inconvenient for the study of excited dye molecules. 

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

It has been necessary to understand the relationship between molecular fine structure of cyanine dyes and important properties such as colour, dye aggregation, adsorption on silver halide and electrochemical potentials in order to design and prepare sensitizers with optimum performance. For general discussion of these topics and the mechanism of spectral sensitization, the reader is referred to recent surveys on the subject (B-77MI11401, 77HC(30)441). 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver halide and the injection of photoelectrons into the conduction band are the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

Self-diffusion of Ag cations in the silver halides involves Frenkel defects (equal numbers of vacancies and interstitials as seen in Fig. 8.116). In a manner similar to the Schottky defects, their equilibrium population density appears in the diffusivity. Both types of sites in the Frenkel complex—vacancy and interstitial— may contribute to the diffusion. However, for AgBr, experimental data indicate that cation diffusion by the interstitialcy mechanism is dominant [4]. The cation Frenkel pair formation reaction is... 

Although silver halide photography dates from 1839 when Daguerre and Talbot disclosed their inventions, there is no general agreement on the mechanism of latent image formation. 

Physical development. The developer used in this process contains a soluble silver salt, and the developed image is obtained by reduction of this salt. The process is chemical, but the term is retained for historical reasons. Development by the same mechanism can occur when silver ions from silver halide grains pass into solution and are subsequently reduced at latent image centers or developed silver. This process is termed solution-physical development. 

In spectral sensitization, conduction electrons appear in the silver halide crystal when the dye absorbs radiation, just as when the silver halide absorbs radiation. According to the mechanism that has received the strongest support in recent years, an electron is transferred to the silver halide from the excited dye molecule, leaving behind an electron-deficient dye molecule (hole). This is analogous to the result of direct absorption of a photon by the silver halide, except that the hole remains in the dye, at least temporarily, instead of in the silver halide. 

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