Third Report of the Committee on Photochemistry Reprint and Circular Series of the National Research Council (1938) / Chapter Skim
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The Action of Optical Sensitizers of the Photographic Plate
The Action of Optical Sensitizers of the Photographic Plate
Pages 97-116
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From page 97... ...
I INTRODUCTION The spectral range of photographic action Since photochemical action in any system is necessarily connected with its light absorption, the light absorption in silver halide emulsions is of fundamental importance for determining the spectral range of photographic action.
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From page 98... ...
The absorption of gelatin is negligible in the x-ray region, but, in the visible and ultraviolet regions, the presence of gelatin in the photographic emulsion distorts the close correlation between absorption and photographic action which can be observed in pure silver halides. Eggert and Noddack (2)
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From page 99... ...
The conflicting results of earlier experiments can be explained partly by the fact that the sensitizers used were often not chemically pure and in some cases were even combined with various admixtures, but, even with pure materials, the complex nature of the phenomena makes it difficult to arrive at simple laws. Desensitizing was frequently regarded as optical sensitizing with a negative sign, and the difference in constitution was connected with this antagonistic behavior.
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From page 100... ...
With concentrations increasing beyond this optimal concentration, desensitizing sets in, and Heisenberg proved that this desensitizing effect is real, since it is much too large to be accounted for merely by the increased light absorption of the dye. This desensitizing effect of sensitizing dyes when used in larger concentrations seems to be of a general nature.
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From page 101... ...
Hilsch and Pohl (31) found in crystals of alkali halides that these F color centers are characterized by sharp absorption bands; in silver halide crystals, the absorption bands are more diffuse and broader, overlapping the proper absorption of the silver halides.
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From page 102... ...
This assumption seems far-fetched, although one fact points in this direction, the strong fluorescence which is found in sensitizing dyes. It is to be regarded, however, as one of the possible solutions for ~ puzzling experimental result 2 It must be borne in mind, however, that it would hardly be possible to discover a slight deviation of the straight line in the region of the latent image.
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From page 103... ...
There is, however, additional evidence for a high, although not equally high, efficiency of the sensitizing dyes, namely, the work of Bokinik and Iljina (44~. These authors investigated the sensitizing action of erythrosin in silver bromide sols without gelatin and with an excess of bromide ions, with the result that the ratio of silver atoms to adsorbed dye molecules was found to lie between 4 and 15, rising with increasing alkalinity.
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From page 104... ...
As early as 1904 Kieser (48) studied a great number of sensitizing dyes in their relations to the surface of silver halide grains, and found that saturation was reached at very low concentrations of the dye, but it was only recently that quantitative measurements were started of the adsorption of sensitizing dyes by silver halide emulsions.
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From page 105... ...
, the authors assumed the surface area to consist mainly of bromide ions, since excess bromide was present. By calculating the number of bromide ions in the surface, they were able to establish the ratio of bromide ions to dye molecules as 2.3.
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From page 106... ...
This value was found to be roughly in agreement with the results of other investigations, on dye adsorption at silver bromide surfaces, which had been carried on in the meantime. Having found eight bromide ions for one molecule of adsorbed methylene blue, Wulff and Seidel (53)
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From page 107... ...
For dye II, an 8-alkylthiocarbocyanine, the surface is assumed TABLE 1 Optimal surface concentration of sensitizing dyes (From Leermakers, Carrot] , and Staud (27)
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From page 108... ...
This transitory state of the crystal was found to be relatively stable in solutions of some salts and in the case of adsorption of the dye on various substances (599. In investigating silver bromide emulsions sensitized by cyanine dyes, Leermakers, Carroll, and Stand found similar sharp characteristic absorption bands for many adsorbed dyes; It was this fact which made them doubtful of the unimolecular layer, since Scheibe, Kandler, and Ecker attributed these spectral characteristics to a polymerized state, and Scheibe, Mareis, and Ecker confirmed this statement.
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From page 109... ...
But some of the results of Leermakers, Carroll, and Stand are not in agreement with the assumption that the sensitizing action is exclusively a property of this "aggregated" state. They found parallelism between sensitivity and absorption in all spectral regions, not merely for the bands belonging to the nematic state, but also for bands in other spectral regions which were obtained in emulsions dyed from alcoholic solutions.
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From page 110... ...
Chibisoff (66) , on the other hand, calls attention to some secondary effects of optical sensitizers whereby they act simultaneously as chemical sensitizers.
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From page 111... ...
strongly objected to accepting this as a general statement and pointed out that, wherever this secondary effect was found, it could be easily explained by preferential absorption of the desensitizer. It would indeed be difficult to explain by this theory the desensitizing effect of optical sensitizers at high concentrations.
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From page 112... ...
The strong influence of halide ions on the desensitizing action had previously been observed by Carroll and Kretchman (86) With safranine either the sensitizing or the desensitizing effect was found, depending upon the concentration of bromide ions.
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From page 113... ...
since van Kreveld and Jurriens (35) have developed a method of determining the latent image by measuring the absorption in the red region of the spectrum.
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From page 114... ...
(68) LUPPO-CRAMER: Neue Beitrage zur Kenntnis der Desensibilizierung, Eders Handbuch III, 3 (1932)
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