Chronicle

Scientific Discussion on the Nature of Photosensitivity and the Mechanism of Formation of the Latent Photographic Image

The Commission on Scientific Photography and Cinematography, organized last year under the Academy of Sciences of the USSR and headed by Corresponding Members of the Academy of Sciences of the USSR K. V. Chibisov and T. P. Kravets, adopted a resolution on the periodic organization of scientific discussions on individual questions of photography and cinematography.

The first discussion took place in Moscow at the beginning of this year and was devoted to the structural properties of photosensitive layers. The second discussion, about which the present note will be concerned, took place in Leningrad on June 23–25, 1949, and was devoted to the nature of photosensitivity and the mechanism of formation of the latent photographic image.

At the first session, a survey report by P. V. Meiklyar was presented on the topic “Analysis of Modern Ideas on the Formation of the Latent Photographic Image,” as well as a report by K. V. Chibisov, A. A. Titov, and A. A. Mikhailova, “On the Nature of Photosensitivity Centers and the Role of Gelatin in Their Formation.”

In the first of these reports, after reviewing modern ideas on the nature of latent-image centers and on the mechanism of their formation (this review was set forth by the author in an article in the May issue of UFN for this year), P. V. Meiklyar proposed a new conception of the second stage of formation of latent-image centers. The widespread theory of latent-image formation (Gurney and Mott) holds that after the first electronic stage there follows a second stage, consisting in the neutralization of electrons, fixed at photosensitivity centers, by interstitial silver ions. The speaker, seeing the impossibility of explaining a number of facts on the basis of this conception, proposed a different mechanism for this second stage, consisting in the neutralization of electrons, fixed at photosensitivity centers, not by silver ions, but by vacancies at sites of ions in the ionic lattice of silver halide, formed in the process of illumination. This theory, in agreement with the ingenious crystallographic ideas of P. D. Dankov, makes it possible to explain the formation of color centers in the interior of the crystal without the necessity of strong deformation of the lattice, which is inevitably, according to

theory of Gurney and Mott. This mechanism is also applicable to the process of coagulation of color centers in alkali-halide crystals. The latter circumstance is quite significant, since the proposed theory gives a unified interpretation of the formation of metal colloidal particles in alkali-halide crystals—an analogy to which was first pointed out by T. P. Kravets, who, about 20 years ago, formulated the foundations of the modern conception of the nature of centers of the latent photographic image. Meanwhile, the Gurney and Mott theory, “specialized” for silver-halide salts, did not provide the possibility of creating a unified photochemical scheme for the coloring of ionic crystals.

In K. V. Chibisov’s report, a summary was given of a large cycle of work carried out by him jointly with A. A. Titov and A. A. Mikhailova to elucidate the chemical nature of the centers of sensitivity in crystals of silver halide of photographic emulsions that are responsible for the high photosensitivity of the latter. These works were recently published in the eighth issue of the Transactions of the Scientific Research Cinema and Photo Institute. By the most precise microanalytical methods, applied to processes of interaction of silver ions with gelatin, on the one hand in a homogeneous phase (in solutions of $\mathrm{AgNO}_3$), and on the other hand in heterogeneous systems (on the solid phase of silver halide photographic emulsions), K. V. Chibisov traced the kinetics of the formation of metallic and sulfurous silver and compared it with the course of the change in photosensitivity and fog density in the process of ripening (holding at elevated temperature) of a photographic emulsion. It turned out that there is no unambiguous correspondence between the quantity of sulfurous silver formed and the magnitude of photosensitivity. At the same time, such a correspondence is in most cases observed between the quantity of metallic silver formed and photosensitivity. These results indicate the decisive role of metallic-silver centers in the formation of photosensitivity and thereby refute Sheppard’s old conception of the determining role of sulfurous silver. Chibisov’s data do not, however, exclude the possibility that sulfurous silver may play some, though auxiliary, role in the formation of photosensitivity.

Both of these very interesting reports provoked a lively and animated discussion among those present.

The second session was devoted to consideration of the physical properties of single crystals of silver halide. At the present time these questions are chiefly being studied by the Odessa school of physicists headed by Prof. E. A. Kirillov.

The first report, delivered by E. A. Kirillov, was entitled “Absorption of color centers and latent-image centers in silver halide.” The results were presented of very precise spectrophotometric measurements of silver halide salts, both photochemically colored and coated with thin layers of metallic silver weakly bound to the crystal surface. Analogous measurements were made on Lippmann emulsions that had been preliminarily exposed.

According to these measurements, for color centers formed both by the action of light and by deposition of metal, there is characteristic a large number (up to 18) of individual narrow absorption bands located in sharply fixed portions of the spectrum, from the ultraviolet to the infrared region. Upon additional illumination of such crystals with red light, a weakening of these bands is observed, which is analogous to the known Herschel phenomenon. As in normal photo-

photography, desensitization by dyes enhances the bleaching effect. His report E. A. Kirillov concluded with a brief presentation of the work of J. D. Brown, who studied the effect of additional weak illumination of the photographic layer on the light sensitivity corresponding to the principal illumination forming the photographic image.

The second communication at this session, on the topic “The internal photoeffect and the absorption of light in thin films of silver halide,” was made by B. A. Barshchevskii and contained a comparison of the results of experiments on determining the spectral course of absorption and of the internal photoeffect in thin and ultrathin films of silver chloride, bromide, and iodide obtained by drawing molten salt into the space between two quartz plates. It turned out that the long-wave absorption boundaries and the photoeffect do not coincide. In the ultraviolet region, a decrease of the photoeffect is observed at extremely high absorption. These results agree with the data of Breido and Gorokhovskii, presented in one of the subsequent reports.

The third report, on the topic “Luminescence of silver halide salts,” was made by the Odessa researcher S. I. Golub. The author studied the luminescence of single crystals of silver chloride, bromide, and bromoiodide at low temperatures and discovered new spectral luminescence bands that had hitherto been unknown. S. I. Golub succeeded in proving for silver chloride that the luminescence has a long-lasting character with a hyperbolic law of decay. The dependence of the intensity of the glow on deformation of the crystal was also investigated.

These reports, as on the first day, gave rise to a lively discussion.

At the third session, reports were heard by I. I. Breido, “The nature of the spectral distribution of the intrinsic light sensitivity of photographic materials,” and P. V. Meiklyar, on the topic “The formation of the latent photographic image under different illumination conditions.”

In her report, I. I. Breido presented the results of an extensive investigation carried out by her together with the author of the present note. The ultraviolet spectral-sensitometry method developed at the GOI made it possible to compare the spectral course of the light sensitivity and absorption of unsensitized photographic layers with different compositions of the solid phase. As in the experiments of B. A. Barshchevskii on the photoeffect, it turned out that, upon achieving a certain very high value of relative absorption, a decrease in wavelength leads to a decrease in the light sensitivity and in the contrast coefficient of the layer. It was clearly shown that this phenomenon is caused by absorption of light in an ever thinner part of the emulsion layer and, consequently, by the enormous discrepancy between the emulsion crystals in terms of their “effective” light sensitivity. This was demonstrated on photographs of transverse microsections of blackenings obtained at different wavelengths. In support of this point of view it was also shown that the curve of spectral light sensitivity of a single-layer photographic-emulsion specimen does not pass through the maximum that occurs in a multilayer specimen, but is analogous in form to the curve of spectral absorption of the silver-halide emulsion layer.

The last report was made by P. V. Meiklyar, who showed that high-intensity and brief (of the order of \(10^{-5}\) sec.) illumination,

corresponding to a comparatively high density of blackening, causes the formation of a large number of undevelopable centers, whose absorption lies in the visible and near-infrared regions of the spectrum. Investigation of deviations from reciprocity has shown that these centers increase the light sensitivity of the photographic layer under subsequent illumination, being not only centers of electron trapping, as is usually assumed, but also sources of photoelectric emission of electrons into the silver-halide lattice.

Yu. Gorokhovsky