Latent Image Centres Research Articles

The Sensitivity Distribution of AgHal-microcrystals

AbstractA study of the AgHal-microcrystal sensitivity distribution is based on the Gurney-Mott theory of latent-image formation. In accordance with that theory a model, co-ordinating the threshold of microcrystal development with the number of surface sensitivity centres, has been introduced. The results of calculation, carried out on the assumption that the latent image centres differ in the quantity of the captured silver in accordance with the polynomial distribution, are represented.

Evaporated AgBr Layers: Response to Soft X-Ray Irradiation

AbstractThe response of evaporated layers of pure and doped AgBr to irradiation with soft A1 Kα X-rays (λ = 8.34 A) is studied. The gamma values and maximum density are determined at various accelerating voltages. The layers’ sensitivity is by 3 to 5 orders of magnitude higher than that of commercial resists used in X-ray lithography. The distribution of latent image centres in the bulk of AgBr microcrystals is similar to that in the case of exposure to visible light. A qualitative estimation of the possibilities for reproduction of microstructures by X-rays is given.

Sensitivity Centers and Latent Image Centers in AgBr Films Created and Visualized in Vacuum

AbstractAll stages of the photographic process arc modelled in vacuum. The preparation of single-crystalline AgBr films and their subsequent dry sensitization with CdBr,, flash exposure and development by vapour deposition of silver are carried out in the same vacuum cycle. Visualization in the electron microscope of the location of sensitivity centres and latent image centers is achieved and the number density of both types of centers is compared. It is shown that similarly to vapour deposition of silver, exposure to light stabilizes the sensitivity centers and prevents their subsequent rearrangement. The connection between the results obtained and the mechanism of latent image formation is discussed.

Silver Halide Single Crystal Films as a Model System for Latent Image Research

Recent discussions on the latent image formation point out the need to know the fine structure of the stiver halide surfaces. We have produced epitaxial films of AgBr with well-defined surface structures by vapour deposition on NaCl cleavage faces.The complete structure of surface steps has been revealed by means of the Au decoration technique. Using these films as substrates, experiments on the vapour deposition of various materials (Se, Te, Bi, Zn and Ag) have been carried out in order to find out materials and conditions for a dry development of model latent image centres. Experiments on the dry development of these centres which are produced by the vapour deposition of Au on unsensitized as well as sensitized AgBr films are described.

Influence of the Space Charge Layer on the Topography of the Internal Latent Image

AbstractAn attempt is made to check the existence of a correlation between the concentration of space charge interstitials and the distribution of the internal latent image centres. It is shown that the topography of the internal latent image found by Malinowski and Karadjov in silver bromide emulsions fits very well the distribution of interstitials in the space charge layer. In this way it is demonstrated that in the subsurface region the probability for trapping and neutralization of the photogenerated electrons is proportional to the concentration of space charge interstitials. It is shown also that the fitting procedure reveals a possibility to calculate the free energies of formation of Frenkel defects on the surface as well as the other parameters of the space charge layer. The results obtained impose a renewed discussion about the influence of the space charge layer on the primary photographic process.

Gold‐Containing Surface States on Photographic AgBr Grains: Roles besides Photoelectron‐Trapping

Gold‐sensitized photographic films have been treated in redox buffers between exposure and development. Under certain conditions the latent image centers (surface states on containing photoproduced Ag atoms and allowing amplification by development) are no more noble than those in unsensitized films. It is tentatively concluded that an increase in electron affinity of these centers (gold incorporation and/or electron trap deepening at their formation sites) is no condition for a sensitivity increase. Gold‐containing surface states (sensitivity centers) might eliminate positive holes and thus, e.g., preclude recombination provoked by hole capture at precursors of latent image centers.

Dye sensitized photoluminescence in silver halides

Measurements of the dye sensitized photoluminescence on iodide doped Ag Hal layers have shown that this luminescence does not decrease drastically at dye coverages above about half a monolayer, contrary to the photographic sensitivity at room temperature. Thus the decrease in photographic sensitivity may originate in a reoxidation of the latent image centres by photoholes percolating in the dye layer. The possibility of increasing sensitivity by FOERSTER energy transfer is investigated by luminescence in a system of organized monolayer assemblies.

Sensitized Photolysis on Semiconductors and the Structure of the Latent Image*

The illumination of semiconductor in the intrinsic region of photosensitivity results in a reduction of silver ions adsorbed on the surface of the semiconductor. After this, the latent image can be developed by physical developers. By scanning electron microscopy the nature of the latent image centres is investigated.

Formazan Dye Physical Development-A Novel High Gain Amplification Process

AbstractA dye-forming physical development amplification process based on the catalytic reduction of tetrazolium salts to corresponding formazan dyes is described. A wide range of image hues can be produced by variations in the structural parameters of the T-salt. Using vacuum-deposited metal nuclei as a model for the latent-image centres, some of these amplification solutions have been shown to require only 2-3 Pd atoms to catalyze the dye-formation reaction.

The Mass and Distribution of Photolytic Silver in the Solarization Region

AbstractThe mass and distribution of photolytic silver in the solarization region of a chlorobromide, halide-exchange emulsion has been determined by using radiochemical and autoradiographic techniques in conjunction with latent-image bleaches.For this particular emulsion, in the region where developed density decreases with increasing exposure, the mass of surface silver also decreases whilst internal silver increases. This is a direct demonstration of the rehalogenation theory of solarization.The behaviour of surface latent image towards bleaching solutions, as determined by non-solvent surface development, indicates that although the number of latent-image centres, taken over the emulsion as a whole, decreases with increasing exposure, the average size of the remaining centres continues to increase.

Recombination in Latent Image Formation

AbstractConflicting viewpoints concerning the importance of recombination in latent image formation are examined with respect to existing experimental evidence. Hole-electron pairs and, in certain circumstances, print-out silver specks are produced efficiently. In contrast, most, though not all, latent image centres form inefficiently. Reduction sensitization, which provides a means of detecting holes, increases efficiency of formation of latent image in emulsions chemically sensitized in various ways. This indicates the importance of recombination during exposure of the emulsions with no reduction sensitization. The wide breadth of most sensitivity distributions means inefficient utilization of absorbed photons. Neither variable critical size of latent image centres nor inefficient aggregation of silver or gold atoms is a likely explanation of this inefficiency, at least in certain circumstances. It is concluded that recombination has much influence on latent image formation. Loss of photoelectrons by el...

The Validity of the Gibbs-Thomson Equation for Latent-Image Centres*

AbstractStudies of the bleaching of latent images by redox buffers show that the Gibbs-Thomson equation, relating the excess energy of small silver particles to their size, is valid down to the smallest latent image centres.

Ellipsometric studies on the photolysis of lead chloride crystal surfaces

The photolytic decomposition of large crystals of lead chloride was studied by ellipsometry. This technique proved to be successful for quantitative measurements in the latent image range of the photolysis. Large differences in the photolytic behaviour were observed between exposures in the presence and in the absence of oxygen. In the presence of oxygen, the photolysis is strongly influenced by physisorbed oxygen leading after photochemisorption to the formation of lead oxychlorides as latent image centers. In the absence of oxygen, latent image centers consist of lead nuclei obtained with a high quantum yield as determined from ellipsometric measurements. The photolysis of lead chloride seems to be governed by surface reactions, contrary to that of pure silver halide crystals where only internal latent images are formed, to which ellipsometric measurements are much less sensitive. A mechanism for the lead chloride photolysis is proposed on the basis of our results obtained from ellipsometry, diffuse reflectance spectroscopy, photosolubilisation and physical development of latent images. The good agreement observed between results by these different methods suggests that ellipsometry is a useful technique to study surface photolysis.

The Initiation of Development

Various experimental procedures used to obtain information on the mechanism of initiation of development are reviewed. These include microscopic examination, use of long wave radiation to follow the growth of optical density in unfixed grains, mathematical analysis of rate of development curves, initiation of development in an experimental developer followed by intensification in a standard developer, and examination of the dependence of the rate at various stages of development and conditions of exposure on the composition, temperature, and oxidation-reduction potential of the developer. The use of low temperature exposures to produce latent image centres of near threshold size and to obtain a controlled growth of latent image centres is discussed. Although not conclusive, the available evidence suggests that, if a change in basic mechanism occurs during the course of development in a low solvent type developer, it occurs much earlier than the start of filamentary growth of the developing silver.

The Topography, Mechanisn1 of Topochemical Transformations and the Sources of Chelnical Sensitization in the Ripening of Photographic En1ulsions

AbstractThe sensitization of a photographic silver-halide emulsion is intimately connected with the micro-structure of the solid phase. This paper summarizes and discusses the knowledge gained during recent years, in particular the work of the author and his collaborators. Chemical sensitization is studied as a surface phenomenon. The importance of intrinsic lattice defects and impurity centres for photographic sensitivity is evaluated. The relation between chemical sensitization and the formation of fog is then discussed. A survey of the sources of chemical sensitization in the ripening of emulsions is followed by a comparison of the characteristics of ripening and photolysis and a discussion of the stability of impurity centres and latent-image centres. Finally an attempt is made at summarizing the lows which can be observed in the phenomena of chemical sensitization.

Some Features in the Absorption Spectra of Silver Halides and Thin Metallic Layers

AbstractKirillov and his school in the Soviet Union, using a differential method, observed a fine-structure in the absorption spectra of photographic emulsions, silver-halide crystals, and thin layers of metallic silver. This fine-structure was ascribed to impurity centres, and the method was taken up by a number of other Soviet authors to study the nature and reactions of the sensitivity centres and latent-image centres of photographic materials. Some authors, however, have questioned the findings and suggested that the fine-structure could be an interference effect. The Russian work is reviewed in some detail, and it is shown that the possibility of the results being due to interference has been eliminated, and that the fine-structure is best explained as due to the absorption of light by impurity centres consisting of a few atoms of metallic silver in an aggregate of sub-colloidal dimensions.

Herschel Effect in Single Crystals of Silver Bromide

Observations on single crystals of AgBr exposed to long-wavelength visible or near-infrared radiation in the presence of applied electric fields show that the Herschel bleaching of latent-image centers at room temperature is caused by destruction of silver centers by photoemission of electrons. Silver can be regenerated at other sites by recombination of the liberated electrons with silver ions. Brominated crystals of AgBr exhibit an action spectrum for the production of mobile positive holes, unaccompanied by mobile electrons, extending from the edge of the intrinsic absorption band to about wavelength 630 mμ, attributed to photoliberation of deeply trapped holes.

Investigation of a Latent-Image Model: Internal Image

A simple model for the mechanism of photographic latent-image formation, described in earlier publications, has been modified to include the production of internal latent-image centers. Analysis of the model by simulation with a digital computer shows that most of the characteristic features of real emulsions are found in the response of the model. The function of sulfur-sensitization assumed in the previous treatments, viz., increasing the stability of trapped electrons and/or single silver atoms at surface traps, is found to affect the surface-to-internal sensitivity in the manner usually observed.

Study of the Mechanism of Sulfur Sensitization by a Development-Center Technique*

An investigation has been made of the distribution of development centers among the grains of a simple experimental emulsion and of the influence of sulfur sensitization on this distribution. The distribution of image centers among the grains which have not been deliberately chemically sensitized is not random, but shows instead a strong preference for only one center per grain, regardless of the intensity of the exposing light. For low-intensity exposure, the same emulsion after sulfur sensitization still shows only one development center per grain. At higher intensities, the number of development centers increases. The transition in the distribution of centers in the sensitized emulsion coincides with the onset of high-intensity, reciprocity-law failure. At very high intensity, the observed distribution agrees with the Poisson formula for a random distribution. Latensification with a gold solution, which eliminates high-intensity, reciprocity-law failure, does not affect the form of the speck distribution. This difference resulting from sulfur sensitization is most easily explained in terms of an increase in stability at the sites where latent-image centers form, e.g., an increase in electron trap depth.Although the emulsion does not solarize, an examination of the development centers reveals that prolonged, low-intensity exposure of the sulfur-sensitized emulsion causes regression of all but one center per grain.

Displacement of Photoelectrons and Positive Holes in Sheet Crystals of Silver Bromide as Shown by Development Techniques

The mobility of electrons in unannealed, large, thin crystals of silver bromide made from the melt was measured from the displacement of latent-image centers formed by flash exposures synchronized with pulsed electric fields; an etching-development technique was used to locate the latent image as reduced silver. Similarly, the motion of positive holes in the crystals was determined by the hole-induced destruction of previously introduced latent-image centers or fog centers, which, on development, yielded a bleached image against a background of reduced silver. At 30°C, the electron mobility in these crystals was 66 cm2 V—1 sec—1, and their lifetime about 0.3 μsec; the mobility of the positive holes was 1.7 cm2 V—1 sec—1 and their lifetime in the undisturbed crystal was greater than 10 μsec. Even in the absence of positive holes, the formation of latent image on a surface of silver bromide may be greatly facilitated by the presence of a sensitizer, such as silver sulfide. In such a case, silver sulfide appears to act as an electron trap or in some other way not associated with the capture of positive holes.