J. F. Hamilton

Twinning and Growth of Silver Bromide Microcrystals

Almost without exception, the shapes of the microcrystals of a negative‐type silver bromide photographic emulsion can be explained on the basis of a simple mechanism of twin‐accelerated layered growth. Multiple twinning, both on parallel and nonparallel [111] crystallographic planes, is required for a full explanation, and evidence that this does occur is found. The location of particles of silver, formed by exposure to light, substantiates this hypothesis, and demonstrates the fact that intersections of the twin planes with the grain surface are preferential regions for the location of photolytic silver.

Electrical Measurements on Photographic Emulsion Grains. I. Dark Conductivity

By exposing with light flashes delayed by short time intervals after the application of electric field pulses, it is possible to measure ionic conduction on large silver bromide grains from a photographic emulsion. Values of conductivity were obtained by an analysis of the transient decay of internal field pulses resulting from the application of known pulses of the external field. The internal field was measured by means of its effect in causing displacement of latent‐image centers formed by the exposure flashes. The room‐temperature ionic conductance of these grains was found to be the equivalent of about 9×10−6Ω−1 cm−1 with an activation energy of about 0.42 ev. This conductance is several powers of ten higher than that reported for large silver bromide crystals of high purity and is thought to be primarily the result of surface effects. The implications of this result in terms of modern theories of photographic latent‐image formation are discussed.

Mechanism of Electron Trapping in Silver Bromide Photographic Grains

The temperature dependence of the electron lifetime and of the time for decay of an internal applied electric field in the silver bromide microcrystals of a simple photographic emulsion has been measured by the technique employing flash exposure of the emulsion in pulsed electric fields. Activation energies were found to be 0.236±0.012 eV for the electron lifetime and 0.245±0.008 for the field‐decay time. The agreement of the temperature dependence of the two processes, together with the observation that the room‐temperature values of these two times are, without exception, nearly equal in measurements made on a variety of other emulsions, supports the hypothesis that the mechanism of permanent electron‐trapping involves the motion of the silver ion to a temporarily trapped electron to form a stable center. These results are not consistent with the alternative suggestion, namely, that the electron is captured at a preexisting deep trap consisting of a silver ion associated with a crystal imperfection.

Motion of Electrons and Holes in Photographic Emulsion Grains

The use of simultaneous pulses of light and voltage to study the motion of electrons and holes in photographic grains and the role played by these charge carriers in the formation of the latent image is discussed. The electric field displacement of conduction electrons forming print‐out as well as latent‐image silver is shown by electron micrographs of the grains. Also, results on the motion of positive holes are presented. It is possible to show at the print‐out level that this carrier is mobile; although it may be that the hole is displaced at the latent‐image level, this behavior has not been demonstrated. The light and voltage pulses used for the exposures are analyzed and related to electron displacement. Indications are that when electrons are forced to the grain surface by an electric field, they are trapped in a time less than 1 μsec.

Electrical Measurements on Photographic Emulsion Grains. II. Photoelectronic Carriers

Pulses of electric field applied to silver bromide grains of a photographic emulsion at short time intervals after the application of short light flashes cause displacement of photoelectrons and holes if their lifetimes are greater than the delay interval. The asymmetry in the location of the photoproducts—microscopically visible silver and bromine or developable latent‐image specks—is an indication of the fraction of carriers free at the time the electric field is applied. As the delay interval is varied, the decay of the number of free carriers may be followed. In the emulsion system studied, the number of photoelectrons was found to decay approximately according to a 1/(1+αt) law, falling to half the initial value in 0.25 μsec. The lack of a dependence on intensity or temperature is taken to indicate temporary trapping in a distribution of shallow traps, perhaps at the grain surface. The number of free holes decays by an exponential law, with a mean lifetime of about 15 μsec. Indications are that hole ...

Photoconductivity decay kinetics in silver bromide photographic films

There is a component of the complex decay of the photoconductivity signal for silver bromide photographic materials that is intermediate in time constant and in temperature dependence between the initial electron trapping and the subsequent ionic processes. Previous attempts to explain this transient have not been totally satisfactory. We propose that it is a thermally activated lattice relaxation of the otherwise shallow coulombic electron states.

Electrical Measurements on Photographic Grains Containing Cadmium

Ionic conductance and electron lifetime measurements have been made on emulsion grains containing cadmium. The reduction in the concentration of interstitial silver ions because of the divalent cationic impurity, decreases the ionic conductivity, correspondingly increases the electron lifetimes in the grains, and introduces a high‐intensity, reciprocity‐law failure in the emulsion. These results confirm the conclusion that silver ions are the dominant carriers of ionic current and that these same silver ions play a major role in the initial trapping of photoelectrons during latent‐image formation. Moreover, they strongly suggest that the silver ions responsible for ionic conductance in grains with no impurity additions are interstitials generated at kink sites on the grain surface and moving in a space‐change layer within the grains.

Influence of Crystal Face on Surface‐Induced Electrical Properties of Silver Bromide Photographic Grains

Measurements of ionic conductance and electron lifetimes have been made on the grains of two AgBr photographic emulsions, one composed of regular octahedral grains bounded by (111) planes and the other composed of regular cubical grains having (100) faces. The field decays more slowly in the cubical grains, indicating that crystal habit affects ionic conductivity. Electron lifetimes were observed to be approximately equal to the field decay in the octahedral grains and shorter than the field decay in the cubical grains, in contrast to the tabular grains measured previously, in which the electron lifetime was longer than the field‐decay time. An explanation for this is proposed, based on the assumption that the interstitial silver ions are inhomogeneously distributed in the volume of the grains.

Etching Studies on Photographic Grains

The application of preferential etching techniques to the study of crystalline imperfection in photographic emulsion grains is described. Both chemical etching and halogen evolution by print‐out exposure in the absence of gelatin form well‐defined pits in most cases. The concentration of chemical etch pits is dependent on the solvent used but is not sensitive to concentration or etch time over the range investigated. A general increase in etching on certain faces is found after intentionally straining the grains, but the effect is not sufficiently strong to establish a one‐to‐one relationship between etch pits and dislocations. In both silver bromide and silver bromoiodide grains, no evidence is found for the existence of any type of polycrystalline substructure. Experiments on pitting by print‐out exposure indicate that iodide ions provide internal hole traps or recombination centers.