Speculations on the Properties of the Silver Halide Crystals

The available experiments on the absorption spectra, photoelectric, and photolytic properties of the silver halide crystals are re-analyzed and interpreted. The conclusions drawn may be regarded in the nature of extensions of the Gurney-Mott theory of photolytic action. The principal speculative viewpoints are as follows:

1. The characteristic absorption spectra of the pure salts derive much of their structure from forbidden optical transitions associated with lattice distortion arising from dislocations. It is proposed that in materials prepared from the melt the dislocations are formed by condensation of lattice vacancies associated with about 0.1 percent of Schottky defects.

2. It is also concluded that some of the absorption structure is associated with "forbidden" $d^{10}\to d^{9}s$ transitions in the silver ions.

3. It is proposed that the electron traps which are responsible for the early deposits of photolytic silver are incipient halogen-ion vacancies which have an effective charge $+\frac{e}{2}$. These vacancies occur at jogs in dislocation lines, where such lines pass from one slip plane to a parallel plane, and may be enhanced by plastic flow. Incipient positive-ion vacancies, having an effective charge $-\frac{e}{2}$, also occur and act as nuclei for forming halogen clusters whose absorption bands are hidden behind the fundamental band.

4. At room temperature, the photoelectrons may be trapped by interstitial silver ions before ending at incipient halogen-ion vacancies; however this process represents only a temporary delay since the entire silver atom may migrate to the dislocation.

5. It is suggested that the addition of sulfur to the silver halides plays a role in enhancing darkenability in three ways: (a) By forming precipitates which produce distortion that enhances the number of incipient vacancies; (b) by extending the absorption spectrum toward the red end of the spectrum; and (c) by increasing the equilibrium density of interstitial silver ions.

6. It is proposed that the luminescent radiation observed in pure silver halides by Farnell, Burton, and Hallama arises from transitions within the silver ion. The emitting ions are at dislocations in AgCl and AgBr.

A sequence of experiments is suggested.