Gurney-Mott

Quantitative Formulations. Computer simulations (213) have been used to put the Gurney-Mott mechanism on a more quantitative basis. Hamilton s recent formulation (214) uses "a more analytical approach. ., that gives a maximum insight into the concepts involved." The method is based on the principle that when there is a branch in a sequence of events allowing two or more possible pathways, a particular event j will be selected with probability p given by... 

In the Gurney-Mott mechanism, the trapped electron exerts a coulombic attraction for the interstitial silver ion. This attraction would be limited to a short distance by the high dielectric constant of the silver bromide. Slifkin (1) estimated that the electrostatic potential of a unit point charge in silver bromide falls to within the thermal noise level at a distance of "some 15 interatomic spacings." The maximum charge on the sulfide nucleus would be 1 e. The charge on a positive kink or jog site after capture of an electron would not exceed e/2. An AgJ would have to diffuse to within the attraction range before coulombic forces could become a factor. 

The Gurney-Mott mechanism has been successful in bringing order to a variety of latent image phenomena and in predicting experimental results. However, uncertainties remain about important details of the mechanism, and in recent years several rival mechanisms have been proposed. The first serious challenge to the Gurney-Mott mechanism was made by J. W. Mitchell. 

Mitchell s views on the mechanism of latent image formation are based on theoretical calculations and extensive experimental work on silver bromide sheet crystals (55,56). He accepts the participation of separate electronic and ionic processes in the formation of latent image centers, which he termed the Gurney-Mott principle, but changes the order of the two processes. 

The electron affinity (EA) values, relative to the conduction band, indicate that nucleus growth by the Gurney-Mott... 

Burge does not exclude the possibility that electrons trapped at centers such as those supplied by sulfur sensitization may form silver atoms by the Gurney-Mott mechanism, but he regards these atoms as mobile with a higher probability of aggregation in the region where they are formed than in other regions. 

The Gurney-Mott theory [97] of chemical development is based on a short-circuited local cell where the latent image acts as an anode for the developer halfcell and as a cathode for the silver ion half-cell reactions. The developer reacts at the solution/latent image interface (Eq. (71)) and silver ion reacts at the latent im-age/silver halide interface (Eq. (72)) ... 

The understanding of the basic mechanism of photolysis of silver halide is incomplete yet vital for the planning and interpretation of experiments. This fact is illustrated by the ramifications inherent in the contemporary discussions of the latent image in silver halide. There are the conventional Gurney-Mott mechanism (1) and the thermodynamic model ((5,9). We have described the Gurney-Mott model. The thermodynamic view envisages nucleation of a supersaturated concentration of silver atoms in silver halide as induced by light. Obviously the effect of external variables is quite different in the two mechanisms. 

Photo-development Mechanisms In photo-development light photons are utilized to liberate electrons. The mechanism is probably best described by the Gurney-Mott photochemical theory When a liberated photoelectron combines with a silver ion metallic silver is formed. It is generally accepted that a single silver atom will rapidly undergo oxidation, reverting to a silver ion, unless additional silver atoms are formed nearby. Once a critical number of silver atoms are formed in a local region, they become an autocatalytic center for the reduction of additional silver ions. The rate of silver ion reduction after the photocatalytic formation of stable metallic silver depends on the electron availability or the local oxidation-reduction potential (41.). 

The principle is that the near-UV radiation generates electron-hole pairs in the halide particles. The electrons are trapped by the interstitial Ag ions producing Ag colloids on the surface of the halide particle. The activation energy for this process is only 0.06 eV so the Ag ion moves easily in the halide in addition, a small amount of Cu increases the effect by at least an order of magnitude. This is the same process used in photography and is explained by the same Gurney-Mott theory. The metal particles formed in the process are 1-5 nm in size, but because the halide must be >5 nm to show the effect the second annealing T is critical. 

---