Although development fog arises from the formation of fog centers (i.e. Ag clusters) owing to the electron injection from a developer to AgX grains, the introduction of strong electron traps into the interior of AgBr grains did not influence the rate of development fog formation. It was therefore considered that electrons did not injected into the conduction band of AgBr grains during the formation of development fog. By taking into account the fact that electrons captured by chemical sensitization centers are easily released to the conduction band, it was also considered that electrons were not injected into chemical sensitization centers from a developer. Chemical sensitization however increased the rate of development fog formation with increase in its activation energy and frequency factor at the same time.
It was suggested from these results that the first step of development fog formation was direct Ag
2 formation by the injection of electrons from a developer to a Ag
+ pair on the grain surface. The higher the electron-accepting levels of Ag
+pairs were, the larger would be the density of states and/or the cross section of the levels. Further, the higher the electronaccepting levels of Ag
+ pairs were, less stable would be the Ag
2 thus formed. It was therefore considered that chemical sensitization accelerated the development fog formation by stabilizing such unstable Ag
2 clusters and enhancing their coagulation to form fog centers.
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