Loss and Restoration of Photoconductivity in Red Mercuric Iodide

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Abstract
Single crystals of red mercuric iodide, which normally display photosensitivity when precipitated, lose their photosensitivity on aging. This loss is accompanied by a change from the mono- to the polycrystalline fibrous state with considerable randomness of orientation of the individual crystallites. The nonsensitive crystals can be resensitized by subjecting them to an electric field for a time of the order of a few minutes. The increase in photosensitivity is accompanied by a decrease in the dark current. These correlated phenomena may not occur till after the lapse of an appreciable time after the field is first applied. The rate of decrease of the dark current with time is greater the lower the temperature. The data also suggest, but do not conclusively demonstrate, that the dark current increases with decreasing temperature. The rate of resensitization and the magnitude of the attained photosensitivity are found to increase with (a) increase in applied field strength and (b) decrease in temperature. When the light is turned on the photoelectric current is found, under certain circumstances, to rise briefly to a maximum and then descend to a permanent value at which the total current is equal to the value of the dark current prevailing when the voltage was first applied.

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