Two Photomechanical Effects in Alkali Halide Crystals

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Abstract
Two photomechanical effects, observed in alkali halide crystals containing color centers, appear as sudden reversible changes in flow stress when a crystal is illuminated. Both effects are related to optical transitions of the F center. In potassium chloride, intensely illuminated at 175°K, hardening appears to be caused by the F → F′ process. Below 80°K, in KCl, even more severe hardening is caused upon illumination, by transport of the F‐center electron to an unspecified trap lying very close to the conduction band; decay of the hardening after extinction of the exciting light requires several seconds and may be caused by dislocation—point defect interactions. The effects have been observed in nine of the alkali halides. Flow stress increases and decreases were both observed, depending upon the particular alkali halide.

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