Generation of Electron Traps by Plastic Flow in Alkali Halides

Vacancy clusters, generated by moving dislocations, act as effective electron traps and change the photographic properties of the alkali halides considerably. This has been investigated by using additively colored KCl and electrolytically colored NaCl. Electrons were released from the $F$-centers by irradiation of $F$-light. In a plastically deformed crystal these electrons are immediately trapped by vacancy clusters and have not much chance to return to a negative ion vacancy. Therefore, the $F$-band bleaches much faster than in the case of undeformed crystals. Measurements of the bleaching rates permit an estimation of the number of traps generated by plastic flow. About ${10}^{17}$ traps per ${\mathrm{cm}}^3$ are formed by a plastic strain of 10 percent. The nature of the traps depends upon the time, which has passed after cold work. In the case of KCl, irradiation of $F$-light during plastic deformation gives rise to a pronounced enhancement of the $M$-band. Filling of the traps 48 hours after cold working results in the formation of a very broad band, centered near 800 mμ. The generation of $F^{\prime }$-centers and the thermal conversion of $F^{\prime }$-centers into $F$-centers are also strongly affected by the presence of these traps.