Proposed Mechanism for the Conversion ofU-Centers toF-Centers below 90°K

Abstract

Experiments with KCl-KH single crystals suggest the following mechanism for the production of $F$-centers from $U$-centers at temperatures below 90°K. The thermal spike, produced after excitation of a $U$-center, is sufficiently energetic to eject the hydride ion into an interstitial position. Excitation of an interstitial hydride ion ($U_1$-center) may result in the transfer of the excited electron via a tunneling process to a negative-ion vacancy ($\alpha$-center) forming an $F$-center and an interstitial hydrogen atom. Excitation of an $F$-center may reverse the latter process, also via a tunneling process. The tunneling probabilities for these processes vary with the tunneling distance, and experimental results indicate the existence of four distance intervals, about an $\alpha$ center, each interval being characterized by the tunneling processes which can occur.