F center aggregation kinetics in low-energy electron irradiated LiF

Results of experimental and theoretical studies describing aggregation of F centers after irradiation and subsequent heating of LiF are presented and analysed. Samples were irradiated in UHV for 30 min with 2.5 keV electrons at a temperature of 300 K and then heated at a rate of 1.5 K min(-1). The evolution of F centers from single defects into metal colloids was monitored by optical extinction spectroscopy. The concentration of F centers is found to decrease monotonously for temperatures above 360 K and to fall below the limit of detectability at about 450 K. Experimental data are compared to results of a microscopic theory of F and H center interaction and aggregation. This theory predicts the activation energy for single F center diffusion to be around 1.5 eV and demonstrates the key role of defect mutual attraction for the aggregation kinetics. (C) 1998 Elsevier Science Ltd. All rights reserved.