The kinetics of CaF₂ metallization induced by low-energy electron irradiation

Results of experimental and theoretical studies on metal colloid growth after irradiation and subsequent heating of CaF₂ are presented. Samples at a temperature of 200 K were irradiated in UHV for 30 min with 2.5 keV electrons. After irradiation samples were heated at a rate of about 1 K/min and colloid formation was investigated by optical extinction spectroscopy. Colloid radii as well as the amount of metal were determined from a Mie scattering analysis. A steep rise in colloid size was observed between 260 and 270 K. The concentration of F centers not aggregated into colloids was below the detection limit for all temperatures. A microscopic theory of F and H center aggregation taking into account defect creation, interaction, diffusion, and annihilation of dissimilar defects is presented. Theory successfully reproduces the main experimental observations of small colloid formation at low temperatures during irradiation and the onset of colloid growth at a temperature where F centers become mobile.