Photoluminescence in lithium fluoride crystals exposed to 5MeV electrons at low temperatures

The paper presents studies of photoluminescence (PL) of anion divacancy <i>F</i>₂ defects generated in optical pure lithium fluoride (LiF) crystals by irradiation with 5 MeV electron to moderate fluencies of 5&#183;10¹⁴ cm^-2 and 10¹⁵ cm^-2 at low temperatures (~165 K), when thermal activated defect aggregation is prevented. The open sample surface was exposed to the electron beam, which was emitted from the “Elektronika U-003” accelerator's exit window. Immediately after each irradiation the sample was cooled down to 77 K, then the PL and its excitation spectra were measured in the optical range of 350-800 nm and several temperatures while slow heating from 165 to 300 K. Since optical pure LiF crystals have no PL detected prior to any irradiation, the emerged broad PL bands with maxima at 530 and 650 nm and their excitation bands at 420 and 470 nm can be ascribed to the radiation induced defects - mainly <i>F</i>₂ centers. At low temperatures (&lt; 210 K), F2 centers predominate, and upon heating to 300 K, the thermally activated formation of <i>F</i>₃⁺ aggregates increases.