Optical Spectra of Gamma-Irradiated LiF Crystals with Anisotropic Lithium Nanoparticles

The optical response of Li anisotropic defects and nanoparticles from three faces of cubic LiF crystals with 10-mm edges to γ radiation from a ⁶⁰Со source with doses of 10⁵–10⁹ R at 320 K in air is studied. After a maximum dose of 10⁹ R, scanning electron microscopy revealed submicron metal lithium flakes ordered into long parallel nanofilaments. The resonance absorption band, which shifts from 272 to 295 nm with increasing dose, is assigned to lithium nanocolloids in the (110) plane. The band at 202–225 nm observed only from the (100) face narrows at doses below 10⁵ R and is assigned to diatomic fluorine molecules in interstitial sites. The appearance of the 209–212–215-nm triplets at 2.7 × 10⁵ R from the (100) plane and the 211–213–215-nm triplets at 0.9 × 10⁵ R from the (010) plane is related to radiation-induced axially symmetric bipolarons with a high oscillator strength. The splitting of the 445-nm band into a 438–445–450-nm triplet is observed only for the (010) plane after a dose of 10⁶ R and is related to surface plasmon polaritons (longitudinal and transverse) of elongated Li nanoparticles. The (F₂–F₃) complexes are oriented in the most imperfect (111) plane and are seen along three cubic axes.