Aggregate colour centres in impurity LiF crystals

LiF crystals with colour centres exhibiting a zero-phonon line (ZPL) at 1080 nm in absorption and luminescence are studied. The decay time of luminescence of colour centres at 10 K is 260 – 280 ns, the ZPL half-width is 4.7 cm^-1, and colour centres are characterised by a weak electron – phonon interaction (the Huang – Rhys factor is S < 0.11). The polarisation analysis of luminescence showed that the transition dipole moments of colour centres are oriented along the crystal axes [100], [010], and [001]. The model of aggregate F₄ colour centres having a spatial structure with three symmetry axes C₂ may correspond to the colour centres studied in the paper.