Self-trapped excitons in LiB3O5 and Li2B4O7 lithium borates: Time-resolved low-temperature luminescence VUV spectroscopy

Results are reported of a coordinated investigation of the dynamics of electronic excitations in LiB₃O₅ and Li₂B₄O₇ crystals by low-temperature luminescence VUV spectroscopy performed with subnano-second resolution under synchrotron photoexcitation. Data on the photoluminescence (PL) decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra obtained at 295 and 9.6 K are reported for the first time; the PL of the borates in the 3.5-eV region caused by radiative annihilation of self-trapped excitons (STE) has been established to have an intrinsic nature; the σ and π STE luminescence bands originating from singlet and triplet radiative transitions have been isolated; the shift of the STE σ band relative to the π band has been interpreted; the LBO recombination luminescence band has been isolated; and the creation and decay channels of relaxed and unrelaxed excitons in lithium borates are discussed.