We propose that the carbon dimer defect CBCN in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 3.8 eV is close to the experimental value, and the deduced Huang-Rhys factor of S ≈ 2.0, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized π-type defects states, with a very short radiative lifetime of 1.7 nanoseconds, in very good accord with experiments.

The lowest excited electronic state of the O2 molecule, a 1 g, the "singlet oxygen", is of utmost importance for photochemistry and photobiology. For O2 trapped in silica glass, the lifetime of this state and the associated a 1 g ⟶ X 3 Σg − photoluminescence (PL) is the longest known for O2 in any condensed medium at room temperature. We studied the temperature dependence, decay kinetics and polarization anisotropy of this PL with 1064 nm excitation to the a 1 g (v=1) state, as well as with excitation to higher energies. PL at this excitation shows non-zero polarization anisotropy at 295 K, which increases with cooling to 14 K. At variance, excitation to higher energies yields depolarized PL. Polarization data indicate weak electric dipole character of the emission of the spin-and parityforbidden a 1 g ⟶ X 3 Σg − transition, enabled by O2-SiO2 cage interactions. Density functional theory calculations indicate that at low temperatures the rotation of O2 is partially or fully frozen even in large silica voids. As the temperature increases, PL is increasingly depolarized by libration movement of O2 molecules. Analysis of O2 optical absorption in optical fibers allows one to obtain the absorption cross sections of X⟶a and X⟶b transitions of O2 in SiO2 glass and to evaluate both radiative and non-radiative rates of a⟶X luminescence. * The 1st index (capital H or V) denotes the excitation polarization direction, the 2nd one (lower case h or v)-the PL emission polarization direction.

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