Anomalous luminescence of impurity-bound excitons in lithium borate crystals doped with cerium ions

The spectra and relaxation kinetics of the anomalous ($\tau<10$ ns) luminescence of Li$_6$GdB$_3$O$_9$:Ce$^{3+}$ crystals have been experimentally detected. The time-resolved vacuum ultraviolet spectroscopy study has shown that optical transitions at $6.2$ eV, caused by the transfer of an electron from the $4f^{1}$ ground state of Ce$^{3+}$ to autoionizing states near the conduction band bottom of a crystal, lead to the formation of an impurity-bound exciton with the hole component localized on the $4f$ state of Ce$^{3+}$ and the electron localized on states of the conduction band bottom. It has been found that the decay of such an exciton in Li$_6$GdB$_3$O$_9$:Ce$^{3+}$ occurs through radiative recombination, leading to fast luminescence at $4.25$ eV. The energy threshold for the formation of the impurity-bound exciton has been determined. The distribution functions of elementary relaxations over the reaction rate constants $H(k)$, which determine the relaxation kinetics and luminescence quenching processes, have been calculated.