Picosecond absorption spectroscopy of self-trapped excitons and transient Ce states in LaBr3 and LaBr3 :Ce

Abstract

Picosecond time-resolved optical absorption spectra induced by two-photon interband excitation of ${\mathrm{LaBr}}_{3}$ are reported. The spectra are similar in general characteristics to self-trapped exciton (STE) absorption previously measured in alkali halides and alkaline-earth halides. A broad ultraviolet absorption band results from excitation of the self-trapped hole within the STE. A series of infrared and red-visible bands results from excitation of the bound outer electron within the STE similar to bands found in alkali halides corresponding to different degrees of ``off-center'' relaxation. Induced absorption in cerium-doped ${\mathrm{LaBr}}_{3}$ after band-gap excitation of the host exhibits similar STE spectra, except it decays faster on the tens-of-picoseconds scale in proportion to the Ce concentration. This is attributed to dipole-dipole energy transfer from STE to ${\mathrm{Ce}}^{3+}$ dopant ions. The absorption spectra were also measured after direct excitation of the ${\mathrm{Ce}}^{3+}$ ions with sufficient intensity to drive two- and three-photon resonantly enhanced excitation. In this case, the spectrum attributed to STEs created adjacent to ${\mathrm{Ce}}^{3+}$ ions decays in 1 ps suggesting dipole-dipole transfer from the nearest-neighbor separation. A transient absorption band at 2.1 eV growing with Ce concentration is found and attributed to a charge-transfer excitation of the ${\mathrm{Ce}}^{3+*}$ excited state responsible for scintillation in ${\mathrm{LaBr}}_{3}$:Ce crystals. This study concludes that the energy transport from host to activator responsible for the scintillation of ${\mathrm{LaBr}}_{3}$:Ce proceeds by STE creation and dipole-dipole transfer more than by sequential trapping of holes and electrons on ${\mathrm{Ce}}^{3+}$ ions.