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 LaBr3 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 LaBr3 after band-gap excitation of the host exhibits similar STE spectra, except decaying faster on the tens-of-picoseconds scale in proportion to the Ce concentration. This is attributed to dipole-dipole energy transfer from STE to Ce 3+ dopant ions. The absorption spectra were also measured after direct excitation of the Ce 3+ ions with sufficient intensity to drive 2-and 3-photon resonantly enhanced excitation. In this case the spectrum attributed to STEs created adjacent to Ce 3+ ions decays in 1 picosecond suggesting dipole-dipole transfer from the nearestneighbor separation. A transient absorption band at 2.1 eV growing with Ce concentration is found and attributed to a charge-transfer excitation of the Ce 3+* excited state responsible for scintillation in LaBr3:Ce crystals. This study concludes that energy transport from host to activator responsible for scintillation of LaBr3:Ce proceeds by STE creation and dipole-dipole transfer more than by sequential trapping of holes and electrons on Ce 3+ ions.

Picosecond absorption spectroscopy of self-trapped excitons and Ce excited states in CeBr3 and La1−xCexBr3Li1, Gridin2, Üçer3 et al. 2019Phys. Rev. BSelf Cite7120View full textAdd to dashboardCite

“…3 was measured after two-photon interband excitation of the host LaBr 3 and the bulk crystal CeBr 3 by 420 nm (2.95 eV) photons which do not directly (one-photon) excite the Ce(4f-5d) transitions. In the left three panels for LaBr 3 and LaBr 3 :Ce excited in the host matrix, we identified the absorption bands centered at 0.46 eV and about 0.9 eV as due to self-trapped excitons with Type I (on-center) and Type II (moderately off-center) relaxation, respectively 6 . It was sug- gested that some of the absorption centered around 1.4 eV may be due to Type III (strongly off-center) STEs.…”

“…3. The decay of the three envelope bands of absorption becomes faster with greater Ce concentration, and this dependence was attributed to dipole-dipole transfer from host STE to Ce dopant before STE thermal migration can occur 6 . The V k -type ultraviolet absorption of the self-trapped hole component of the STE was also observed in host-excited LaBr 3 , corresponding to the schematic representation in the lower left of Fig.…”

“…When a LaBr 3 sample is doped with 4% or 20% Ce for example, the 4.43-eV 3 rd harmonic photons are strongly absorbed by Ce 3+ (4f-5d transitions) before the beam can come to a sufficient focus to produce 2-photon interband excitation. We used this as a method to see effects of direct excitation of isolated Ce 3+ ions in LaBr 3 :Ce 6 . Figure 2 displays the luminescence excitation spectrum of CeBr 3 19 along with the absorption spectrum of the CeBr 3 sample studied in this work and absorption of a thin film of CeBr 3 16 .…”

“…3 were analyzed for STE decay time in Ref. 6 , finding that in undoped, 4.4%-, and 22.2%-doped LaBr 3 :Ce, about 50% of the absorption decayed with time constants of 92 ps, 69 ps, and 38 ps, respectively versus Ce concentration. The increase of STE decay rate with Ce concentration is clear, and was interpreted as indicating energy transfer from Br 2 -STEs to Ce 3+ dopant ions within the effective dipole-dipole transfer radius of STEs at their place of creation in the host lattice.…”

Role of electron and hole centers in energy transfer in BaBrI crystals

“…Excitation of Eu 2+ emission in the region of exciton peak [5] can indicate the possibility for energy transfer from exciton to Eu 2+ ions as well as in LaBr 3 :Ce, where resonance exciton energy transfer is dominant [14]. Absorption of 4f-5d transition of Eu 2+ ions and exciton emission spectra overlap in BaBrI crystals (Fig.4).…”