Radiative energy transfer in alexandrite

Abstract

Energy transfer between chromium ions at mirror sites in BeAl2O4 has been investigated with fluorescence line-narrowing techniques. At helium temperatures the crystals exhibit strong phonon trapping and the radiative transfer is found to provide the dominant transfer mechanism. The trapping has the effect of delaying the relaxation of the 2E excited state from its intrinsic value of 1.3 ms to 2 ms, and a recently reported lifetime of 2.3 ms is attributed to the same effect. The temperature dependence of the radiative transfer at the mirror site follows a T3 dependence. This indicates a two-phonon non-resonant process, whereas simple theoretical arguments predict a dominant two-phonon resonant process. The zero-field splitting of the ground state is found to be 14.68 GHz in contrast with the recently reported value obtained by fluorescence line-narrowing measurements.