Spectroscopic properties of alexandrite crystals II

Abstract

Several aspects of the optical spectroscopic properties of alexandrite crystals not previously investigated are reported here. For Cr 3+ ions occupying the Al 3+ lattice sites with mirror symmetry in BeAl 2O 4, the positions of the zero-phonon lines for absorption transitions to the 2T 1g and 4T 2g levels are identified, and vibronic transition peaks in the fluorescence spectrum are compared to transitions appearing in the Raman spectrum and Stokes excitation spectrum. In addition, the effects of radiation trapping are shown to lengthen the fluorescence lifetime of the 2E g - 4A 2g transition for ions in these sites at low temperatures. For Cr 3+ ions occupying the Al 3+ lattice with inversion symmetry, the ground state splitting of the 2E g - 4A 2g transition is reported and the decrease of the fluorescence lifetime with temperature is shown to be due to the increase in vibronic emission probability as well as increased probability of direct radiationless decay.