Polarization of emission spectra from Ti³⁺-doped oxide crystals - II. Zero-phonon emission lines

The Ti³⁺ ion in YAlO₃ (YAP), Y₃Al₅O₁₂ (YAG), and Al₂O₃ crystals occupies distorted octahedral sites relative to the nearest neighbour ligand ions. Such distortions are of even-parity in YAG where the zero-phonon lines in emission occur via magnetic dipole transitions. In contrast, the zero-phonon transitions occur by electric dipole processes in Ti³⁺:YAP and Ti³⁺:Al₂O₃ where there are odd-parity distortions from octahedral symmetry. This paper reports measurements of the zero-phonon lines of Ti³⁺ ions in YAP, YAG, and Al₂O₃ at 10 K. The zero-phonon lines of Ti³⁺:YAP are strongly polarized perpendicular to the tetragonal axis and those of Ti³⁺:Al₂O₃ parallel to the trigonal axis. The experimental results are shown to be in accord with a molecular orbital model of the radiative transition according to which the transition intensities derive from odd-parity ligand wavefunctions induced into even-parity ground and excited Ti³⁺ wavefunctions by odd-parity crystal distortions.