Symmetry assignments of one-phonon sidebands in the emission spectrum of the F+centre in calcium oxide

Stress-induced dichroism in the emission spectrum of the CaO F + centre has been used to identify the symmetry of the vibrational modes coupling to the 2 T 1u excited electronic state of the defect. The polarization of the main one-phonon sidebands under (001) and (110) stress shows that a peak at a separation of h(cross) omega =205 cm -1 from the zero phonon line is due predominantly to A 1g modes and a sharp peak at h(cross) omega =302 cm -1 to a composite of E g and T 2g modes. A separation of the E g and T 2g components shows that the frequencies of these modes are equal to within about 15 cm -1 , and that they couple equally strongly to the electronic state. The relevance of this result to the lattice dynamics and Jahn-Teller effect of the F + centre is discussed.