Vibronic properties of the F+absorption band in calcium oxide

The shape and first four moments of the F+absorption band in CaO have been studied as a function of temperature between 4.2 K and 300 K. Analysis of these results shows that the properties of the band can be accounted for very well in terms of Jahn-Teller coupling of the excited2T1ustate of the F+centre to both Egand T2gvibrational modes, with weaker coupling to A1gmodes. The shape of the band agrees remarkably well with the calculations of O'Brien (1969). Values for the Huang-Rhys factors of S=3.3 for the Egand T2gmodes and SA=1.6 for the A1gmodes are derived from a model where all modes have the same effective frequency of h(cross) omega =264 cm-1. An attempt to allow different frequencies for the various modes is only partially successful, due to the lack of detailed information on the frequency spectra of the coupled modes. Nevertheless such an analysis does suggest that the vibrational sidebands observed at frequencies of about 205 cm-1and 302 cm-1may be associated with A1gand (Eg, T2g) modes respectively. Measurements of the intensity, position and halfwidth of the zero phonon line as a function of temperature cannot be interpreted adequately in terms of conventional theories, unless the frequency of the interacting modes is made much smaller than the value used for the broad band.