Vibronic aspects of the CaO and MgOF bands

A useful extension is made to the existing theory of the optical absorption and emission bands of defects and impurities in low concentrations in crystalline host materials. This extension is able to account for the shape of the low-temperature absorption and emission bands in both the details of the sharp-line phonon-related peaks and the broad-band characteristics-as applications to defect centers in the alkaline-earth oxides (and alkali halides) demonstrate. A lifetime-broadening mechanism is introduced to explain the observed smearing and broadening of higher-energy multiple-phonon-assisted transitions, the manifold of such transitions giving rise to the broad band. We then specialize to the case of F centers in cubic (O-h group) materials; a short group-theoretical discussion shows for this case that only local modes belonging to the Gamma(+)(1), Gamma(+)(12), and Gamma(+)(25) representations may be active and that both transverse and longitudinal acoustic lattice modes near the L band point are expected to be most active in coupling the single trapped electron to the lattice. Comparison is made with recent experimental results for the F center in CaO and MgO.