Temperature Dependence of Luminescence ofF+andFCenters in CaO

Abstract

The principal moments of the ${F}^{+}$-center absorption and emission bands in CaO have been studied as a function of temperature in the range 5-300 K. The absorption results are shown to be in excellent agreement with the very recent results of Escribe and Hughes. Moment analysis is less useful for emission than absorption bands and gives no further information about the nature of the dynamic Jahn-Teller effect in the excited state of the ${F}^{+}$ center. Nevertheless, certain features of the temperature dependence of the first four moments of the broad band shape indicate that the Jahn-Teller interaction is still important even though the luminescence process samples the orbitally nondegenerate ground state. Analysis of the results shows that in the range 5-78 K the moments ${M}_{1}(0)$ and ${M}_{2}(T)$ can be accounted for with an effective Huang-Rhys factor $S=6$, the phonon modes effective in broadening the transition having a mean frequency $\ensuremath{\hbar}\ensuremath{\omega}=273$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The first four moments of the $^{3}P\ensuremath{\rightarrow}^{1}S$ transition of the $F$ center have been measured between 5 and 300 K. The results are interpreted as being consistent with $S=5.5$ and $\ensuremath{\hbar}\ensuremath{\omega}=155$ ${\mathrm{cm}}^{\ensuremath{-}1}$. Surprisingly, the band shape, as indicated by the ratio $R=\frac{{M}_{4}}{{({M}_{2})}^{2}}$, varies with temperature, being Gaussian below 90 K ($R=3.1$) and rather asymmetric with $R<3$ between 90-300 K. The experimentally measured line shapes in electronirradiated crystals are well represented by Voigt profiles, which is interpreted as a convolution of the Gaussian component of the unirradiated crystal and that Lorentzian component due to point-defect broadening. The shape, shift, and broadening due to electron irradiation are qualitatively in agreement with the statistical theory of strain broadening.