Influence of random intrinsic strain upon the zero-field double-resonance spectra of the CaOFcenter in its excited Jahn-Teller states

In previous studies of the photoexcited state of CaO $F$ centers it was shown that the ${}_{}{}^3{}_{}{}^{}T_{1u}^{}{}_{}{}^{}$ state exhibits a Jahn-Teller coupling to $e_g$ vibrational modes in the strong-coupling regime. The present work examines how, at low temperatures, coupling to a randomly distributed (intrinsic) strain affects zero-field optically detected magnetic resonance (ODMR) and phosphorescence microwave double resonance spectra. The ODMR spectra reflect microwave absorptive and emissive parts. As a result of this, different portions of the inhomogeneously broadened lines could be studied selectively as a function of the microwave-field polarization and small applied magnetic fields. The data are consistent with a proposed model in which the coupling between the vibronic system and randomly distributed strain is considered. It will be shown that singularities in the spectral density of the microwave resonances arise when a random angular distribution in the strain components $e_{\theta }$ and $e_{\epsilon }$ is adopted. It turns out that the resonances associated with these extrema are restricted to just a few subensembles of $F$ centers. In each subensemble the vibronic degeneracy is lifted by the additional effect of the spectral distribution in strain magnitude, ${(e_{\theta }^2+e_{\epsilon }^2)}^{\frac{1}{2}}$. This gives rise to the occurrence of several zero-phonon lines in the optical emission, inhomogeneous broadening of the ODMR line, population inversion, and the observations of internal conversion with conservation of spin state.