Optically detected magnetic resonance and spin coherence in the phosphorescent state of exchange-coupledF+-center pairs in CaO

In yellow-colored CaO crystals a sharp zero-phonon line emission at 682.8 nm is observed. From optically-detected-magnetic-resonance experiments conducted at 1.3 K and at low- and high-magnetic-field strengths, it is concluded that the emission is due to a photo-excited triplet state of an orthorhombic $I$ center. Optical polarization, spin alignment, and spin coherence decay phenomena were studied, the latter by means of techniques for the optical detection of spin echoes and spin locking, in order to determine the radiative and nonradiative properties of the triplet sublevels. The triplet state is assocaited with a $1s2p$ excited state of two electrons captured in two nearest-neighbor oxygen-anion vacancies (an ${F_2}^{2+}$ center) with possibly a ${\mathrm{Ca}}^{2+}$ vacancy nearby for charge compensation (i.e., an ${F_2}^{2+}-V^{2-}$ center). The observed linear dichroism in the excitation microwave double-resonance spectra is considered in connection with the mechanism for the triplet-state production. It is discussed briefly why the lowest ${\mathrm{}\left(1s\right)\mathrm{}}^2$ triplet state, which is predicted in the $M$-center model, could not be observed.