The fine structure of the optical fluorescence spectrum arising from the fourth-nearest-neighbor chromium ion pair system in ruby is studied using high-resolution optical spectroscopy, ordinary electron-spin resonance, and optically detected electron-spin resonance. The ground-state energy levels of this system are found to be describable by a simple spin Hamiltonian of the form

$\mathcal{H}=g\beta \mathbf{H}·\mathbf{S}+\frac{J}{2}\mathrm{}\left[S\right(S+1\mathrm{})-\frac{15}{2}]+D_S[S_z^{}{}_{}{}^2-\frac{1}{3}S(S+1\mathrm{}\left)\right]+E_S[S_x^{}{}_{}{}^2-S_y^{}{}_{}{}^2]$

where the directions of the symmetry axes, $D_S$, and $E_S$ each depend on the spin $S$ in a predictable way, requiring only two adjustable parameters: $D_c$ (the usual second-order crystal-field term of axial symmetry) and $D_E$ (a similar term arising from the anisotropic exchange interaction). The value of $D_c$ is found to be -0.191±0.005 ${\mathrm{cm}}^{-1\mathrm{}}$, which is equal to that for the isolated ion. The value of $D_E$ is found to be -0.021±0.005 ${\mathrm{cm}}^{-1\mathrm{}}$. A phonon-assisted energy-transfer mechanism is postulated to account for the existence of the optically detected spin-resonance spectrum.

• Received 19 June 1969

DOI:https://doi.org/10.1103/PhysRev.188.675