Optical and Electron-Spin-Resonance Studies of Fourth-Nearest-Neighbor Chromium Ion Pairs in Ruby

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\ensuremath{\beta}\mathbf{H}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}+\frac{J}{2}[S(S+1)\ensu