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 $H=g{β}H·{}S+J/2[S(S+1){-}15/2]+{D}S[Sz^{}{}_{}{}²{-}1/3S(S+1)]+{E}S[Sₓ^{}{}_{}{}²{-}Sy^{}{}_{}{}²]$ 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 ${D}cis found to be -0.191±0.005{cm}^{{-}1}$, which is equal to that for the isolated ion. The value of ${D}Eis found to be -0.021±0.005{cm}^{{-}1}$. A phonon-assisted energy-transfer mechanism is postulated to account for the existence of the optically detected spin-resonance spectrum.