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=gβH•S+J2[S(S+1)-152]+DS[Sz2-13S(S+1)]+ES[Sx2-Sy2] where the directions of the symmetry axes, DS, and ES each depend on the spin S in a predictable way, requiring only two adjustable parameters: Dc (the usual second-order crystal-field term of axial symmetry) and DE (a similar term arising from the anisotropic exchange interaction). The value of Dc is found to be -0.191±0.005 cm-1, which is equal to that for the isolated ion. The value of DE is 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.