Energy transport in ruby via microwave-optical experiments

The process which results in enhanced pair fluorescence relative to single-ion fluorescence for ruby samples containing more than 0.2-wt% ${\mathrm{Cr}}^{3+}$ has been studied using microwave-optical double resonance. These experiments confirm that the enhancement is associated with energy transport among excited states of the system. However, the primary donor-level complex is not the normal ion excited state as postulated previously. Microwave-optical experiments indicate the existence of an additional excited level which is postulated to be the actual donor level. Evidence is presented which suggests that this additional level may be connected with exciton formation above a critical ${\mathrm{Cr}}^{3+}$ concentration.