EPR in the Excited Optical States of Exchange-Coupled Chromium Ion Pairs in Ruby

Abstract

By means of an optical detection technique, we have studied EPR in the excited metastable optical level of exchange-coupled chromium ion pairs in ruby with concentration of ∼0.2% Cr2O3. In such excited pair states one ion is in an excited optical state while the other is in its ground state. We have confined our attention mainly to fourth-nearest-neighbor pairs. We find evidence that when one ion of the pair is excited, it is antiferromagnetically coupled to its fourth-nearest-neighbor, while both ions are ferromagnetically coupled in the ground state. In the optical detection method we monitor the fluorescence from the particular center being studied and note the change in fluorescence signal which occurs when resonant microwave power changes the population in the excited state. Besides the resonance signals expected from our knowledge of the excited pair state, we find a number of additional resonances. Some of these have been identified as single chromium ion resonances which affect the pair fluorescence because of the transfer of optical excitation from single ion to pair. The other signals appear to be fourth-nearest-neighbor ground-state resonances which are detected because of their effect upon the optical pumping of the pair.