Optical properties of strongly coupled Yb2+ and CN- ions in alkali halide crystals:: Electronic absorption and emission

We have studied absorption and emission spectra of the 4f(14)<->4f(13)5d electronic transitions of Yb2+:(CN-)(n) defect complexes in NaCl, KCl, RbCl, and KBr host crystals under concentration variation of both dopants Yb2+ and CN-. Due to strong coupling between Yb2+ and (CN-)(n), the Yb2+ electronic transitions shift drastically towards lower energies (from near uv to near ir) as the n value increases. Applying thermal quenching and excitation spectroscopy of the emission, the observed absorption spectra can be decomposed into contributions from the various defect complexes Yb2+:(CN-)(n), with n = 0,1, and n greater than or equal to 2. Within the framework of ligand field theory we can estimate a splitting parameter D-q as high as similar to 3450 cm(-1) for the Yb2+:(CN-)(6) complexes in KCN, a value three times higher than for isolated Yb2+ defects in KCl. These large D-q values can be understood by a comparison with other metal ion complexes and by the special role of CN- within the spectrochemical series. For our system we try to interpret the latter by a charge redistribution from the n CN- molecules to the 5d orbital of their Yb2+ defect partner.