Optical transitions ofRh2+in NaCl crystals studied by electron spin resonance and magnetic circular dichroism

Abstract

Electron spin resonance, magnetic circular dichroism, and optical absorption spectra of a paramagnetic ${\mathrm{Rh}}^{2+}$ complex are investigated in ${\mathrm{Rh}}^{3+}$ doped solution-grown NaCl single crystals. x irradiation of the crystals at 77 K followed by a warm-up to room temperature results in the formation of a stable low-spin Jahn-Teller distorted ${\mathrm{RhCl}}_{6}^{4\ensuremath{-}}$ complex possessing ${D}_{4h}$ symmetry. The unpaired electron is largely localized in the ${4d}_{{z}^{2}}$ orbital of the ${\mathrm{Rh}}^{2+}{(4d}^{7})$ which is involved in a $\ensuremath{\sigma}$ type molecular bond with two symmetrically placed ${\mathrm{Cl}}^{\ensuremath{-}}$ ions along $〈001〉$. The magnetic circular dichroism and optical absorption measurements at 1.4 K reveal two main bands around $12200{\mathrm{cm}}^{\ensuremath{-}1}$ and $16200{\mathrm{cm}}^{\ensuremath{-}1}.$ Within the octahedral Tanabe-Sugano formalism of the ${\mathrm{Rh}}^{2+}$ these two absorptions are associated with two vibronically allowed ${}^{2}{\stackrel{\ensuremath{\rightarrow}}{{E}_{g}}}^{2}{T}_{1g}{,}^{2}{T}_{2g}$ molecular transitions. This assignment is in quantitative agreement with the analysis of the $\mathbf{g}$ matrix components. A small MCD feature at $15200{\mathrm{cm}}^{\ensuremath{-}1}$ is tentatively associated with two coinciding ${}^{2}{\stackrel{\ensuremath{\rightarrow}}{{E}_{g}}}^{2}{T}_{1g}{,}^{2}{T}_{2g}$ transitions of another, probably a ligand-exchanged, ${\mathrm{Rh}}^{2+}$ center.