Electric dipole intensity parameters for the samarium 4f → 4f transitions in Na₃[Sm(oxydiacetate)₃] · 2NaClO₄ · 6H₂O

Abstract

Optical intensity data available from axial and polarized orthoaxial absorption measurements on single crystals of Na₃[Sm(oxydiacetate)₃] · 2NaClO₄ · 6H₂O are analysed in terms of a 13-parameter expression for 4f → 4f transition dipole strengths. Twelve of the parameters in this expression are related to one-electron/one-photon electric-dipole transition processes occurring within the 4f ⁵ electronic configuration of Sm³⁺ (in a crystal field of D ₃ symmetry), and one parameter reflects bulk sample refractivity effects on the magnetic-dipole contributions to observed line strengths. Both the electric-dipole and magnetic-dipole terms in the dipole strength expression contain matrix elements that require, for evaluation, eigenvectors of the crystal-field states involved in any given transition. These eigenvectors, expressed in a |(4f ⁵)SLJM_J > basis, are obtained from energy level calculations parametrized to yield optimal fits between calculated and observed crystal-field levels. The empirical intensity data are fitted with the parametrized dipole strength expression, allowing all 13 parameters to vary freely. Very good agreement is achieved between calculated (fitted) and observed absorption intensities, both inside and outside transition regions represented in the data fits, which indicates that the intensity parameters are reasonably well-determined by the ‘best-fit’ calculations. The electric-dipole intensity parameterization scheme employed in this study is perfectly general within the one-electron/one-photon approximation for f → f crystal-field transitions. Six of the 12 parameters in this scheme (for a system with D ₃ symmetry) must vanish if a point-charge crystal-field model is assumed for the f-electron/ligand interactions. In the present study, a six-parameter scheme for electric-dipole intensities is found to be inadequate for fitting the empirical intensity data, and good fits are achieved only when all 12 of the generalized intensity parameters are considered. This is the first study in which a complete set of generalized electric-dipole intensity parameters has been reasonably well characterized for the 4f → 4f transitions of a lanthanide complex.