Parametric analysis of f-f transition intensities in trigonal ${\mathrm{Na}}_3$[Ho(${\mathrm{C}}_4$${\mathrm{H}}_4$${\mathrm{O}}_5$${)}_3$]⋅${2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{O}}_4$⋅${6\mathrm{H}}_2$O

Optical-intensity data available from axial and polarized orthoaxial absorption measurements on single crystals of trigonal ${\mathrm{Na}}_3$[Ho(${\mathrm{C}}_4$ ${\mathrm{H}}_4$ ${\mathrm{O}}_5$ ${)}_3$]⋅${2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{O}}_4$⋅${6\mathrm{H}}_2$O are analyzed in terms of a parametric model for 4f-4f transition line strengths. The model contains 12 parameters related to the electric-dipole intensities of transitions between crystal-field (Stark) levels, and these parameters are used to fit calculated-to-empirical line strengths for 42 transitions observed in axial absorption spectra and 19 transitions observed in π-polarized orthoaxial absorption spectra at 10 K. The parameters obtained from these line-strength fits are then used to calculate line intensities for all transitions occurring between 8000 and 37 000 ${\mathrm{cm}}^{\mathrm{-}1}$, and comparisons are made between simulated and observed absorption spectra over this frequency range. Possible structural and mechanistic interpretations of the derived parameters are discussed, with particular emphasis on the parameters that imply lanthanide–ligand–radiation-field interactions not subsumed by the standard superposition model theories of lanthanide 4f-4f electric-dipole intensity. These latter parameters are of crucial importance in the intensity analyses reported here, and they reflect the structural complexity of the ligands coordinated to ${\mathrm{Ho}}^{3+}$ in ${\mathrm{Na}}_3$[Ho(${\mathrm{C}}_4$ ${\mathrm{H}}_4$ ${\mathrm{O}}_5$ ${)}_3$]⋅${2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{O}}_4$⋅${6\mathrm{H}}_2$O.