VIBRONIC INTENSITIES IN CENTROSYMMETRIC LANTHANIDE COMPLEX-IONS .1. ACOMBINED CRYSTAL FIELD-LIGAND POLARIZATION APPROACH

A theoretical model to calculate the vibronic intensities induced by t he odd vibrational modes in centrosymmetric lanthanide complexes is de veloped and applied to octahedral complex ions, LnX6(3-) such as occur in the hexachloroelpasolites Cs2NaLnCl6. Both the crystal field and t he ligand polarisation contributions are evaluated using a standard se t of symmetry coordinates. For the crystal field term a truncated expa nsion of the intermediates states is employed rather than the more con ventional closure approximation. Special care is necessary to ensure t hat the phases of the contributions are correctly determined since the cross-term between the ligand polarisation and crystal field contribu tions is signed. General equations applicable to any f(n) complex ion are derived and an example of their application to the PrCl6(3-) ion i s given The agreement with experiment is satisfactory.