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

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