THE THEORY OF VIBRONIC TRANSITIONS IN RARE-EARTH COMPOUNDS

The theory of vibronic transitions in rare earth compounds is re-exami ned in the light of a more reliable representation for the ligand fiel d Hamiltonian than the crude electrostatic model. General expressions that take into account the relevant contributions from the forced elec tric dipole and dynamic coupling mechanisms are derived for the vibron ic intensity parameters. These include additional terms, from charge a nd polarizability gradients, which have not been considered in previou s work. Emphasis is given to the relative signs of these various contr ibutions. Under certain approximations these expressions may be conven iently written in terms of accessible ligand field parameters. A compa rison with experimental values for the compounds Cs2NaEuCl6 and LiEuF4 is made and satisfactory agreement between theory and experiment is f ound. A discussion is given on the sensitivity of the calculated inten sities to the values of radial integrals, interconfigurational energy differences and ligand field parameters that may be used. Finally, the problem in which a vibronic and an electronic level are in resonance, or near resonance, is analyzed. Suitable expressions to describe the effects of the even-rank components of the vibronic Hamiltonian are ob tained. It is found that, depending on the strength of the vibronic in teraction and the resonance conditions, the admixture between these tw o levels may lead to intensities of nearly equal values.