Journal of Chemical Physics vol:98 issue:12 pages:9372-9378
The role of spin-orbit and crystal field interactions in the intensity mechanism for orbit- and spin-forbidden transitions in two-photon absorption spectroscopy of lanthanide compounds is investigated. The existing third-order analysis of the crystal field contribution to the transition moment has been restated in a more compact way using a 9j symbol. New general expressions for the fourth-order combination of spin-orbit and crystal field interactions are developed. The highest recoupling is found to require a 12j symbol. The associated selection rules allow a distinction between the various coupling mechanisms.