3RD-ORDER AND 4TH-ORDER ANALYSIS OF ORBIT-FORBIDDEN AND SPIN-FORBIDDEN TRANSITIONS IN 2-PHOTON SPECTROSCOPY OF LANTHANIDE COMPOUNDS

The role of spin-orbit and crystal field interactions in the intensity mechanism for orbit- and spin-forbidden transitions in two-photon abs orption spectroscopy of lanthanide compounds is investigated. The exis ting third-order analysis of the crystal field contribution to the tra nsition moment has been restated in a more compact way using a 9j symb ol. New general expressions for the fourth-order combination of spin-o rbit and crystal field interactions are developed. The highest recoupl ing is found to require a 12j symbol. The associated selection rules a llow a distinction between the various coupling mechanisms.