Third-order contributions to spin-forbidden rare earth optical transition intensities

uantitative calculations show that numerous parity- and spin-forbidden linear optical transitions observed in trivalent rare earth ions acquire a major fraction of their intensity from hitherto neglected contributions involving spin-orbit linkages within excited configurations. Motivated by the importance of analogous linkages previously demonstrated in two-photon absorption, a general expression for these new "third order" terms, applicable for all 1;Presenting this revised analysis of observed linear absorption intensities in aqueous solution for the entire trivalent lanthanide series, we show that these new contributions are important throughout the lanthanides, and selectively modify spin-forbidden ;In addition, we extend this analysis to selected trivalent actinides. Other extensions examined include the possibility of g-orbital contributions and contributions involving the linear crystal field for selected hypersensitive transitions, in particular the...