F-7(6)(GAMMA(1G))-](GAMMA(1G))-D-5(4) 2-PHOTON TRANSITION OF TB3+ IN CS2NATBCL6

Two-photon spectroscopy has expanded the scope of studies of excited s tates of ions and has also enabled the examination of the validity of conventional theories. In this study, the direct theoretical calculati on of the two-photon intraconfigurational crystal field transitions of F-7(6)(Gamma(1g)) --> D-5(4) of Tb3+ in Cs2NaTbCl6 has been performed , based on third-order perturbation theory including electric dipoles and spin-orbit coupling. The core 4f(7)(S-8(7/2), Gamma(6), Gamma(7), Gamma(8)) and 4f(7)(P-6(7/2) Gamma(6), Gamma(7), Gamma(8)) states coup led with 5d(Gamma(3), Gamma(5)) are taken as the intermediate states. The calculated transition intensity ratios are in good agreement with the experimental results. In particular the two-photon transition F-7( 6)(Gamma(1g)) --> (Gamma(1g))D-5(4) is allowed in third-order perturba tion instead of tile proposed fourth-order process by Ceulemans ct al, using the Judd-Pooler-Downer model. The inconsistency between tile tw o studies arises from the failure of application of the Judd-Ofelt clo sure approximation. The closure approximation does not only simplify t he two-photon calculation but also sacrifices the physical accuracy by changing the selection rule of the two-photon transition from that of two electric dipole transitions to that of one electric quadrupole tr ansition.