Polarization dependence of two-photon excitation spectra in the 8S7/2 → 6P7/2, 6P5/2 and 6P3/2 transition regions of Gd3+ in Na3[Gd(C4H4O5)3] · 2NaClO4 · 6H2O

The relative intensities and polarization properties of two-photon absorption (TPA) transitions are examined throughout the 8S7/2 → 6P7/2, 6P5/2 and 6P3/2 excitation regions of Gd3+(4f7) in single crystals of Na3[Gd(oda)3] · 2NaClO4 · 6H2O (where oda denotes an oxydiacetate ligand of chemical formula C4H4O2 − -- −OOCCH2OCH2COO−). In these crystals the Gd3+ ions are located at sites of D3 point symmetry, and the Gd(oda)3−3 coordination complexes have chiral, tris-terdentate chelate structures. These crystals exhibit natural optical activity, and they differentially absorb left- and right-circularly polarized light in both one-photon and two-photon excitations of the Gd3+ 4f → 4f (intraconfigurational) electronic transitions. In the present study, results are reported for fluorescence-detected TPA measurements performed under three different excitation polarization conditions: left-circularly polarized, right-circularly polarized, and linearly polarized. In each case, excitation was with a single beam of laser radiation directed along the unique axis of a uniaxial single-crystal sample. The results include relative intensities and polarization properties determined for transitions to each of the nine Stark levels split out of the 6P7/2 6P5/2, and 6P3/2 multiplets of Gd3+ (4f7). Experimental measurements are analyzed in terms of a modified Judd-Pooler perturbation model that explicitly incorporates spin-dependent interaction terms. Calculated versus experimentally determined ratios of circularly polarized to linearly polarized transition intensities are used as a diagnostic of the efficacy of the various calculational methods in rationalizing the experimental results. The calculations show some modest success in reproducing many of the qualitative features of the experimental results, but they fail to account for many of the quantitative details