Evidence for a spin-correlated crystal field - two-photon spectroscopy of thulium III in the elpasolite Cs2NaYCl6:Tm

We present two-photon excitation spectra for thulium III in the elpasolite Cs 2 NaYCl 6 :Tm. 37 out of the total of 40 crystal-field (CF) levels have been assigned, with the aid of a one-electron CF Hamiltonian, representing the most extensive data set so far reported for Tm III at a cubic site. Deviations from the predictions of the one-electron CF model are unusually large, the effective fourth-rank CF parameter being 60% larger for the singlet states than for the triplet states. This is discussed in terms of a spin-polarized covalency that is more pronounced in the singlet states of the metal ion.