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 in in the elpasolite C s2NaYCl6: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 m ost extensive data set so far reported for Tm in at a cubic site. Deviation s from the predictions of the one-electron CF model are unusually large, th e effective fourth-rank CF parameter being 60% larger for the singlet state s than for the triplet states. This is discussed in terms of a spin-polariz ed covalency that is more pronounced in the singlet states of the metal ion .