Two-photon excitation spectroscopy of lanthanide elpasolites - Implications for the correlation crystal field

In centrosymmetric sites most f-f optical transitions of lanthanide ions are allowed by two-photon selection rules. Polarisation data permit an unambiguous determination of excited state symmetries, and these may be related to crystal field calculations. Ambiguities can often be resolved by the Zeeman effect, using both experimental g-values and, in second order, the appearance of forbidden transitions. Together with emission spectroscopy, these techniques can establish enough data to explore the origin of the correlation crystal field (CCF). Here we illustrate the utility of two-photon spectroscopy in the terbium(III) elpasolites Cs₂NaTbX₆ (X = F, Cl).