Rare earth crystal field spectra as a probe of librational motions in BaY₂F₈ solid state laser crystals

The fine structure (FS) accompanying a few lines, originated by crystal field (CF) transitions of rare earths (RE), as Er³⁺ and Tm ³⁺, in BaY₂F₈ single crystals, is analyzed as a function of the RE³⁺ concentration (0.520 at%) and temperature (9-300 K), by using high resolution (as fine as 0.02 cm^-1) Fourier transform spectroscopy and linear dichroism measurements. The 9 K absorption spectra show that FS includes weak, narrow, and closely spaced (0.40.8 cm ^-1) lines, covering a few cm^-1 range on both sides of the narrowest among the CF lines. The FS increases by increasing the RE³⁺ concentration and vanishes at rather low temperature (40 and 60 K for Er ³⁺ and Tm³⁺, respectively). The polarized light spectra confirm the association of a given set of FS lines to a specific CF line. The FS is ascribed to the simultaneous excitation of an electronic CF transition and of a local librational (or hindered rotation) mode of the RE³⁺-F ^- group. The attribution is supported 1) by specific features of the host matrix and guest rare earths, which allow some mobility of F^- ions, and 2) by the spacing of the FS lines, which is in excellent agreement with the calculated RE³⁺-F^- group rotational constant.