Spectral signatures of hyperfine and isotopic effects and of Tm3+-Tm3+ pairs in LiYF4:Tm

We report on the high-resolution optical Fourier-transform spectroscopy of the LiYF4:Tm3+ crystals. Splitting of several lines in the optical low-temperature polarized spectra was observed. We show that these splittings are caused by (i) the hyperfine interaction, (ii) the isotopic disorder in the lithium sublattice, and (iii) the interionic interaction between neighboring Tm ions. It is the first observation of the hyperfine splitting in the spectra of the Tm3+ ions in crystals. From the experimentally measured hyperfine splitting we evaluate the magnetic field at the thulium nucleus and calculate the magnetic g-factors of the excited crystal-field levels