Direct observation of hyperfine level anticrossings in the optical spectra of a LiYF47:Ho3+ single crystal

We have investigated the hyperfine structure in high-resolution broadband optical absorption spectra of a strongly diluted paramagnet $^{7}\mathrm{LiYF}_{4}:{\mathrm{Ho}}^{3+}$ (0.1 at. %) in external magnetic fields parallel to the tetragonal symmetry axis of the crystal. Anticrossings in the hyperfine spectral pattern were observed. It is the first direct observation of the hyperfine level anticrossings in optical spectra. Analysis of the spectral envelopes corresponding to transitions between the electron-nuclear sublevels of crystal-field non-Kramers doublets and singlets of the ${\mathrm{Ho}}^{3+}$ ions, based on the microscopic model of the electronic $4{f}^{10}$ configuration, allowed us to retrieve information on the hyperfine structure of electronic singlets, nuclear quadroupole interactions, and random lattice strains.