Local symmetry distortions during f-f transitions in HoAl₃(BO₃)₄ single crystal

The polarized absorption spectra of the HoAl₃(BO₃)₄ single crystal were studied in the region of f–f transitions ⁵I₈→⁵F₅, ⁵S₂ + ⁵F₄, ⁵F₃ and ⁵F₂ inside the 4f shell of the holmium ion as a function of temperature in the range of 5–90 K. In a number of absorption lines the splitting which are not provided by the local point group D₃ symmetry of holmium ion in the crystal are found. This phenomenon is explained by local changes in the structure in both the excited and ground states of holmium under the influence of a perturbation caused by a photon. This perturbation mixes the ground and excited states and changes the interaction of the atom with its environment. It was also shown that the magnitude of the splitting depends on the polarization of the incident light and on the temperature of the sample.