Optical absorption spectrum and local symmetry of Tm3+ ion in TmAl3(BO3)4 crystal

With the help of group theory analysis of absorption spectra of the transition 3H6 → 3F3 of Tm3+ ion in TmAl3(BO3)4 crystal, measured at several temperatures from 1.8 till 293 K, it has been shown, that the local symmetry of the Tm3+ environment is C3C3 and it decreases to C1C1 at a low temperature. Effective selection rules and polarizations of lines at high enough temperatures (when the line-width is larger than the splitting in C3C3 symmetry) have been obtained.