Low temperature absorption spectra of Tm³⁺ ion in TmAl₃(BO₃)₄ crystal

Polarized absorption spectra in the region of electron transitions ³H₆ → ³H₅, ³H₄, ³F₃, ³F₂, ¹G₄, and ¹D₂ of Tm³⁺ ion in TmAl₃(BO₃)₄ single crystal were investigated at different temperatures from 1.8 to 293 K at two light polarizations (π and σ) relatively to the C₃ crystal axis. Polarization properties of the absorption bands have been shown to be inconsistent with the generally accepted idea on the D₃ local symmetry of Tm³⁺ ion in this crystal. With the help of the group theory analysis it has been shown, that the local symmetry of the Tm³⁺ environment is C₃ and it decreases to C₁ at a low temperature. Effective selection rules and polarizations of lines at enough high temperatures (when the line-width is larger than the splitting in C₃ symmetry) have been obtained. There were found absorption lines having small width at high temperatures, what is of importance for the realization of effective laser generation.