Low temperature absorption spectra of Tm 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 (pi and sigma) 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.