Absorption Spectrum of Tm3+:LaF3

The absorption spectrum of Tm³⁺:LaF₃ was measured in the region 0.2–2.0 $\mu$ which encompasses all but one of the excited states in the ${\text{4f}}^{12}$ configuration. Centers of gravity of the observed crystal‐field components of each level were computed and used as the basis for determining the free ion energy level parameters. In addition to the electrostatic and spin–orbit interactions, the effects of configuration interaction and certain two‐body magnetic interactions were considered. The theoretical analysis of the results yielded an excellent agreement between calculated and observed free ion levels.