One example of molecular hyperfine structure in theΠ1state: Study of theBΠ1→XΣ1transition of the LuF molecule

The accurate measurements obtained with Fabry-Perot spectrometry allowed a detailed determination of the main hyperfine interactions in the (0-0) $B{}_{}{}^1{}_{}{}^{}\Pi \to X{}_{}{}^1{}_{}{}^{}\Sigma$ transition of the LuF molecule. The corresponding parameters are $Q_{0,0}=-35\mathrm{}\pm 5$ mK, $Q_{1,1}=-30\mathrm{}\pm 4$ mK, $Q_{1,-1}=-34\mathrm{}\pm 3$ mK, $G_{\mathrm{Lu}}=53\mathrm{}\pm 3$ mK, and $G_{\mathrm{F}}\le 7$ mK. Then it is shown that these parameters give important information on the LuF bond and its origin: high ionicity (> 95%), high $c$ tendency for the excited ${}_{}{}^1{}_{}{}^{}\Pi$ state, probable atomic origin of molecular configurations: $6s6p\left(\mathrm{Lu}\mathrm{II}\right)\to \left(6s\sigma \right)\left(6p\pi \right)$ and $6s^2\left(\mathrm{Lu}\mathrm{II}\right)\to {\left(6s\sigma \right)}^2$.