Anomalous Spin-Rotation Coupling in theX2Σ+State of YbF

We have measured the spin-rotation and hyperfine structure in the ground state of the molecular radical YbF using Doppler-free laser spectroscopy in a molecular beam. The spectrum reveals that the spin-rotation constant $\gamma$ of the $X^2{\Sigma }^{+}$ $\left(\nu =0\mathrm{}\right)$ state is exceptionally small. At the same time, the centrifugal distortion is so enormous that $\gamma$ changes sign at rotational quantum number $N=60\mathrm{}$. To the best of our knowledge, this behavior is unprecedented.