Undergrad Is the Spin-Rotation Constant Always Positive in Diatomic Molecules?

[Malamala]

Hello! Can the spin-rotation constant, $\gamma$, have a negative value in a diatomic molecule, or does it have to be positive (in my case the state is a ground state $^2\Sigma_+$, which is about 14000 cm$^{-1}$ away from the next electronic level). I don't think I have ever seen negative values for $\gamma$ in the papers I came across, but I am also not sure if there is a reason for it to be always positive. Thank you!

 

[Herzberg137]

Hello! Sure it can, see for example the YbF and YbOH molecules that are being pursued for precision measurements. In a lot of cases, the spin-rotation constant is mostly determined by a second order perturbation interaction involving spin-orbit and rotational operators. This can be either positive or negative depending on the sorts of states involved.