We present calculations of the Zeeman effect of narrow photoassociation lines of $^{ 40}$Ca near the $^3$P$_1$ + $^1$S$_0$ asymptote. Using a coupled-channel model we find a nonlinear Zeeman effect that even at low fields of a few mT amounts to several kHz. With this model we analyze previous measurements and give corrected long range dispersion coefficients of the $^3\Pi_{ u}$ and $^3\Sigma^+ _{ u}$ states.
Categories: physics.atom-ph cond-mat.quant-gas