We measure and theoretically determine the effect of molecular rotational splitting on Zeeman relaxation rates in collisions of cold ${}^3\Sigma$ molecules with helium atoms in a magnetic field. All four stable isotopomers of the imidogen (NH) molecule are magnetically trapped and studied in collisions with ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$. The ${}^4\mathrm{He}$ data support the predicted $1/B_e^2$ dependence of the collision-induced Zeeman relaxation rate coefficient on the molecular rotational constant $B_e$. The measured ${}^3\mathrm{He}$ rate coefficients are much larger than the ${}^4\mathrm{He}$ coefficients, depend less strongly on $B_e$, and theoretical analysis indicates they are strongly affected by a shape resonance. The results demonstrate the influence of molecular structure on collisional energy transfer at low temperatures.

• Received 2 April 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.013003