Mechanism of Collisional Spin Relaxation in \(^3\)Σ Molecules

Abstract

We measure and theoretically determine the effect of molecular rotational splitting on Zeeman relaxation rates in collisions of cold (^3)Σ 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)He and (^4)He. The (^4)He data support the predicted 1/B(^{2}_{e}) dependence of the collision-induced Zeeman relaxation rate coefficient on the molecular rotational constant B(_e). The measured (^3)He rate coefficients are much larger than the (^4)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.