Mechanism of Collisional Spin Relaxation in (3)Sigma Molecules

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 He-3 and He-4. The He-4 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 He-3 rate coefficients are much larger than the He-4 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.