  We measure and theoretically determine the effect of molecular rotational
splitting on Zeeman relaxation rates in collisions of cold Triplet-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 3He and 4He. The 4He data support the predicted inverse square dependence
of the collision induced Zeeman relaxation rate coefficient on the molecular
rotational constant B. The measured 3He rate coefficients are much larger than
4He and depend less strongly on B, and the 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.

Atomic Physics