Spin-flipping transitions in 2Σ molecules induced by collisions with structureless atoms

It is shown that spin-flipping transitions in low energy collisions of 2Σ diatomic molecules in the lowest rotational N=0 level with structureless atoms proceed through coupling to the rotationally excited N>0 levels, and are determined by the spin-rotation interaction in the rotationally excited molecule. We carry out calculations for collisions of CaH and 3He. For the spin-flip rate coefficient corresponding to an interaction potential that does not generate a shape resonance, we obtain 1.20x10-17 cm3 s-1 at a temperature of 0.4 K consistent with the measured value of 10-17±1 cm3 s-1. We make predictions as to which molecules should have the least probability of spin-flipping in collisions with atoms