Collision‐induced transitions between molecular hyperfine levels: Quantum formalism, propensity rules, and experimental study of CaBr(X ²Σ⁺)+Ar

The general quantum treatment of collisions of a ²Σ⁺ molecule with hyperfine structure is presented. The recoupling technique introduced by Corey and McCourt into the field of molecular collisions [J. Phys. Chem. 87, 2723 (1983)] allows us to represent hyperfine‐state‐resolved tensor opacities, and hence cross sections, in terms of the corresponding nuclear‐ and also electron‐spin‐free quantities. The formalism also predicts (independent of the dynamical limit) that the largest F→F^′ cross sections will be those for which ΔF=ΔJ, a rule well known for radiative transitions. Hyperfine‐state‐resolved scattering involving collisions of CaBr(X ²Σ⁺) with Ar is also studied here experimentally by electric quadrupole state selection and cw dye laser fluorescence detection. The relative final F^′ distributions were determined for the N=3,e→N=5,e and N=2,e→N=1,e collisional transitions. These results clearly exhibit the ΔF=ΔJ propensity rule. Moreover, the F^′ distributions were predicted with nearly quantitative accuracy using our previously determined CaCl(X ²Σ⁺)‐Ar tensor opacities. By contrast, the M_J‐ randomization model, first proposed to treat the influence of hyperfine structure in atomic collisions, is shown to disagree with both our experimental data and theoretical predictions.