Rotational alignment of the product BaO in the crossed-beam reaction Ba+SO2 → BaO+SO was studied as a function of collision energy. The collision energy was varied from 3.0 to 8.0 kcal/mol by changing the stagnation condition of the supersonic SO2 beam. The rotational alignment was probed by means of the polarized laser-induced fluorescence (PLIF) of the product. The distribution of the rotational vector was isotropic at the collision energy of 3.0 kcal/mol, but anisotropic at a higher collision energy, with the most probable alignment being perpendicular to the relative velocity vector. These results indicate that the reaction proceeds via a long-lived complex at a low collision energy, whereas at a higher collision energy, the lifetime of the complex is short enough to couple the rotational angular momentum to the total angular momentum.
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