Unexpected Collisional Rotational Excitation via Long-Range Capture and Orbiting

Collisional rotational excitation is a fundamental process in many gaseous environments. The textbook hard-sphere model stipulates that high rotational excitation results from head-on collisions, leading primarily to backward scattering, whereas long-range glancing collisions in the forward direction are inefficient for rotational energy transfer. Here, we report rotational state resolved product imaging for a system with strong attractive interaction, the charge-transfer collision between spin-orbit selected Ar+(2P3/2) ions and para/ortho-H2 molecules. Surprisingly, the H2+ products are rotationally excited and dominated by forward scattering, in sharp contrast to conventional wisdom. Quantum dynamical calculations on a first-principles diabatic potential energy matrix reproduce the observations. Trajectory surface hopping analysis further reveals that rotational excitation occurs mostly with large impact parameters, and the captured complex undergoes orbiting motion owing to the strong attractive interaction between the two collision partners before they break up. This novel mechanism should be general for collisional systems featuring strong attractive interactions, which undermine the hard-sphere assumption.