Observation of van der Waals resonances in low-energy F + H2(v = 0, j = 1) reaction.

Quantum resonances exert a crucial impact on chemical reactivity, but experimental evidence has so far been restricted to transition-state or product-channel features. Here we provide the first direct evidence of van der Waals (vdW) resonances in the entrance channel of a neutral molecular reaction. Using fully quantum-state resolved crossed molecular beam scattering, we studied the benchmark F + H2(v = 0, j = 1) reaction over collision energies from 4.0 to 26.4 cm-1. A pronounced forward scattering peak of the HF(v' = 2) product at 6.8 cm-1 reveals a resonance arising from quasi-bound states trapped in the entrance-channel vdW well, facilitated by centrifugal barriers. This signature is reproduced by quantum dynamical calculations on an open-shell diabatic potential energy surface, revealing partial wave resonances with total angular momentum (J) = 6.5-7.5 and demonstrating that spin-orbit coupling decisively shifts the energies of these quasi-bound states, shaping both the resonance position and scattering distribution. This combined experimental and theoretical study establishes a general quantum mechanism likely to influence a wide range of elementary reactions at low temperatures, including those relevant to interstellar chemistry and cold controlled systems.