Communication: State-to-state inelastic scattering of interstellar O₂ with H₂

Molecular oxygen (O₂) is predicted to be a major reservoir of elemental oxygen in dense interstellar molecular clouds. However, the abundance of O₂ derived from astronomical observations is much lower than expected. Solving the discrepancies between models and observations requires a review of the chemistry and collisional excitation of O₂ in space. In particular, O₂-H₂ collisions are crucial to derive O₂ abundance in space from the interstellar spectra. A crossed molecular beam experiment to probe the rotational excitation of O₂ due to H₂ collisions at energies of 650 cm^-1 is reported. Velocity map imaging was combined with state-selective detection of O₂( $$ ) by (2 + 1) resonance-enhanced multiphoton ionization. The obtained raw $$ images were corrected from density to flux and the differential cross sections (DCSs) were then extracted. Exact quantum mechanical calculations were also performed. Very good agreement between experimental and theoretical DCSs was found. The agreement demonstrates our ability to determine inelastic processes between O₂ molecules and H₂ both theoretically and experimentally and that the excitation of O₂ in the interstellar medium can be correctly modeled. Consequences on the astrophysical modeling are briefly evaluated.