Molecular beam experiments on collisions between oxygen molecules were performed at low energy and high angular resolution to permit observation of the ``glory'' interference effect. A novel technique for aligning the rotational angular momentum of the colliding molecules is exploited. Analysis of total scattering cross section data yields for the O2-O2 bond an energy of 1.65+/-0.08 kJ˙mol-1 for the most stable configuration (parallel molecules) at a distance of 0.356+/-0.007 nm. These results indicate that most of the bonding in the dimer comes from electrostatic (van der Waals) forces but chemical (spin-spin) contributions are not negligible.This work was supported by the Italian CNR and MURST, and the European Union (Contract No. ERG-FMRX-CT96-0088)Peer Reviewe
