Molecular beam experiments are reported for collisions between oxygen molecules. Total integral cross sections have been measured as a function of the collision energy with the control of molecular alignment. The low collision energy (in the thermal and subthermal range) and the high angular resolution permit observation of the “glory” effect, manifestation of quantum-mechanical interference, which allows an accurate probe of intermolecular interactions. This first complete experimental characterization of the interaction yields a ground (singlet) state bond energy of 17.0 ± 0.8 meV for the most stable dimer geometry (the two oxygen molecules lying parallel at a distance of 3.56 ± 0.07 Å). Also the splittings among the singlet, the triplet, and the quintet surfaces are obtained, and a full representation of their angular dependence is reported via a novel harmonic expansion functional form for diatom−diatom interactions. These results indicate that most of the bonding in the dimer comes from van der Waals forces, but chemical (spin−spin) contributions in this open-shell/open-shell system are not negligible (∼15% of the van der Waals component of the interaction).

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a b s t r a c tArrhenius plots, which are used to represent the effects of temperature on the rates of chemical and biophysical processes and on various transport phenomena in materials science, may exhibit deviations from linearity. Account of curvature is provided here by a formula which involves a deformation of the exponential function, of the kind recently encountered in treatments of non-extensivity in statistical mechanics. We present here examples on diverse topics -respiration rates of plants, speed of gliding of bacteria, quantum mechanical tunneling in a chemical reaction -illustrating the variety of possible applications and the additional insight that can be gained.

CommunicationsScattering cross sections for collisions of water and oxygen with the five noble gases reveal the onset of hydrogen bonds in water-noble-gas complexes, which increase in strength from He to Xe. Details on the determination of the strength of the interaction in excess of that expected for pure van der Waals forces are given by D. Cappelletti and co-workers on the following pages.