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 O 2 -O 2 bond an energy of 1.65 6 0.08 kJ ? mol 21 for the most stable configuration (parallel molecules) at a distance of 0.356 6 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. [S0031-9007(98) The interactions of molecular oxygen are of vital importance in a number of fields such as combustion and, particularly, atmospheric physics and chemistry. Despite continuing efforts, some of the processes linked to the release and recombination of oxygen in its various forms are still far from being properly understood. Among these, those involving the oxygen dimer ͑O 2 ͒ 2 are particularly interesting both for the peculiar nature of the bond [1,2] and for their relation to the atmospheric ozone and atomic oxygen balance, through the highly endothermic reaction [10]. For the latter, three different phases, two of which are paramagnetic, are known but their modeling is not fully understood. Finally, spectra of the dimers show several weak absorption bands [11]. They are observed in the atmosphere and in oxygen under pressure or as a liquid and their assignment is far from complete. Some occur in the same wavelength range as the Chappuis bands of ozone, hence affecting measurements of stratospheric ozone [11]. The intermolecular potential in the oxygen dimer continues to pose a challenge to the theory of weak chemical bonds, beyond van der Waals forces. Indeed, because of the open shell nature of the oxygen molecules in their 3 S 2 g ground electronic state, the interaction in the dimer depends not only on the intermolecular distance and on the relative orientation between two molecules, but also on the coupling of their spins [12]. This originates a singlet ground potential energy surface and two excited ones, of triplet and quintet character.Ewing et al. [13] were the first to reveal clearly the presence of ͑O 2 ͒ 2 dimers in the gas phase, by spectroscopic studies at low temperature in the IR and visible range. From the analysis of the collision induced absorption spectrum they concluded that the dimer, a floppy molecule, is stabilized both in the ground and the excited electronic state [dissociating to O 2 ͑ 1 D͒ 1 O 2 ͑ 1 D͒] by a weak bond of van der Waals nature with well depths of 1.0 and 0.6 kJ ? mol 21 , respectively. They also give an equilibrium distance of ϳ0.35 nm tentatively associated with a parallel (H-like) geometry but provide no information on spin coupling and on the dependence of the interaction on the mutual orientation of the molecules.As for the magnetic properties of oxygen dimers in the gas phase an early study [14] of Stern-...

Experimental investigations on the collisional alignment of the rotational angular momentum, occurring in supersonic seeded beams and in drift tubes, have recently documented a strong dependence of the observed effects on the final molecular velocity. The present investigation aims at elucidating the possible mechanisms at the molecular collision level. Quantum state-to-state differential scattering cross sections, calculated for the prototype system O2–He, for an interaction potential previously obtained in this laboratory, exhibit propensities relevant to reveal nature and selective role of the elastic and inelastic scattering events, participating in the overall mechanisms which lead to molecular alignment and cooling. The present analysis shows that the dynamics of such phenomena crucially depends on the initial and final rotational state, on the collision energy, on the involved orbital angular momentum and therefore alternative routes are possible for molecular polarization and relaxation. These routes lead to scattering into specific angular cones and therefore observations from different experiments provide complementary pieces of information which, exploiting studies of various molecular systems under diverse experimental conditions, can be correlated in a single mosaic.

By using the time-of-flight technique under crossed-beam conditions, the collision energy dependence of the Ca(1D) + HCl → CaCl(A) + H reaction cross-section has been measured over the 0.2−0.5 eV collision energy range and found to have a broad peak at 0.3 eV. An empirical description of entrance covalent channelsincluding orbital alignment effects due to the open-shell nature of calcium atomsand a harpooning mechanism, involving ionic charge-transfer states, account for the observed translational energy dependence of the reaction cross section.

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