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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.

We report molecular-beam measurements of the total integral cross sections for the scattering of water molecules by helium atoms. A combined analysis of the new experimental data together with available differential cross section results has allowed an accurate determination of the isotropic component of the interaction potential for this prototypical system. The potential well shows a depth of 0.265 +/- 0.010 kJ/mol at a distance between He and the center of mass of the water molecule of 0.345 +/- 0.02 nm. An effective isotropic long-range attraction constant C(LR) = (6.3+/-0.3) x 10(-4) kJ mol(-1) nm(-6), including both dispersion and induction contributions, has also been determined. The most recent and accurate ab initio potential-energy surfaces have been tested against these new experimental results.

The characterization of the non covalent interaction potential, responsible for the intermolecular bond in NO-N(2) and (NO-N(2))(+) molecular aggregates, has been achieved by coupling the predictions of a semiempirical method with the results of a scattering experiment and ab initio calculations. The potential wells for the most stable configurations of the neutral and ionic state, having approximatively a T shape in both cases, fall in the same intermolecular distance range. In addition, in the ionic state, the charge is completely localized on the NO partner. Important implications on the dynamics of the neutralization process, occurring as a vertical transition from ionic to neutral state, are obtained by exploiting the analytical formulation of the interaction and calculating energy spacings and relevant Franck-Condon factors for both intramolecular and intermolecular vibration modes.

The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl(2+) and Cl(2+) product ions have been detected by a photoelectron-photoion-coincidence technique, while the H(+)+Cl(+) formation, which follows the double ionization of HCl, has been studied by photoelectron-photoion-photoion coincidence. The photon energy threshold for the production of HCl(2+) ions has been found to be 35.4+/-0.6 eV, while for the dissociative channel leading to H(+)+Cl(+), it has been measured a threshold at 36.4+/-0.6 eV and a change in the slope of the cross-section energy dependence at 38.7+/-0.7 eV. The production of H+Cl(2+) occurs with a threshold photon energy of 42.8+/-1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory.