Velocity selection and mass spectrometric detection of an H2S molecular beam and a collisional study of its interactions with rare gases

Total integral cross section experiments, for collisions of rotationally hot H₂S projectile molecules by He and Xe targets, have been performed in the thermal energy range by using a molecular beam apparatus, which operates under high resolution conditions and exploits a quadrupole mass spectrometer as a detector. Information on the radial dependence of the intermolecular interaction, averaged over all relative orientations of colliding partners, is obtained. This study completes the investigation of H₂S–rare gas family [see D. Cappelletti, A.F.A. Vilela, P.R.P. Barreto, R. Gargano, F. Pirani, V. Aquilanti, J. Chem. Phys. 125 (2006) 133111, for Ne, Ar and Kr] and permits us to discover similarities and differences with the analogous H₂O–Rg series of systems. In particular, the analysis of the scattering data suggests that while the complexes of H₂S–rare gases from He to Kr are essentially bound by nearly isotropic noncovalent interactions of van der Waals type, the H₂S–Xe system exhibits a weak, but measurable, additional component which emerges at intermediate intermolecular distance (in proximity of the potential well), and provides a bond stabilization. This observation is of possible interest for the investigation of the phenomenology of the hydrogen bond formation.     

Stereodynamic effects in the adsorption of propylene molecules on Ag(001)

We report on the experimental evidence of the role of rotational alignment of the gas-phase molecules in the interaction of propylene with Ag(001). Molecular alignment has been controlled by a velocity selection of the impinging molecules, flying in a supersonic seeded molecular beam. The experimental findings indicate that at low surface coverage the sticking probability is independent of molecular alignment, while when coverage exceeds few percent of a monolayer, molecules impinging rotating parallel to the surface (helicopter-like configuration) achieve a higher chance to be trapped than those which impinge rotating perpendicularly (cartwheels). The sudden appearance of a large stereodynamic effect suggests that the adsorption proceeds via a mobile precursor state and is tentatively correlated with a change in the configuration of the added propylene molecules, which adsorb tilted rather than flat at the surface.     

Intermolecular interaction potentials for the Ar-C2H2, Kr-C2H2, and Xe-C2H2 weakly bound complexes: information from molecular beam scattering, pressure broadening coefficients, and rovibrational spectroscopy

Integral cross sections and pressure broadening coefficients have been measured for the acetylene-krypton complex, by molecular beam scattering and by high resolution IR spectroscopy, respectively. A new potential energy surface (PES) is proposed to describe structure and dynamical properties of this prototypical weakly bound complex. The PES has been parametrized exploiting a novel atom-bond pairwise additive scheme and has been fitted to the experimental data. A similar PES has been obtained for the acetylene-xenon system by a proper scaling of the interaction parameters of the krypton case, based on empirical considerations. These PESs together with that recently proposed by the same authors [J. Phys. Chem. 109, 8471 (2005)] for the acetylene-argon case have been employed for close coupling calculations of the pressure broadening cross sections and for a characterization of the rovibrational structure of the complexes.     

The Different Story of π Bonds

We revisit “classical” issues in multiply bonded systems between main groups elements, namely the structural distortions that may occur at the multiple bonds and that lead, e.g., to trans-bent and bond-length alternated structures. The focus is on the role that orbital hybridization and electron correlation play in this context, here analyzed with the help of simple models for σ- and π-bonds, numerically exact solutions of Hubbard Hamiltonians and first principles (density functional theory) investigations of an extended set of systems.