Gian Gualberto Volpi

Crossed beam studies of four‐atom reactions: The dynamics of OH+CO

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center‐of‐mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty‐two percent of the available energy is released as product translational energy.

The dynamics of the reaction OH + D2 → HOD + D: Crossed beam experiments and quantum mechanical scattering calculations on ab initio potential energy surfaces

Abstract The dynamics of the reaction OH + D 2 → HOD + D has been investigated in a crossed molecular beam experiment at a collision energy of 6.3 kcal/mol. From laboratory product angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were determined. The HOD angular distribution is strongly backward scattered (with respect to the OH direction), which reflects a direct rebound dynamics, and the average fraction of total available energy released into translation is 0.34, indicating a high degree of product internal excitation. These results are compared with the predictions of two different ab initio potential energy surfaces within quantum mechanical scattering calculations based on the rotating bond approximation. It is concluded that a new surface, obtained from large scale high-quality electronic structure calculations, represents a significant improvement with respect to the previous Walch-Dunning-Schatz-Elgersma potential energy surface.

High‐resolution total differential cross sections for scattering of helium by O2, N2, and NO

High‐resolution crossed molecular beam measurements of the total differential cross sections (DCS) for the scattering of He by O2, N2, and NO in the thermal energy range are reported. The data show well resolved diffraction oscillations which appear damped with respect to the corresponding isotropic He–Ar case. Information on the anisotropy of the interaction is indirectly obtained from the damping of the diffraction oscillations within the framework of the infinite‐order‐sudden (IOS) approximation. While large discrepancies, within the same scheme of analysis, are found with respect to anisotropy estimates from similar experiments, but performed at a lower resolution, good agreement is observed with respect to those obtained for He–O2 and He–N2 from state‐to‐state rotationally inelastic DCS measurements. Potential energy surfaces (PES) are derived for all systems by also simultaneously fitting absolute total integral cross sections and second virial coefficient data. Comparisons are made with previous bo...

Molecular beam studies of weak interactions for open‐shell systems: The ground and lowest excited states of ArF, KrF, and XeF

Absolute integral cross sections for scattering of ground state fluorine atoms by argon, krypton, and xenon have been measured in the thermal velocity range. Information has been obtained on the long range interaction and using a technique for magnetic analysis of substates of F atoms, a characterization is given for the bonding in the ground and the two lowest excited states of these rare gas fluorides. The potentials are represented as a spherical part and an anisotropic component, which have been obtained in an adiabatic decoupling treatment, including also information from other scattering data. Nonadiabatic coupling matrix elements and other general features of these interactions are also presented.

Crossed molecular beams and quasiclassical trajectory studies of the reaction O(1D)+H2(D2)

The dynamics of the reactions O(1D)+H2→OH+H and O(1D)+D2→OD+D have been investigated in crossed molecular beam experiments with mass spectrometric detection at the collision energies of 1.9 and 3.0 kcal/mol, and 5.3 kcal/mol, respectively. From OH(OD) product laboratory angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were derived. The angular distributions are nearly backward–forward symmetric with a favored backward peaking which increases with collision energy. About 30% of the total available energy is found to be channeled into product translational energy. The results are compared with quasiclassical trajectory calculations on a DIM (diatomic-in-molecules) potential energy surface. Related experimental and theoretical works are noted. Insertion via the 1 1A′ ground state potential energy surface is the predominant mechanism, but the role of a second competitive abstraction micromechanism which should evolve on one of (or both) the ...

Dynamics of the Simplest Chlorine Atom Reaction: An Experimental and Theoretical Study

Angular distributions and time-of-flight spectra for the reaction Cl + H2 → HCl + H obtained from a high-resolution, crossed-molecular beam experiment were compared to differential cross sections calculated by both converged quantum mechanical scattering and quasi-classical trajectory methods. Good agreement was found between the experimental results and each theoretical prediction. The results demonstrate that excellent agreement can be obtained between state-of-the-art simulations and experiments for the detailed dynamical properties of this prototype chlorine atom reaction.

Multiproperty determination of a new N2–Ar intermolecular interaction potential energy surface

A new multiproperty potential energy surface for the N2–Ar intermolecular interaction is reported. The present determination is based upon molecular beam total differential and integral scattering data, taken together with the temperature dependence of the interaction second virial coefficient, transport properties, transport property field effects, and relaxation phenomena, such as pressure broadening of the depolarized Rayleigh line and longitudinal nuclear spin relaxation. The primary fit has been made to the beam scattering and virial data, and refinements to the potential parameters thus determined have been made by employing the data available for the gas phase transport and relaxation phenomena. The potential energy surface employed is an empirical Morse–Morse–spline–van der Waals form, in which the potential parameters depend upon the angle between the N2 figure axis and the line joining the centers of mass of N2 and Ar. No N2 stretching dependence has been included in the present determination. C...

Comparative dynamics of Cl(2P) and O(3P) interactions with a hydrocarbon surface

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbon liquid, squalane, were investigated and compared to the results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as a function of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCl time-of-flight (from the surface to the detector) distributions revealed thermal and hyperthermal interaction channels, in analogy to the dynamical behavior of the O and OH signals observed in the previous study. The thermal HCl product may arise from two mechanisms: (i) desorption of trapped HCl product and (ii) reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O atoms with squalane led to a thermal OH signal, which could only come from desorption of trapped OH. The hyperthermal HCl s...

Dynamics of the reaction O(1D) + HCl → ClO + H from crossed-beam experiments

Abstract The angular velocity distribution of the ClO product from the reaction O( 1 D) + HCl at 12.2 kcal/mol collision energy has been obtained in a crossed-molecular-beam study. The product center-of-mass angular distribution is found to be almost backward—forward symmetric, with backward scattering being slightly favored, from which it is deduced that part of the reaction occurs via a long-lived complex and part via direct abstraction of the halogen atom. The derived large fraction (≈ 43%) of energy released into translation indicates the existence of a barrier in the exit channel. A lower limit of the branching ratio between ClO + H and OH + Cl channels is derived and is compared to recent bulk results.

ION-MOLECULE REACTIONS IN HYDROGEN-RARE-GAS MIXTURES

Ion—molecule reactions in rare‐gas—hydrogen systems have been studied by high‐pressure mass spectrometry, using a specially designed ion source which employs the beta rays of tritium as ionizing medium. The rate constants for reactions of H3+ with rare gases and of rare‐gas ions with hydrogen have been measured. The results are in qualitative agreement with radiolysis experiments on hydrogen—deuterium exchange, and if a quantitative comparison is attempted, it is possible to estimate the rate constant of the chain‐propagation reaction. Experiments are also reported for N2–H2 and O2–H2 mixtures.