L. Beneventi

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...

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...

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.

Improved potential energy surface for He–CO2

The anisotropic potential energy surface of He–CO2 is determined by the simultaneous analysis of newly measured high resolution total differential cross sections, differential energy loss spectra, new low temperature second virial coefficients, new diffusion, and viscosity data. The calculations are carried out in the infinite‐order‐sudden approximation. The repulsive anisotropy of the potential is determined from the rotationally inelastic cross sections and the quenching of the diffraction oscillations, while the absolute scale is fixed by the position of these oscillations. The second virial coefficient data are essentially sensitive to the general features of the spherical effective potential well. The transport data are then correctly predicted by this potential surface which differs both in the anisotropy and the spherical part from the recently derived multiproperty fit potential for this system.

The HeCl2 potential: A combined scattering‐spectroscopic study

Total differential scattering angular distribution measurements for He scattering from Cl2 are reported. The scattering data are combined with previous excitation spectroscopy and vibrational predissociation product state distributions for He–Cl2 to determine the potential‐energy surfaces for the ground X and excited B electronic states. The potentials are somewhat deeper, De=−38.2 cm−1 for the X state, than previously estimated. The X state potential minimum is for the ‘‘T’’ shaped configuration with a Rmin value of 3.55 A. The potentials are quite successful at describing the scattering data, the rotationally resolved spectroscopy of the complex, and the product state distributions for vibrational predissociation. The B state potential underestimates the dependence of the rate of vibrational predissociation on the initial vibrational level.

The Ne–N2 potential energy surface from high‐resolution total differential scattering experiments and a close‐coupling and infinite‐order‐sudden analysis

High‐resolution crossed molecular beam measurements of the total differential cross section (DCS) for the scattering of Ne by N2 at a collision energy of E=75.8 meV are reported. Strongly quenched diffraction oscillations superimposed on the falloff of the main rainbow structure are clearly resolved. A reliable Ne–N2 potential energy surface (PES) is derived by simultaneously fitting second virial, diffusion, and viscosity coefficient data taken from the literature. Information on the anisotropy of the interaction is obtained from the quenching of the diffraction oscillations in the total DCS within the framework of the infinite‐order‐sudden (IOS) approximation. The reliability of the IOS approximation in deriving a fully anisotropic potential energy surface from the measured scattering dynamics is examined and demonstrated by performing exact close‐coupling calculations for the present experimental conditions and then comparing both integral and differential total and rotationally inelastic cross section...

The effect of reagent electronic energy on the dynamics of chemical reactions: A high‐resolution crossed beam study of O(3P,1D)+H2S

Crossed beam reactive scattering studies of the H displacement reaction of both ground 3P and excited 1D oxygen atoms with H2S show that the reaction dynamics changes dramatically upon electronic excitation: while the reaction of O(3P) is direct, that of O(1D) proceeds via a long‐lived complex.

On the N2–He potential energy surface

A detailed comparison of the predictive powers of two recently determined empirical and two recently proposed theoretical potential energy surfaces for the N2–He interaction has been carried out. In particular, the following properties have been tested: at the microscopic level, total and state‐to‐state differential cross sections and absolute total integral cross sections, while at the macroscopic level, interaction second virial, diffusion, viscosity, and thermal conductivity coefficients, as well as the rotational relaxation time, depolarized Rayleigh spectral collision broadening, and shear viscosity and thermal conductivity field‐effect data in N2–He mixtures. Exact calculations have been employed, from the close‐coupling method for treating scattering data at low energies to the classical trajectory method with second‐order corrections to compute the effective cross sections that determine the bulk transport and relaxation phenomena.The empirical exponential‐spline–Morse‐spline–van der Waals surface...

Coupling schemes for atom–diatom interactions and an adiabatic decoupling treatment of rotational temperature effects on glory scattering

The quantum mechanical theory for scattering of a particle by a rigid rotor is formulated in five alternative diabatic representations, corresponding to alternative coupling schemes. Use is made of a recently introduced procedure for obtaining discrete representations by artificial quantization. In order to develop an efficient computational scheme for obtaining information on the interaction potential from atom–diatom scattering experiments, decoupling approximations are developed. An adiabatic representation in the coupled states framework is applied to the computation of integral cross sections and nonadiabatic coupling effects are analyzed. The approach provides an accurate description of the experimentally observed dependence of glory scattering from the rotational temperature of the diatom.

The Ne-O2 potential energy surface from high-resolution diffraction and glory scattering experiments and from the Zeeman spectrum

The full anisotropic potential energy surface for the Ne–O2 van der Waals molecule has been obtained from the simultaneous analysis of total differential cross section data with well resolved diffraction oscillations and of absolute total integral cross section data with glory structure measured in Perugia, of the Zeeman spectrum measured in Nijmegen, and of transport coefficients taken from literature. The anisotropy of the interaction is obtained from the Zeeman spectrum using both numerical and analytical approaches for the energy levels, and from the quenching of the diffraction oscillations in the total differential cross section within the infinite‐order‐sudden approximation. The latter approximation is practically exact under the experimental conditions, as previously shown for the similar Ne–N2 system by comparison with exact close‐coupling scattering calculations. The derived potential energy surface represents a dramatic improvement with respect to a previous approximate estimate.