M. Menéndez

Experimental and theoretical differential cross sections for the reactions Cl+H2/D2

Experimental and theoretical differential cross sections for the reactions between Cl atoms and two isotopic variants of molecular hydrogen (H2 and D2) are presented. The experimental results have been obtained by using the crossed molecular beam method with mass spectrometric detection. The theoretical results have been computed using both the quasiclassical trajectory and quantum mechanical (QM) methods. The potential energy surface employed for the calculations is the ab initio BW2 surface by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The theoretical results have been directly compared to the experiments in the laboratory frame at a collision energy (Ec) of 4.25 and 5.85 kcal/mol for the Cl+H2 reaction and of 4.9 and 6.3 kcal/mol for the Cl+D2 reaction. The agreement between QM results and experiment is quite satisfactory for the Cl+D2 reaction, especially for the low collision energy, while for Cl+H2 is less good, especially when considering data at the lower Ec.

A quantum mechanical and quasi-classical trajectory study of the Cl+H2 reaction and its isotopic variants: Dependence of the integral cross section on the collision energy and reagent rotation

Quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations have been performed for the Cl+H2, Cl+D2, Cl+HD→ HCl(DCl)+D(H) reactions in order to determine integral cross sections as a function of collision energy and for different reagent rotational quantum numbers using the recent ab initio BW2 potential energy surface (PES) by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The results are compared with experimental data obtained by using the Doppler-selected time-of-flight technique. It has been found theoretically by both the QM and QCT methods that reagent rotation enhances reactivity in agreement with experiment. The QM results are found to be in quantitative agreement with the experimental excitation functions for the Cl+p-H2 and Cl+n-H2 reactions, whereas those obtained quasi-classically fail to reproduce the experimental data. These results are in strong contrast with those reported on the previous G3 PES, in which QM and QCT calculations predicted that reactivity decreases with r...

Dynamics of the Cl+D2 reaction: a comparison of crossed molecular beam experiments with quasi-classical trajectory calculations on a new ab initio potential energy surface

Abstract The dynamics of the Cl+D2 reaction has been studied experimentally at the collision energies 4.9 and 6.3 kcal mol−1 by the crossed molecular beam technique. The experimental results have been compared with theoretical predictions based on quasi-classical trajectory (QCT) calculations on the new BW potential energy surface (PES), and a good general agreement has been obtained. The QCT results obtained on the BW PES have been compared with those obtained on the previous semiempirical G3 PES, and striking differences regarding the effect of reagent rotation j on the reactivity have been found.

QUASICLASSICAL TRAJECTORY STUDY OF THE LI + HF(V = 0) LIF + H REACTION

Abstract Quasi-classical trajectory (QCT) calculations for the Li+HF( v=0, j )→LiF+H reaction have been performed on a recent ab initio potential energy surface (PES). Integral and differential cross-sections, as well as angle–velocity polar maps, have been calculated at the collision energies and initial rotational states of HF( v=0, j=0 –3) relevant to the experiment of Becker et al. (J. Chem. Phys. 73 (1980) 2833). With these theoretical results, the laboratory angular distributions (LAB-AD) have been simulated and compared with experiment. The main features of the experimental LAB-AD and energy-dependent cross-section are qualitatively reproduced. In addition, the QCT total reaction cross-section as a function of the collision energy is compared with an approximate quantum mechanical calculation on the same PES.

Effect of rotational energy on the reaction Li+HF(υ=0,j)→LiF+H: An experimental and computational study

In a crossed molecular-beam study we have measured angular and time-of-flight distributions of the product LiF from the reaction Li + HF(upsilon = 0)-->LiF + H at various collision energies ranging from 97 to 363 meV for three markedly different rotational state distributions of HF obtained at nozzle temperatures close to 315, 510, and 850 K. Particularly, for the low and intermediate collision energies we observe significant effects of the varying j-state populations on the shape of the product angular distributions. At 315 K an additional feature appears in the angular distributions which is interpreted as being due to scattering from HF dimers. The experimental data are compared with simulations of the monomer reaction based on extensive quasiclassical trajectory calculations on a new state-of-the-art ab initio potential energy surface. We find an overall good agreement between the theoretical simulations and the experimental data for the title reaction, especially at the highest HF nozzle temperature.

Collision energy and product polarization effects in the Ca*(1D2) + HCL → CaCl*(A, B) + H reaction

The collision energy dependence of the Ca(1D2)+ HCl → CaCl(A)+ H reaction cross-section has been measured over the low-energy range using the time-of-flight technique under crossed-beam conditions. The excitation function shows a step-like functionality which seems to be of a non-classical nature. These steps could be associated with the opening of internal transition states that become available as the total energy increases.In addition beam–gas experiments have been carried out to measure the polarization of the CaCl*(B) emission to the ground electronic state. The collision-energy effects and the deviation from the kinematic limit of the product polarization have been also measured and they show a strong product rotational momentum alignment. The reaction stereodynamics are discussed with the aid of a dynamical model for kinematically constrained reactions. Finally a comparison of the present (full-collision) cross-beam reaction with the van der Waals (half-collision) photoinitiated reaction is also made.

Low temperature rotational relaxation of N2 in collisions with He

Abstract The rotational relaxation of N 2 in collisions with He has been investigated in free jets with a combination of resonance-enhanced-multiphoton ionization spectroscopy and time-of-flight techniques. From the measured data, a global cross-section for rotational relaxation has been derived. The cross section grows from a value of ≈8 A 2 at 100 K to ≈18 A 2 at 5–15 K. For the weakest supersonic expansions investigated, a breakdown of the translational equilibrium between He and N 2 has been observed, that could be well accounted for by an isentropic jet model and classical collision cross-sections.

Reaction Dynamics of Electronically Excited Calcium Atom

Absolute values of the total chemiluminescence cross-section for the beam-gas Ca(3P, 1D) + Cl4C → CaCl(A, B) + Cl3C and Ca(3P, 1D) + SF6 → CaF(A, B) + SF5 reactions have been measured at low collision energy, E T = 0.15 and 0.14eV, respectively. Both metastable atomic calcium states Ca(3P, 1D) were produced under low voltage dc-discharge conditions. By changing the discharge conditions, different metastable concentrations were produced to measure the state-to-state cross-section for both 3P and 1D reactions. The following values for the total chemiluminescence cross-sections were obtained: σ D 1 = 1.77 A and σ P 3 = 0.25 A for the Ca(3P, 1D) + Cl4C → CaCl(A, B) + Cl3C reaction. σ D 1 = 0.59 A2 and σ P 3 = 0.56 A2 for the Ca(3P, 1D) + SF6 → CaF(A) + SF5 reaction. σ D 1 = 0.04 A2 and σ P 3 = 0.12 A for the Ca(3P, 1D) + SF6 → CaF(B) + SF5 reaction.In addition, beam-beam experiments were carried out at the same average low collision energy that of the beam-gas, and therefore, normalization between both experiments was possible. This procedure allowed us to obtain the excitation function of the Ca(1D) + SF6 reaction in absolute values over the 0.15–0.60eV collision energy range.On the other hand, by simulation, the ratio of CaCl(B-X/A-X) emissions intensities was found to be of 0.15. The variation of this ratio with the relative concentration of 1D/3P in a Broida oven leads to the conclusion that this state favours the formation of the B state in the chemiluminescent Ca(3P, 1D) + CH3CHCl2 → CaCl(A, B) + CH3CHCl reaction.

Crossed‐beam (full collision) versus van der Waals (half collision) studies. Application to the determination of the D00(Ca...HCl)

The dissociation energy of the van der Waals molecule Ca...HCl in its ground state, D00(Ca...HCl), has been determined using energy balance arguments. By combining collisional information from the Ca(3P)+HCl→CaCl(A 2Π)+H reaction with spectroscopic information from the Ca...HCl+hν→CaCl(A 2Π)+H reaction we found D00(Ca...HCl)≤150 meV.

The influence of rotational energy on the cross-section of direct reactions: comparison of a simple model with classical trajectories☆

Abstract A simple dynamical model which incorporates angular momentum conservation has been developed to account for the rotational-energy dependence of the reaction cross-section in elementary chemical reactions. The model was tested against classical-trajectory calculations for the Na + ICH 3 and CH 3 + H 2 → CH 4 + H reactions, and it seems to be useful for the diagnosis and parametrization of the rotational-energy dependence of the reaction cross-section. A rationale of this rotational-energy dependence is presented and discussed in terms of the so-called adiabaticity parameter, i.e. the ratio between the collision time and the rotational period of the colliding particles.