E. Verdasco

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.

REACTION DYNAMICS OF TRANSLATIONAL AND ELECTRONIC EXCITATION IN Ca + SF6 COLLISIONS

Relative values of the total chemiluminescence cross.section for the crossed molecular beam Ca∗

Reaction cross sections by time profile measurements under crossed‐beam conditions: The Ca(3P,1D)+N2O→CaO*+N2 reaction

The collision energy dependence of the Ca(3P,1D)+N2O→CaO*+N2 reaction has been determined by time profile measurements under crossed‐beam conditions. A narrow velocity distribution N2O beam collides with a pulsed beam of excited metastable Ca(3P,1D) atoms, produced by low‐voltage discharge. Time‐of‐flight spectra were used to monitor both the reagent translational and the chemiluminescence yield of the CaO* product (green arc band). The chemiluminescence cross section for excitation of the CaO green arc bands decreases with increasing translational energy with 1D reagent, but increases for the 3P state. The combination of a narrow velocity distribution of the supersonic N2O beam with short pulses of the Ca* beam leads to collision energy resolution of the order of magnitude of a few meV. This could be important not only for determining high resolution reaction thresholds, but perhaps for the onset of quantum effect in the total reaction cross section.

Experimental and theoretical study of the Li+HF (v=1)→LiF+H reaction

Laboratory angular distributions (LAB ADs) have been measured for the Li+HF (v=1, j=1) reaction in a crossed molecular beam experiment at the collision energies 0.231 eV and 0.416 eV and compared with the results of extensive quasi-classical trajectory (QCT) calculations performed on the most recent ab initio potential energy surface (PES) for this system. The calculations also include the collision energy dependence of the integral and differential cross sections in the range 0.025–0.5 eV (2.4–48.2 kJ mol−1). In particular, the total QCT integral reactive cross sections have been found to be in very good agreement with recent quantum mechanical (QM) calculations carried out on the same PES by Lara et al. (J. Chem. Phys., 1998, 109, 9391). In addition, the triple scattering angle–recoil velocity differential cross section has been calculated in order to simulate the experimental LAB AD. An excellent concordance between both sets of data has been found, indicating that the reaction of Li with HF in v=1 can be very well described by QCT calculations on the mentioned PES.

Reaction cross-section and product polarization in the Ca(1D2) + HBr → CaBr(A,B) + H reaction☆

Abstract The Ca ( 1 D 2 ) + HBr → CaBr(A,B) + H reaction has been studied by measuring its chemiluminescence at an average collision energy of ET = 0.16 eV under beam-gas conditions. Absolute values of the reaction cross-section for both chemiluminescence channels (e.g., A and B electronic states) were found to be σ A = 2.2 ± 0.9 A 2 and σ B = 0.52 ± 0.24 A 2 . In addition, the product polarization of the chemiluminescence emission was also measured, indicating a strong product rotational angular momentum alignment. The results are discussed in the light of a recent stereodynamical model for these, heavy + heavy-light, kinematically constrained reactions.

Maximum in the translational energy dependence of the cross section for the Na+CH3I→NaI+CH3 reaction

Relative values of the total reaction cross section for the crossed molecular beam reaction Na+CH3I→NaI+CH3 have been measured over the collision energy ET range of 0.11–0.40 eV. The measured excitation function is of a mechanistically bimodal type showing a maximum at ET ∼0.14 eV. This functionality is compared to those of closely related reactions and is discussed in the light of different dynamical models.

Influence of the radical size on the Ca∗ + ROH → CaOH∗ + R (RCH3, C2H5 and C3H7) reaction cross section

Abstract Absolute values of the reaction cross section, σ R , for the title reaction family have been measured by chemiluminescence emission under beam-gas conditions. The σ R values diminish as the radical group R increases in size, showing an important overall steric effect. These results are discussed in the light of simple models for reaction stereodynamics.

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.