M. Sizun

Quantum dynamics of H2 formation on a graphite surface through the Langmuir Hinshelwood mechanism

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar, and a model potential is obtained in a semiempirical way to reproduce the experimental properties of H physisorption on graphite. The reaction probability has been computed in the case when the angular momentum of the relative H-H motion lies parallel to the surface plane. Three-dimensional wave packet calculations have been performed for collision energies ranging from 2 to 50 meV. It is shown that the reaction occurs with a significant probability and produces the H2 molecule with a considerable amount of vibrationnal energy. A simple mechanical model is presented, where desorption of the nascent H2 molecule results from two successive binary elastic collisions.

Semiclassical coupled wave packet study of the nonadiabatic collisions Ar+(J)+H2: Zero angular momentum case

The title reaction is investigated for total angular momentum I=0 using a semiclassical coupled wave packet method where the Smith–Whitten‐type hyperspherical angles θ and φ are treated quantally, and the hyperspherical radius ρ is treated classically. The wave function is expanded over an electronic basis set which includes 28 states. The diabatic potential energy surfaces are determined by DIMZO calculations. Probabilities for reaction, charge transfer, collision induced dissociation, dissociative charge transfer, and fine structure transitions are obtained in the energy range 0.3 eV≤E≤30 eV. A comprehensive analysis of the reaction mechanisms is presented.

The dynamics of H2 formation on a graphite surface at low temperature

The catalytic formation of H2 on carbonaceous grains in interstellar conditions is studied theoretically. The grain is modelled by a coronene molecule. The coronene–H–H interaction is described at the DFT level. The dynamics study is limited to the collinear case, with two degrees of freedom (Cor–H and H–H) handled by a wavepacket description. The collision energy range extends between 0.3 meV and 0.5 eV. The main results are: (i) the collision time has the same order of magnitude as the lattice relaxation time; (ii) the vibrational excitation is very large; (iii) the reaction probability is very sensitive to the small potential barrier in the entrance valley predicted by the DFT calculation.

Theoretical investigation of the Ar+(J) + H2 → ArH+ + H reaction: semiclassical coupled wavepacket treatment

Abstract The title reaction is investigated using a semiclassical coupled wavepacket method where the hyperspherical radius ϱ is treated classically and the other coordinates quantally. The wavefunction is expanded over electronic states, whose potential energy surfaces and couplings are determined within a DIMZO framework. Dynamical calculations are performed in a coplanar-like approximation using eight coupled electronic states. State to state reaction cross sections are obtained in the energy range 0.3 eV ≤ E ≤ 5 eV. A comprehensive analysis of the reaction mechanisms is presented. It shows that the collision can be viewed as a two step process: the sharing of the system between the electronic adiabatic states, followed by the reaction along the uncoupled ground adiabatic potential energy surface. Two reaction mechanisms are exhibited, one of them occurring at low collision energy only.

Vibrational state‐to‐state calculations of H++O2 charge transfer collisions

A comprehensive theoretical investigation of vibrational excitation and vibronic charge transfer in the H++O2 collision at ECM=23 eV is reported. The calculations of differential and integral scattering observables are undertaken within both the quantal infinite order sudden (QIOS) and the vibronic semiclassical (VSC) approximations. They involve 2×15 vibronic expansions associated with the diabatic states determined by Grimbert et al. [Chem. Phys. 124, 187 (1988)] using a so‐called effective model potential (EMP) method. A quadripartite comparison involving experimental data of Noll and Toennies [J. Chem. Phys. 85, 3313 (1986)], results of a QIOS treatment of Gianturco et al. [Phys. Rev. A 42, 3926 (1990)] based on DIM potentials and the present QIOS, and VSC results is presented. From the comparison of the theoretical and experimental results we find that the present calculations based on the EMP reproduce much better the experimental data than those based on the DIM potentials. Though differences are f...

Theoretical study of the reactions of Ar++H2 and Ar++HD using the trajectory surface hopping method

Trajectory surface hopping calculations have been carried out for collisions of Ar++H2 and Ar++HD on three low-lying potential energy surfaces projected from the original six in the Kuntz and Roach diatomics in molecules surface for this system. The location and probability of hops between surfaces were determined using the new algorithm developed by Parlant and Gislason. In addition to the reactive channel and total charge transfer to H2+ and HD+, dissociative channels to, for example, Ar++H+H, and Ar+H++H have been studied. Particular attention was paid to the dissociative charge transfer isotope effect for the processes Ar++HD→Ar+H++D, or Ar+H+D+; near threshold the D+ product is favored over H+ which we attribute to preferential dissociation of excited ArD+ products. This is the first theoretical study of these dissociation processes.

Calculation of the total scattering cross section for the collision of hard spheres: The atom–diatom case

A simple expression for the total scattering cross section Q for the collision of A+BC, where A, B, and C are hard spheres, is derived. It is assumed that B and C are initially touching. The result represents an average over all spatial orientations of the diatomic BC. The cross section depends only on the radii of the three spheres. If RB=RC, the total cross section is given, to a good approximation, by Q=π(RA+RB)(RA+2RB). A number of combining rules for total cross sections involving hard spheres are also derived. In addition, the results for hard spheres are used to develop a very simple procedure for computing classical atom–diatom total cross sections on realistic potential energy surfaces. The method is applied to collisions of H+H2, Li++N2, and Li++CO, and is seen to work very well.

Theoretical study of the reactions of Ar++HX(v=0) and Ar+HX+(v) (X=H and D) at E=0.1 eV using the trajectory surface hopping method

Trajectory surface hopping calculations have been carried out for collisions of Ar++H2 (v=0), Ar++HD (v=0), H2+(v)+Ar, and HD+(v)+Ar, where v=0, 1, and 2 on the Kuntz–Roach diatomics-in-molecules potential surfaces at a relative energy of 0.1 eV. The importance of the mutual “capture” of the two particles on the attractive ground potential energy surface is shown clearly. The fact that capture does not occur on every collision is attributed to an effect of the vibrational phase of the H2 or HD molecule. This vibrational phase effect can explain the drop in the experimental rate constant seen at very low temperatures in the Ar++H2 system. For H2+(v=2)+Ar and HD+(v=2)+Ar we also find that many trajectories hop to the first excited potential surface as the particles approach. Since these trajectories cannot reach small separations, this further reduces the reactive cross section for v=2 and higher levels. The ground potential energy surface has a fairly deep well, particularly when the Ar–H–H angle is near 9...

Theoretical study of the isotope effects in the non-adiabatic reaction of Ar+(J) with H2, D2, HD

Abstract We use a wavepacket method to investigate isotope effects on the reactive collision between an Ar+ ion in a well defined spin orbit state and the isotopes of the hydrogen molecule: H2, HD and D2 in their ground rovibronic state. The collision energy range is 0.3 to 5 eV. Because eight electronic states play a role during this non-adiabatic reaction, a number of approximations have been made: Use of the diatomic in molecule method to determine the diabatic potential energy surfaces and couplings, semi-classical and coplanar approximation in the treatment of the dynamics. The total reaction cross section are obtained and compared the to the available experimental data. We also present the first state-to-state reaction cross sections for these systems. Finally, the results are analysed in terms of reaction mechanism.

Theoretical investigation of differential cross sections for vibrational excitation and vibronic charge transfer in H+ + H2 collisions

The multitrajectory semiclassical method combined with the fixed rotor approximation is used to investigate the differential state‐to‐state scattering in H++H2 collisions at Ec.m.=20 eV. Little differences are found with respect to previous quantal infinite order calculations of Baer et al. [J. Chem. Phys. 91, 4169 (1989)] when similar DIM (diatomics in molecules) potential energy surfaces are used. Adjustment of the ground state DIM potential energy surface to ab initio data considerably improves the comparison with the experiment of Niedner et al. [J. Chem. Phys. 87, 2685 (1987)].