V. Sidis

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.

Ground and excited state adiabatic 2A″ and 2A′ potential energy surfaces of the (Kr–O2)+ cluster ion

The seven lowest adiabatic potential energy surfaces (PES) of the (Kr–O2)+ cluster ion in each of the 2A″ and 2A′ symmetries are calculated. The computational method involves configuration interaction calculations in a basis of a thousand projected valence‐bond state functions. It resorts to diagonal corrections of the Hamiltonian matrix prior to configuration interaction and makes use of an l‐dependent pseudopotential for Kr. The results are characterized by the shallowness of the 1 2A″ potential well and the absence of wells in the other PES investigated. The 1 2A″ equilibrium characteristics differ significantly from those proposed in other work. Notable effects on all the PES are observed when the O–O bond is stretched beyond 2.5a0. No 1 2A′−2 2A′ (nor 1 2A″−2 2A″) pseudocrossings are found that could explain, on the basis of mere electronic structure arguments, the available thermal energy charge transfer data. A 2 2A″−1 2A′ crossing actually exists but the related Coriolis coupling mechanism cannot ...

Diabatic excited states of the (HeH2)+ molecular ion for the charge exchange-excitation reaction: He+ + H2 → HeH+ + H∗

Abstract An extensive set of data has been determined for a few low lying diabatic states of the (HeH 2 ) + system with a view to make possible future theoretical investigations of the He + + H 2 → HeH + + H ∗ ( n =2) reaction dynamics The data are produced in two steps. Firstly, ab initio calculations are carried out which follow a similar scheme to that proposed by Kubach et al.: SCF, IVO and CIs in blocks. Secondly, due to the modest sizes of the considered CIs, the latter results are corrected in an ad hoc manner that insures the proper placing and exact shapes of the asymptotic potential energy curves and introduces shape corrections of the potential energy surfaces when the triatomic system forms. The adequacy of the proposed correction scheme is attested by various comparisons with earlier work. Selected samples of the results show the behaviour of the potential energy surfaces, the diabatic curve crossing pattern, the crossing seams and the diabatic couplings, in various geometries for both reactant HeHeH arrangements.

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)].

Angular and energy analysis of HeH+ products from the charge exchange excitation reaction He+ + H2 → HeH+ + H∗

The angular and energy distributions of the HeH+ products resulting from He+ + H2 collisions have been measured in the center-of-mass energy range 9 eV < Ecm < 34 eV. These measurements confirm those performed previously by us at Ecm = 20 eV and lead themselves to an investigation of the reaction mechanism. The HeH+ molecule is formed in its first ν ≤ 3 vibrational levels while the hydrogen atom is found essentially in the H∗ (n = 2) state. At all energies the angular distribution is strongly peaked; the peak maximum moves from the cm angle χ ≈ 55° to χ ≈ 30° when the energy increases within the above range. The results are related to two-electron transitions occurring at diabatic potential surface crossings of the (HeH2)+ transient molecule. Accordingly a theoretical framework involving a distorted wave — infinite order sudden approach is devised to rationalize the observations. This leads to a “generalized reflection principle” which, when submitted to a stationary phase treatment, leads to a description of the reaction mechanism in terms of two mere elastic half-collisions. The model which is arrived at explains the general trends of the measured vibrational and angular distributions.

Quantal characteristics of an ion-molecule complex formed by translation-to-vibration energy exchange: The H+⋯O2 system

Abstract Complex formation via translation-to-vibration energy exchange is investigated in the H+ + O2 collisional system. The study is undertaken in the quantal infinite order sudden approximation framework using a basis of 14 vibronic states associated with the electronically elastic H+ + O2 (v) channels. For collision energies below the first vibrational excitation channel, Feshbach as well as shape resonances are found whose lifetimes are generally smaller than a rotation period of the O2 molecule. The total elastic cross-section, at a fixed relative angle, as a function of energy shows numerous structures that can be analyzed in terms of the found resonances.

2Σ+ potential energy curves of the (Na19–Na)+ jellium‐cluster‐ion system

A prototypical description of the (Na19–Na)+ system is reported. The Na atom, in its ground and first two excited states, is treated by a one‐electron pseudopotential method. The cluster is first described in the spherical jellium background model (SBJM). A numerical Hartree–Fock approach is used to calculate the electronic wave function of the cluster in its ground state configuration. Singly excited Na19* states are obtained using an improved virtual orbital technique to allow for the distortions of the cluster electron cloud during the Na19–Na approach. The matrix of the electronic Hamiltonian in a (diabatic) basis of projected valence bond configuration state functions are determined with an effective model potential method. As a first model case, the (Na19–Na)+ system is treated holding the isolated positive background of the jellium cluster unchanged. This description pertains to rapid displacements of the atom relative to the cluster. As a second case, we consider distortion and reconstruction of t...

Multitrajectory semiclassical treatment of vibronic excitation and charge transfer in the fixed rotor approximation

Abstract The multitrajectory semiclassical method first introduced and tested in the IOS approximation [Florescu et al., Phys. Rev. A 47 (1993) 2943] is transposed in the fixed-rotor framework. The method is applied to the study of the impact parameter and scattering angle dependences of vibrationally inelastic and vibronic charge transfer processes in the H++O2 collision at Ecm=23 eV. Trajectory effects on the orientation dependence of transition probabilities and differential cross sections are evidenced. The present calculations confirm the earlier VSC results of Sizun et al. [J. Chem. Phys. 96 (1992) 307].