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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. The interaction potential is modeled to reproduce the experimental properties of H physisorption on graphite. Extending our previous work [S. Morisset, F. Aguillon, M. Sizun, and V. Sidis, J. Chem. Phys. 121, 6493 (2004)], full-dimensionality quantum calculations are presented for collision energies ranging from 4 to 50 meV. It is shown that the reaction occurs with a large cross section and produces the H2 molecule with a considerable amount of vibrational energy. The mechanism is either direct or involves the formation of an intermediate complex. [ABSTRACT FROM AUTHOR]

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 he 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. [ABSTRACT FROM AUTHOR]

Three-body fragmentation of Na[sup +][sub 3] ions to Na[sup +] + Na(3s)+Na(3s) following excitation by He is studied experimentally and theoretically. The three reduced kinetic energies of the products in the center-of-mass are determined for each fragmentation event, and the results are displayed in a Dalitz plot. The fragmentation involves three adiabatic ¹A' electronic states of Na[sup +][sub 3] that become degenerate at the detector. It is possible to determine the final electronic state for each event, and here we show that each of the three product states appears in a particular sector of the Dalitz plot. Theoretical and experimental Dalitz plots for the three-body fragmentation of Na[sup +][sub 3] are presented, and the results are related to various mechanisms for three-body fragmentation of this system. [ABSTRACT FROM AUTHOR]

The process of Na[sub 3][sup +] cluster ion fragmentation following excitation by a fast He atom is studied using a theoretical procedure developed earlier. The collision with He leaves Na[sub 3][sup +] in any of three electronic states, and each of these can fragment into three product channels (Na[sub 2][sup +]+Na, Na[sub 2]+Na[sup +] and Na+Na+Na[sup +]). Attention is focused on understanding both the dynamics of the Na[sub 3][sup +]-He interaction and the post-collisional fragmentation of the excited cluster. Four simple fragmentation mechanisms are proposed to describe the major features of the process. Contributions of these mechanisms to different fragmentation pathways are determined and their dependence on the initial internal energy of the cluster is studied. Fragmentation intensity maps are calculated and good agreement with experiment is obtained. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A time dependent quantal study of the dynamics of the H2 formation through a collinear ER mechanism, which explicitly handles substrates relaxation effects, are investigated. The study concluded that this process, which is still open to investigation, is undoubtedly very slow compared to the time scale of the title reaction, still it could be very fast compared to the time scales of the chemical evolution of interstellar.

Studies the experimental reaction of hydrogen molecular ion with a hydrogen molecule at a relative collision energy of 20 eV. Analysis of the ionic products and energy in coincidence with the excited atomic products; Explanation of the mechanism of the D[sup +][sub 2] + H[sub 2] using an adaptation of the interpretation framework proposed by Dhuicq.

Examines the effects of complex formation and nonadiabatic transition on low energy ion-molecule collision. Analysis on the exothermic charge transfer process at collision energies below vibrational excitation threshold; Formation of translation-to-vibration and orbiting complexes; Anisotropy of the interactions in the (H-O[sub 2])[sup +] 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 the positive background as the Na+-ion core approaches and penetrates into the cluster core. The latter description is appropriate for slow displacements of the atom relative to the cluster. Potential energy curves are reported for the 2Σ+ states. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A coupled wave packet method is presented which allows us to treat exactly the vibrational and dissociative motions in nonadiabatic atom–molecule collisions, whose relative motion is described by a classical trajectory. It consists of a time propagation of the coupled vibrational wave functions defined over a grid. It is applied to the dissociative charge exchange (DCE) process in atom–molecule collisions. Model cases are investigated that put forward the basic characteristics of the DCE process. Analysis of the time evolution of the wave packets leads to a direct view of the collision dynamics and to an interpretation of the dissociation mechanisms. Striking phenomena are revealed, that were not predictable by the previously existing approximate methods. [ABSTRACT FROM AUTHOR]

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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 be invoked for low energy <1 eV encounters. This draws attention to alternative more subtle dynamic charge transfer mechanisms possibly involving quartet-doublet spin–orbit transitions and/or (near) degeneracies of rovibronic, instead of mere electronic, energy levels. Evidence is shown for pseudocrossing mechanisms of population sharing between the 2,3A″ and 2A′ states. Similar evidence is shown for an O2(X 3Σ-g)→O2(a 1Δg) excitation involving two electron rearrangement. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The (HO2)+ molecular ion is used to experiment two approximate procedures which aim at reducing the computation effort that is needed for the determination of potential energy surfaces of ion–molecule charge transfer systems. The two procedures involve configuration interaction (CI) calculations of moderate sizes and are based on diagonal corrections of the electronic Hamiltonian matrix in a basis of projected-valence bond (PVB) configuration-state functions (CSF). The PVB-CSF’s used in practice correspond to a full valence CI for each ionic or neutral partner as well as single excitations accounting for polarization and electron transfer. The diagonal corrections are of two sorts: (i) if insufficiently large orbital expansion bases are used they remove the relative ion–molecule basis set superposition error; (ii) if asymptotic energy levels of the involved neutrals or ions in their ground or valence excited states are misplaced they properly adjust these levels. When applied to (HO2)+ using a minimal or an extended orbital basis set expansion the proposed approaches yield concording results. The results also agree with the effective model potential (EMP) data of Grimbert et al. [Chem. Phys. 124, 187 (1988)] which have proved successful in the description of the H++O2 charge transfer dynamics. Comparison with fragmentary results from MRD-CI calculations by Vazquez et al. [Mol. Phys. 59, 291 (1986)] and Schneider et al. [Chem. Phys. 128, 311 (1988)] is somewhat mitigated. The method should be particularly useful for bulky ion–molecule systems. [ABSTRACT FROM AUTHOR]

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 found between the present QIOS and VSC results, the experimental data do not help deciding between the two theories. The relative merits of the QIOS and VSC approaches are put forward and discussed. By scrutinizing intermediate results of lengthy calculations we have been able to bring out elementary models in them. Pure vibrational excitation is found to be a prominent process in this system and charge transfer substantially affects it. [ABSTRACT FROM AUTHOR]