Excitation and charge transfer in He++H collisions

For the conflictive case of He⁺+H collisions, we present a norm-method optimization of the parameters included in the (often used) two-electron translation factor of Errea et al. As surmised in a previous publication, a strong cut-off is needed at short internuclear distances to prevent the translation factor from marring the properities of the molecular expansion there. With a basis of 16 molecular states, we present the first calculations including translation factors, of total and partial charge exchange and excitation cross sections in He⁺+H collisions, as well as the alignment parameter A₂₀ for hydrogen excitation. Good agreement with experiment is reached up to the energy range where ionization and charge exchange cross sections are comparable.     

Charge transfer in S2++H collisions at intermediate energy

Partial cross-sections for the charge transfer process of S²⁺ ions in collision with atomic hydrogen at impact energies up to 8 keV have been calculated by means of a semi-classical method using ab initio potential energy curves and couplings. The results are in relatively good agreement with experiment and improve significantly previous Landau-Zener calculations.     

Charge transfer inp-H and H2 collisions at low and high energies

The previous modified two-orthogonal state expansion method with the initial united-atom effect is extended to study the charge transfer for the resonant and nonresonant reactions at low and high energies and the theory is no longer restricted to low capture probabilities. The impact energy covers a wide range: 0.17 eV–200 keV and 0.1–200 keV forp-H and H₂ collisions, respectively. The present calculations show good agreement with experimental data at energies 0.17 eV–10 keV and 75–200 keV for thep-H reaction and 0.2–200 keV for thep-H₂ reaction.     

Elastic and charge transfer processes in H+ + CO collisions

Proton and hydrogen atom time-of-flight spectra in collision energy range of E(trans) = 9.5-30 eV show that the endoergic charge transfer process in the H+ + CO system is almost an order of magnitude less probable than the elastic scattering [G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)]. Ab initio computations at the multireference configuration interaction level have been performed to obtain the ground- and several low-lying excited electronic state potential energy curves in three different molecular orientations namely, H+ approaching the O-end and the C-end (collinear), and H+ approaching the CO molecule in perpendicular configuration with fixed CO internuclear distance. Nonadiabatic coupling terms between the ground electronic state (H+ + CO) and the three low-lying excited electronic states (H + CO+) have been computed and the corresponding diabatic potentials have been obtained. A time-dependent wavepacket dynamics study is modeled first involving only the ground and the first excited states and then involving the ground and the three lowest excited states at the collision energy of 9.5 eV. The overall charge transfer probability have been found to be approximately 20%-30% which is in qualitative agreement with the experimental findings.     

Quantum dynamics of inelastic excitations and charge transfer processes in the H+ +NO collisions

State-resolved differential cross section, integral cross section, average vibrational energy transfer, and the relative transition probability are computed for the H(+)+NO system using our newly obtained ab initio potential energy surfaces (PES) at the multireference configuration interaction level of accuracy employing the correlation consistent polarized valence triple zeta basis set. The quantum dynamics is treated within the vibrational close-coupling rotational infinite-order sudden approximation using the coupled ground state and first excited state ab initio quasidiabatic PES. The computed collision attributes for the inelastic vibrational excitation are compared with the state-to-state scattering data available at E(c.m.)=9.5 eV and E(c.m.)=29.03 eV and are found to be in overall good agreement with those of the experiments. The results for the vibrational charge transfer processes at these collision energies are also presented.     

A model for energy transfer in inelastic molecular collisions applicable at steady state or non-steady state and for an arbitrary distribution of collision energies

A new model for energy exchange between translational and internal degrees of freedom in atom-molecule collisions has been developed. It is suitable for both steady state conditions (e.g., a large number of collisions with thermal kinetic energies) and non-steady state conditions with an arbitrary distribution of collision energies (e.g., single high-energy collisions). In particular, it does not require that the collision energies be characterized by a quasi-thermal distribution, but nevertheless it is capable of producing a Boltzmann distribution of internal energies with the correct internal temperature under quasi-thermal conditions. The energy exchange is described by a transfer probability density that depends on the initial relative kinetic energy, the internal energy of the molecule, and the amount of energy transferred. The probability density for collisions that lead to excitation is assumed to decrease exponentially with the amount of transferred energy. The probability density for de-excitation is obtained from microscopic reversibility. The model has been implemented in the ion trap simulation program ITSIM and coupled with an Rice-Rampsberger-Kassel-Marcus (RRKM) algorithm to describe the unimolecular dissociation of populations of ions. Monte Carlo simulations of collisional energy transfer are presented. The model is validated for non-steady state conditions and for steady state conditions, and the effect of the kinetic energy dependence of the collision cross-section on internal temperature is discussed. Applications of the model to the problem of chemical mass shifts in RF ion trap mass spectrometry are shown.     

H++Xe low-energy collisions: Opposite-phase oscillations of the elastic and charge transfer differential cross sections

Elastic as well as charge transfer collisions of H⁺+Xe have been investigated in a crossed beam experiment atE _CM ≈30 and 50 eV. Opposite-phase oscillations have been observed in the elastic differential cross section with respect to the charge transfer differential cross section for the formation of Xe⁺(² P _1/2). Taking advantage of the asymptotic quasi-degeneracy of the channels in question, this behavior has been qualitatively interpreted in terms of a simplified two-curve crossing model. The conditions of the validity of the model are discussed and its relation to the potential symmetry scattering in homonuclear systems is pointed out.     

Vibrational inelastic and charge transfer processes in H(+)+H2 system: an ab initio study

State-resolved differential cross sections, total and integral cross sections, average vibrational energy transfer, and the relative probabilities are computed for the H(+)+H2 system using the newly obtained ab initio potential energy surfaces at the full CI/cc-pVQZ level of accuracy which allow for both the direct vibrational inelastic and the charge transfer processes. The quantum dynamics is treated within the vibrational close-coupling infinite-order-sudden approximation approach using the two ab initio quasidiabatic potential energy surfaces. The computed collision attributes for both the processes are compared with the available state-to-state scattering experiments at E(c.m.)=20 eV. The results are in overall good agreement with most of the observed scattering features such as rainbow positions, integral cross sections, and relative vibrational energy transfers. A comparison with the earlier theoretical study carried out on the semiempirical surfaces (diatomics in molecules) is also made to illustrate the reliability of the potential energy surfaces used in the present work.     

Crossed beam study of He+-O2 charge transfer reactions in the collision energy range 0.5–200 eV

Energy spectra and angular distributions of the O⁺ and O ₂ ⁺ product ions resulting from the He⁺-O₂ charge transfer reaction have been measured in the collision energy range 0.5–200 eV using the crossed-beam method. The O ₂ ⁺ ions represent only a minor fraction of the reaction products (0.2–0.6% over the energy range measured). In the dissociative charge transfer reaction, four main processes are identified leading to O+O⁺ reaction products in different electronic states. Two different mechanisms can be distinguished, each being responsible for two of the observed processes:(i) a long-distance energy-resonant charge transfer process involving thec ⁴∑ _u ^? (v′=0) state of O ₂ ⁺ and(ii) a slightly exothermic charge transfer process via the (III)²∏ _u state of O ₂ ⁺ (with the exothermicity depending on the collision energy). Angle-integrated branching ratios and partial cross sections (in absolute units) have been determined. The branching ratios of the individual processes show a pronounced dependence on the collision energy. At low energies, the O⁺ product ions are preferentially formed in the² P ⁰ and² D ⁰ excited states. The angular distributions of the O⁺ product ions show an anisotropic behaviour indicating an orientation-dependent charge transfer probability in the He⁺ ?O₂ reaction.     

Total cross sections for electron scattering by polyatomic molecules at 10–1000 eV: C2H2, C2H4, C2H6, C3H6, C3H8 and C4H8

A model complex optical potential (composed of static, exchange, polarization and absorption terms) is employed to calculate the total (elastic and inelastic) electron-atom scattering cross sections from the corresponding atomic wave function at the Hartree-Fock level. The total cross sections (TCS) for electron scattering by their corresponding molecules (C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈ and C₄H₈) are firstly obtained by the use of the additivity rule over an incident energy range of 10–1000 eV. The qualitative molecular results are compared with experimental data and other calculations wherever available, good agreement is obtained in intermediate-and high-energy region.     

Mechanism and control of the F+H2 reaction at low and ultralow collision energies

This article uses theoretical methods to study the dependence on stereodynamical factors of the mechanism and reactivity of the F+H2 reaction at low and ultralow collision energies. The impact of polarization of the H2 reactant on total and state-to-state integral and differential cross sections is analyzed. This leads to detailed pictures of the reaction mechanism in the cold and ultracold regimes, accounting, in particular, for distinctions associated with the various product states and scattering angles. The extent to which selection of reactant polarization allows for external control of the reactivity and reaction mechanism is assessed. This reveals that even the simplest of reactant polarization schemes allows for fine, product state-selective control of differential and (for reactions involving more than a single, zero orbital angular momentum partial wave) integral cross sections.     

Molecular treatment of charge transfer cross sections in N5+ collisions with H2

We present cross sections for electron capture in N⁵⁺+H₂ collisions in the energy range 100 eV/amu≤E≤6 keV/amu. We employ a model potential aproximation to treat the interaction of the active electron with the cores, and a recently proposed method, which applies the independent particle model to evaluate the Hamiltonian matrix elements. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Importance of long-range forces and short-range forces in electron scattering: Elastic scattering by N2 at 10 and 30 eV

Polarized Born and fixed-nuclei approximation (sudden approximation) quantum mechanical calculations of the sum of the elastic scattering and rotational excitation differential cross sections are presented, compared to published experimental values, and interpreted in terms of the forces.     

Vibrational mode and collision energy effects on reaction of H2CO+ with C2H2: charge state competition and the role of Franck-Condon factors in endoergic charge transfer

The effects of collision energy (E(col)) and six different H(2)CO(+) vibrational states on the title reaction have been studied over the center-of-mass E(col) range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H(2) (+) transfer, hydrogen transfer, and proton transfer channels is strongly affected by E(col) and H(2)CO(+) vibrational excitation, providing insight into the factors that control competition and charge state "unmixing" during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion.     

Total cross sections for K + Br2 at relative energies between 0 and 8000 eV

Trajectory Surface Hopping (TSH) calculations have been applied to the non-elastic scattering in the K + Br₂ collision system over a wide range of relative kinetic energies from 0 to 8000 eV. Absolute total cross sections have been computed for the formation of various collision products with an accuracy of 5% with respect to statistical errors. The following non-elastic processes have been studied: chemical reaction, inelastic neutral scattering, neutral dissociation and ion pair formation, yielding atomic as well as molecular negative bromine ions together with PC ions. The absolute values of the respective total cross sections, obtained from the TSH calculations, are in close agreement with the available experimental data, both for chemical reaction and for ion pair formation, over the whole energy range considered. The three particle character of the collision system is important in describing the experimental results quantitatively at relative kinetic energies below 100 eV.     

Application of fixed-nuclei scattering theory to electron methane elastic and inelastic differential cross sections at 10 eV impact energy

Quantum mechanical calculations are reported for electron-methane elastic scattering and rotational excitation cross sections at 10 eV impact energy. The calculations employ a fixed-nuclei close coupling formalism with full incorporation of symmetry and are used to test previous laboratory-frame calculations employing a direct coupling approximation. Good agreement is obtained. Additional comparisons to previous theoretical and experimental work are also presented, and the contributions of the various symmetries to the cross sections are analyzed in terms of representatve matrix elements of the interaction potential.     

Simultaneous vibration-vibration and vibration-translation transitions in H2 + D2 at high collision energies

The coupling between VV and VT transitions in H₂(2) + D₂(0) → H₂(1) + D₂(1) and D₂(2) +H₂(0) → D₂(1) + H₂(1) at high collision energies is investigated by use of the solution of the Schrodinger equation of motion. The coupling is sufficiently important that the energy exchange processes cannot be described by the VV mechanism alone.     

Polarization energies,transport gap and charge transfer states of organic molecular crystals

A self-consistent calculation of electronic polarization in organic molecular crystals and thin films is presented in terms of charge redistribution in nonoverlapping molecules in a lattice. The polarization energies P₊ and P₋ of a molecular cation and anion are found for anthracene and perelynetetracarboxylic dianhydride (PTCDA), together with binding energies of ion pairs and transport gaps of PTCDA films on metallic substrates. The 500 meV variation of P₊+P₋ with film thickness agrees with experiment, as do calculated dielectric tensors. Comparisons are made to submolecular calculations in crystals.     

Theoretical study of the dynamics of AR collisions with C2H6 and C2F6 at hyperthermal energy

We present a classical-trajectory study of the dynamics of high-energy (5-12 eV) collisions between Ar atoms and the C2H6 and C2F6 molecules. We have constructed the potential-energy surfaces for these systems considering separately the Ar-molecule interactions (intermolecular potential) and the interactions within the molecule (intramolecular potential). The intermolecular surfaces consist of pairwise empirical potentials derived from high-accuracy ab initio calculations. The intramolecular potentials for C2H6 and C2F6 are described using specific-reaction-parameters semiempirical Hamiltonians and are calculated "on the fly", i.e., while the trajectories are evolving. Trajectory analysis shows that C2F6 absorbs more energy than C2H6 and is more susceptible to collision-induced dissociation (CID). C-C bond-breakage processes are more important than C-H or C-F bond breakage at the energies explored in this work. Analysis of the reaction mechanism for CID processes indicates that, although C-C breakage is mostly produced by side-on collisions, head-on collisions are more efficient in producing C-F or C-H dissociation. Our results suggest that high-energy collisions between closed-shell species of the natural low-Earth-orbit environment and spacecraft can contribute to the observed degradation of polymers that coat spacecraft surfaces.