Ab initio potential energy surfaces and nonadiabatic collision dynamics in H(+)+O(2) system

The adiabatic potential energy surfaces for the lowest five electronic states of (3)A" symmetry for the H(+)+O(2) collision system have been obtained at the multireference configuration interaction level of accuracy using Dunning's correlation consistent polarized valence triple zeta basis set. The radial nonadiabatic coupling terms and the mixing angle between the lowest two electronic states (1 (3)A" and 2 (3)A"), which adiabatically correlate in the asymptotic limit to H((2)S)+O(2) (+)(X (2)Pi(g)) and H(+)+O(2)(X (3)Sigma(g)(-)), respectively, have been computed using ab initio procedures at the same level of accuracy to yield the corresponding quasidiabatic potential energy matrix. The computed strengths of the vibrational coupling matrix elements reflect the trend observed for inelastic vibrational excitations of O(2) in the experiments at collision energy of 9.5 eV. The quantum dynamics has been preformed on the newly obtained coupled quasidiabatic potential energy surfaces under the vibrational close-coupling rotational infinite-order sudden framework at the experimental collision energy of 9.5 eV. The present theoretical results for vibrational elastic/inelastic excitations of O(2) are in overall good agreement with the available experimental data obtained from the proton energy-loss spectra in molecular beam experiments [F. A. Gianturco et al., J. Phys. B 14, 667 (1981)]. The results for the complementary charge transfer processes are also presented at this collision energy.     

Communication: theoretical exploration of Au(+)+H2, D2, and HD reactive collisions

A quasi-classical study of the endoergic Au(+)((1)S) + H(2)(X(1)Σ(g)(+)) → AuH(+) ((2)Σ(+)) + H((2)S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au(+)+H+H, is the dominant reaction channel for energies above the H(2) dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion.     

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.     

Influence of vibrational excitation on the nonadiabatic reactions of metal atoms with H2

The reactions of alkaline earth metal atoms, Mg(3s3p 1P1) and Ca(4s4p 1P1), with H2(v = 1, j) are studied using a pump-probe technique combined with stimulated Raman pumping and coherent anti-Stokes Raman spectroscopy. For the Ca(4 1P1) case, the energy deposited in the v = 1 level enlarges the H2 bond distance to help facilitate the reaction without opening an additional pathway. For the Mg(3 1P1) case, the vibrational excitation of H2 leads to enhancement of the low rotational component of the rotational distribution and the MgH(v = 0)/MgH(v = 1) ratio. These results can be predicted with quasi-classical trajectory calculations and interpreted with a kinematic collision model.     

State-selective electron capture processes in collisions involving boron ions

A full theoretical treatment of electron capture processes using ab initio configuration interaction methods within, according to the collision energy range concerned, a semiclassical or a quantal collisional formalism including translation effects has been developed recently. An application for collisions involving boron, an important impurity in fusion reactors, is presented on examples of the ground state: B³⁺(1s²) + He, B⁴⁺(1s) + H, and the metastable ion B³⁺(1s2s) + H reactions. © 1996 John Wiley & Sons, Inc.     

Isotopic exchange processes in cold plasmas of H2/D2 mixtures

Isotope exchange in low pressure cold plasmas of H(2)/D(2) mixtures has been investigated by means of mass spectrometric measurements of neutrals and ions, and kinetic model calculations. The measurements, which include also electron temperatures and densities, were performed in a stainless steel hollow cathode reactor for three discharge pressures: 1, 2 and 8 Pa, and for mixture compositions ranging from 100% H(2) to 100% D(2). The data are analyzed in the light of the model calculations, which are in good global agreement with the experiments. Isotope selective effects are found both in the surface recombination and in the gas-phase ionic chemistry. The dissociation of the fuel gas molecules is followed by wall recycling, which regenerates H(2) and D(2) and produces HD. Atomic recombination at the wall is found to proceed through an Eley-Rideal mechanism, with a preference for reaction of the adsorbed atoms with gas phase D atoms. The best fit probabilities for Eley-Rideal abstraction with H and D are: γ(ER H) = 1.5 × 10(-3), γ(ER D) = 2.0 × 10(-3). Concerning ions, at 1 Pa the diatomic species H(2)(+), D(2)(+) and HD(+), formed directly by electron impact, prevail in the distributions, and at 8 Pa, the triatomic ions H(3)(+), H(2)D(+), HD(2)(+) and D(3)(+), produced primarily in reactions of diatomic ions with molecules, dominate the plasma composition. In this higher pressure regime, the formation of the mixed ions H(2)D(+) and HD(2)(+) is favoured in comparison with that of H(3)(+) and D(3)(+), as expected on statistical grounds. The model results predict a very small preference, undetectable within the precision of the measurements, for the generation of triatomic ions with a higher degree of deuteration, which is probably a residual influence at room temperature of the marked zero point energy effects (ZPE), relevant for deuterium fractionation in interstellar space. In contrast, ZPE effects are found to be decisive for the observed distribution of monoatomic ions H(+) and D(+), even at room temperature. The final H(+)/D(+) ratio is determined to a great extent by proton (and deuteron) exchange, which favours the enhancement of H(+) and the concomitant decrease of D(+).     

R-matrix theory of inelastic processes in low-energy electron collisions with HCl molecule

The processes of vibrational excitation and dissociative attachment in low-energy electron collisions with HCl molecule are considered. TheR matrix theory of these processes incorporating nuclear dynamics is developed. The input data of the theory are characteristics of the systeme-HCl calculated in the fixed-nuclei approximation. The comparison with experimental data and other theories is given. It is shown that the account of the nuclear motion leads to the significant increase of the vibrational excitation cross sections near threshold. However, in the present variant of the theory, the cross sections for the excitation of the first level are essentially lower than the experimental ones. The dissociative attachment cross sections are in good agreement with the experiment.     

Rotational inelasticity in high-energy H2H2 collisions

Rotational cross sections for transitions in the H₂H₂ system have been calculated for energies up to ≈ 2.0 eV and for rotor levels up to j = 11 in the effective potential approximation. The cases of para H₂-para H₂, ortho H₂-ortho H₂ and ortho H₂-para H₂ are considered. Correlations and trends in the cross sections have been examined, and it is shown that the high-energy collisions are dominated by coupling effects. The results of this analysis also suggest that the collision process may be profitably viewed as a diffusion of probability among the levels.     

Natural orbital analysis of nonadiabatic H2+ wave functions

Previously determined nonadiabatic wave functions for H₂⁺ (containing several hundred terms) are analyzed by using natural orbitals. This is the first time that the natural orbital concept has been applied to other than purely electronic wave functions. We find that the natural orbital expansion converges rapidly and that five or six terms are sufficient to reproduce the exact expectation values. Several plots are given of the orbitals so found and these allow a visualization of the somewhat abstract nonadiabatic wave function in a format more reminiscent of everyday quantum-mechanical pictures.     

Quantum nonadiabatic dynamics of hydrogen exchange reactions

In continuation of our earlier effort to understand the nonadiabatic coupling effects in the prototypical H + H₂ exchange reaction [Jayachander Rao et al. Chem. Phys. 333 (2007) 135], we present here further quantum dynamical investigations on its isotopic variants. The present work also corrects a technical scaling error occurred in our previous studies on the H + HD reaction. Initial state-selected total reaction cross sections and Boltzmann averaged thermal rate constants are calculated with the aid of a time-dependent wave packet approach employing the double many body expansion potential energy surfaces of the system. The theoretical results are compared with the experimental and other theoretical data whenever available. The results re-establish our earlier conclusion, on a more general perspective, that the electronic nonadiabatic effects are negligible on the important quantum dynamical observables of these reactive systems reported here.     

Vibrational energies of H2+ using fully nonadiabatic wavefunctions

Using variational Monte Carlo methods, we examine a number of fully nonadiabatic trial wavefunctions to determine which features best describe the lowest several vibrational states of H₂⁺. Our final energies are in excellent agreement with previous calculations. © 2012 Wiley Periodicals, Inc.     

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.     

Dynamics of the Rydberg electron in H*+D2-->D*+HD reactive collisions

Experimental crossed-beam studies carried out previously have indicated that the dynamics of the Rydberg-atom-molecule reaction H*+D2-->D*+HD are very similar to those of the corresponding ion-molecule reaction H++D2-->D++HD. The equivalence of the cross sections for these related systems would open up a new approach to the experimental study of ion-molecule reactions. However, a recent experimental and theoretical study has brought to light some important qualitative differences between the Rydberg-atom reaction and the ion-molecule reaction; in particular, the experimental cross section for the Rydberg-atom reaction exhibits a higher degree of forward-backward scattering asymmetry than predicted by a quasiclassical trajectory study of the ion-molecule reaction. In this paper, the authors consider the dynamics of the Rydberg-electron over the course of a reactive collision and the implications of these dynamics for the Rydberg-atom-molecule crossed-beam experiment. Using an approach based on perturbation theory, they estimate the attenuation of the experimental signal due to the Rydberg-electron dynamics as a function of the scattering angle. They show that at least part of the experimental asymmetry can be ascribed to this angle dependent attenuation. Their results offer general insight into the practical aspects of the experimental study of ion-molecule reactions by means of their Rydberg-atom counterparts.     

A nonadiabatic lower bound calculation of H 2 + and D 2 +

Accurate nonadiabatic lower and upper bounds for groundstate energies of H ₂ ⁺ and D ₂ ⁺ are calculated with the linearized method of variance minimization. The results in a.u. are –0.59713906₃<E ₀(H ₂ ⁺ )<–0.59713899₄ –0.59878877₅<E ₀(D ₂ ⁺ )<–0.59877873₈ i.e. the values are determined with an absolute error smaller than 0.02 cm^–1 for H ₂ ⁺ and 0.01 cm^–1 for D ₂ ⁺ .     

Effects of anharmonicity on nonadiabatic electron transfer: a model

The effect of anharmonicity in the intramolecular modes of a model system for exothermic intramolecular nonadiabatic electron transfer is probed by examining the dependence of the transition probability on the exoergicity. The Franck-Condon factor for the Morse potential is written in terms of the Gauss hypergeometric function both for a ground initial state and for the general case, and comparisons are made between the first-order perturbation theory results for transition probability for harmonic and Morse oscillators. These results are verified with quantum dynamical simulations using wave-packet propagations on a numerical grid. The transition-probability expression incorporating a high-frequency quantum mode and low-frequency medium mode is compared for Morse and harmonic oscillators in different temperature ranges and with various coarse-graining treatments of the delta function from the Fermi golden rule expression. We find that significant deviations from the harmonic approximation are expected for even moderately anharmonic quantum modes at large values of exoergicity. The addition of a second quantum mode of opposite displacement negates the anharmonic effect at small energy change, but in the inverted regime a significantly flatter dependence on exoergicity is predicted for anharmonic modes.     

State-to-state rotational transitions in H2+H2 collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the superthermal region. The two most recently published potential energy surfaces for the H(2)-H(2) complex, the so-called Diep-Johnson (DJ) [J. Chem. Phys. 112, 4465 (2000); 113, 3480 (2000)] and Boothroyd-Martin-Keogh-Peterson (BMKP) [J. Chem. Phys. 116, 666 (2002)] surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H(2)+H(2) collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H(2)-H(2) interaction. We found significant differences in rotational excitation/deexcitation cross sections computed on the two surfaces in collisions between two para-H(2) molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment B. Mate et al., [J. Chem. Phys. 122, 064313 (2005)] for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.     

Classical trajectory study of orientation and alignment effects for charge exchange processes in H+- Na*(3p m) collisions

The orientation and alignment effects for charge exchange in H⁺ + Na*(3p) collisions are studied using the classical trajectory Monte-Carlo method in the energy range from 1 to 8 keV. For Na*(3p _-1) → H*(2s, 2p _±1) transitions a large orientation effect is predicted by the probability functions, in very good agreement with semiclassical calculations. Angular differential cross sections are also calculated and interpreted using the impact parameter dependence of the proton deflection angle. They predict left-right asymmetry in agreement with semiclassical calculations or experimental results, but slightly smaller. Another geometry, not experimentally realized, is considered, where the proton velocity is parallel to the quantization axis of the p _±1 oriented states. Charge exchange from different aligned states with respect to the direction of the projectile velocity is also investigated, but the alignment effects are not as well described as the orientation effects. Total cross sections from oriented or aligned states with cylindrical symmetry around the projectile velocity direction are calculated and allow the hypothesis of velocity matching to be tested.     

Thermal electron attachment processes in gas mixtures containing H2S,HCl and HBr

The kinetics of thermal electron capture reactions by H₂S, HCl and HBr in mixtures with CO₂ have been investigated using the electron swarm method. A mechanism involving van der Waals molecules (H₂S)₂, (HCl·CO₂) and (HBr·CO₂) has been proposed and the rate constants have been calculated. Some tentative conclusions have been made concerning the influence of electronegativity and the dipole moment of the electron scavenger on the mechanism of the electron capture process.     

Reactive collisions in quadrupole cells. 2. H/D exchange reactions of enolate ions with CH3od and C2H5OD

The H/D exchange reactions of enolate ions with CH₃OD and C₂H₅OD at low collision energies in the quadrupole collision cell of a hybrid tandem BEqQ mass spectrometer have been studied. The optimum parameters for exchange were elucidated using the enolate ion of acetone as a test reactant ion. With the exception of the enolate ions of 2,4-pentanedione and dimethylmalonate, all the enolate ions studied underwent exchange with CH₃OD and C₂H₅OD. Ion signals were observed corresponding to exchange of from one to all of the enolic hydrogen atoms. The H/D exchange reaction was more facile with C₂H₅OD as the reagent gas, and the results with this gas clearly permit a count of the number of enolic hydrogen atoms present in the molecule.