H atom scattering from W(110): A benchmark for molecular dynamics with electronic friction.

Molecular dynamics with electronic friction (MDEF) at the level of the local density friction approximation (LDFA) has been applied to describe electronically non-adiabatic energy transfer accompanying H atom collisions with many solid metal surfaces. When implemented with full dimensional potential energy and electron density functions, excellent agreement with experiment is found. Here, we compare the performance of a reduced dimensional MDEF approach involving a simplified description of H atom coupling to phonons to that of full dimensional MDEF calculations known to yield accurate results.

An Experimental and Theoretical Investigation of the Gas-Phase C(P) + NO Reaction. Low Temperature Rate Constants and Astrochemical Implications.

The reaction between atomic carbon in its ground electronic state, C(P), and nitrous oxide, NO, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.

Random Force in Molecular Dynamics with Electronic Friction.

Originally conceived to describe thermal diffusion, the Langevin equation includes both a frictional drag and a random force, the latter representing thermal fluctuations first seen as Brownian motion. The random force is crucial for the diffusion problem as it explains why friction does not simply bring the system to a standstill. When using the Langevin equation to describe ballistic motion, the importance of the random force is less obvious and it is often omitted, for example, in theoretical treatments of hot ions and atoms interacting with metals.

Ab initio molecular dynamics of hydrogen on tungsten surfaces.

The dissociation process of hydrogen molecules on W(110) was studied using density functional theory and classical molecular dynamics. We have calculated the dissociation probability for molecules with energies below 300 meV and analyzed the dynamics of the adsorption process. Our results show that the fate of each trajectory is determined at distances relatively far from the surface, at roughly 2-2.

Experimental and theoretical studies of the N(D) + H and D reactions.

This study reports the results of an experimental and theoretical investigation of the N(2D) + H2 and N(2D) + D2 reactions at room temperature and below. On the experimental side, a supersonic flow (Laval nozzle) reactor was employed to measure rate constants for these processes at temperatures as low as 127 K. N(2D) was produced indirectly by pulsed laser photolysis and these atoms were detected directly by pulsed laser induced fluorescence in the vacuum ultraviolet wavelength region.

When classical trajectories get to quantum accuracy: II. The scattering of rotationally excited H on Pd(111).

The classical trajectory method in a quantum spirit assigns statistical weights to classical paths on the basis of two semiclassical corrections: Gaussian binning and the adiabaticity correction. This approach was recently applied to the heterogeneous gas-surface reaction between H2 in its internal ground state and Pd(111) surface e.g.

Experimental and Theoretical Study of the O(D) + HD Reaction.

This work addresses the kinetics and dynamics of the gas-phase reaction between O(D) and HD molecules down to low temperature. Here, measurements were performed by using a supersonic flow (Laval nozzle) reactor coupled with pulsed laser photolysis for O(D) production and pulsed-laser-induced fluorescence for O(D) detection to obtain rate constants over the 50-300 K range. Additionally, temperature-dependent branching ratios (OD + H/OH + D) were obtained experimentally by comparison of the H/D atom atom yields with those of a reference reaction.

The Intricate Dynamics of the Si(P) + OH(XΠ) Reaction.

The dynamics of the Si(P) + OH(XΠ) → SiO(XΣ,',') + H(S) reaction is investigated by means of the quasi-classical trajectory method on the electronic ground state XA' potential energy surface in the 10-1 eV collision energy range. Although the reaction involves the formation of a long-lived intermediate complex, a high probability for back-dissociation to the reactants is found because of inefficient intravibrational redistribution of energy among the complex modes. At low collision energies, the reactive events are governed by a dynamics with mixed direct/indirect features.

Energy dissipation to tungsten surfaces upon hot-atom and Eley-Rideal recombination of H.

Adiabatic and nonadiabatic quasi-classical molecular dynamics simulations are performed to investigate the role of electron-hole pair excitations in hot-atom and Eley-Rideal H2 recombination mechanisms on H-covered W(100). The influence of the surface structure is analyzed by comparing with previous results for W(110). In the two surfaces, hot-atom abstraction cross sections are drastically reduced due to the efficient energy exchange with electronic excitations at low incident energies and low coverage, while the effect on Eley-Rideal reactivity is negligible.

Dynamics of N sticking on W(100): the decisive role of van der Waals interactions.

The reactive dynamics of N2 on W(100) has been investigated by means of quasi-classical trajectory calculations using an interpolated six-dimensional potential energy surface (PES) based on density functional theory energies obtained employing the vdW-DF2 functional. The dynamics are compared to those obtained using the PW91 functional and to experimental data. The results show that the new PES provides a significant improvement in the description of the reactivity in this system.

The dynamics of the C(D)+H/D/HD reactions at low temperature.

We present results of a theoretical investigation on the dynamics of the C(D)+H reaction and the corresponding isotopic variants in which the carbon atom collides either with D or HD. Statistical techniques have been tested in comparison with the recent experimental information at low temperature (T < 300 K) and exact quantum mechanical calculations reported on the title reactions in an attempt to establish their possible complex-forming character. Our study includes the calculation of probabilities, rotational distributions, integral cross sections, differential cross sections, and rate constants.

A combined theoretical and experimental investigation of the kinetics and dynamics of the O(D) + D reaction at low temperature.

The O(D) + H reaction is a prototype for simple atom-diatom insertion type mechanisms considered to involve deep potential wells. While exact quantum mechanical methods can be applied to describe the dynamics, such calculations are challenging given the numerous bound quantum states involved. Consequently, efforts have been made to develop alternative theoretical strategies to portray accurately the reactive process.

Communication: Hot-atom abstraction dynamics of hydrogen from tungsten surfaces: The role of surface structure.

Adiabatic and non-adiabatic quasiclassical molecular dynamics simulations are performed to investigate the role of the crystal face on hot-atom abstraction of H adsorbates by H scattering from covered W(100) and W(110). On both cases, hyperthermal diffusion is strongly affected by the energy dissipated into electron-hole pair excitations. As a result, the hot-atom abstraction is highly reduced in favor of adsorption at low incidence energy and low coverages, i.

Hydrogen abstraction from metal surfaces: when electron-hole pair excitations strongly affect hot-atom recombination.

Using molecular dynamics simulations, we predict that the inclusion of nonadiabatic electronic excitations influences the dynamics of preadsorbed hydrogen abstraction from the W(110) surface by hydrogen scattering. The hot-atom recombination, which involves hyperthermal diffusion of the impinging atom on the surface, is significantly affected by the dissipation of energy mediated by electron-hole pair excitations at low coverage and low incidence energy. This issue is of importance as this abstraction mechanism is thought to largely contribute to molecular hydrogen formation from metal surfaces.

S((1)D) + ortho-D2 Reaction Dynamics at Low Collision Energies: Complementary Crossed Molecular Beam Experiments and Theoretical Investigations.

The excitation function of the S((1)D) + D2 reaction was determined in a crossed molecular beam apparatus for collision energies ranging from 1817 to 47 J mol(-1) in the near-cold regime. A very good overall agreement was found between experimental data and the theoretical results obtained using the ab initio potential energy surface built by Ho and coworkers and different methods: time-independent quantum dynamics (QM), semiclassical mean potential capture theory (sc-MPCT), and quasi-classical trajectories (QCT). The general trend of the experimental excitation function is well reproduced in most of the range by a simple capture calculation with an R(-6) dispersion potential.

Quantum state-resolved differential cross sections for complex-forming chemical reactions: Asymmetry is the rule, symmetry the exception.

We argue that statistical theories are generally unable to accurately predict state-resolved differential cross sections for triatomic bimolecular reactions studied in beam experiments, even in the idealized limit where the dynamics are fully chaotic. The basic reason is that quenching of interferences between partial waves is less efficient than intuitively expected, especially around the poles.

Recombination and chemical energy accommodation coefficients from chemical dynamics simulations: O/O2 mixtures reacting over a β-cristobalite (001) surface.

A microkinetic model is developed to study the reactivity of an O/O(2) gas mixture over a β-cristobalite (001) surface. The thermal rate constants for the relevant elementary processes are either inferred from quasiclassical trajectory calculations or using some statistical approaches, resting on a recently developed interpolated multidimensional potential energy surface based on density functional theory. The kinetic model predicts a large molecular coverage at temperatures lower than 1000 K, in contrary to a large atomic coverage at higher temperatures.

Theoretical sensitivity of the C((3)P) + OH(X(2)Pi) --> CO(X(1)Sigma(+)) + H((2)S) rate constant: the role of the long-range potential.

Faced with the lack of experimental data on the C(3P) + OH(X2Pi) --> CO(X1Sigma+) + H(2S) reaction, we propose here to compare rate constant values and their behavior with temperature following various dynamical models and, in particular, to check the sensivity of these quantities with the long-range part of the potential energy surface. For that, we have evaluated the C + OH rate constant using the quasiclassical trajectory (QCT) method, the adiabatic capture centrifugal sudden approximation (ACCSA), and the mean potential capture theory (MPCT) based on a full ab initio potential energy surface fitted with q12,5 kernels or on a perturbative multipolar expansion (MPE) potential including the monomer spin orbit splittings (MPE-SO) or not. Despite the various approximations involved in the different methods and PESs, an excellent agreement is obtained in a subset of three models: the ACCSA method with PME-SO or ab initio PESs and the QCT method with the latter PES.

Quasi-classical statistico-dynamical description of polyatomic photo-dissociations: state-resolved distributions.

An alternative methodology to investigate indirect polyatomic processes with quasi-classical trajectories is proposed, which effectively avoids any binning or weighting procedure while provides rovibrational resolution. Initial classical states are started in terms of angle-action variables to closely match the quantum experimental conditions and later transformed into Cartesian coordinates, following an algorithm very recently published [J. Chem.

On the local relaxation of solid neon upon Rydberg excitation of a NO impurity: the role of the NO(A)-Ne interaction potential and zero-point quantum delocalization.

The local relaxation of solid neon subsequent to the impulsive excitation of the NO chromophore to its A(3s sigma) Ryberg state is investigated using molecular dynamics simulations. This study makes use of empirical NO(X,A)-Ne isotropic pair potentials as well as a recently developed ab initio triatomic potential energy surface for the excited state. The role of these interaction potentials is analyzed, including many-body effects.

On the statistical behavior of the O + OH --> H + O2 reaction: a comparison between quasiclassical trajectory, quantum scattering, and statistical calculations.

The dynamics of the O + OH reaction on the ground state potential energy surface (PES) is investigated by means of the quasiclassical trajectory method and two statistical methods: phase space theory and statistical quantum method. Preliminary calculations with an exact quantum method are also reported. The quasiclassical trajectory calculations show evidence for a phase space bottleneck inhibiting the intramolecular energy transfer between the O-H and O-O bonds.

Study of the H+O2 reaction by means of quantum mechanical and statistical approaches: the dynamics on two different potential energy surfaces.

The possible existence of a complex-forming pathway for the H+O(2) reaction has been investigated by means of both quantum mechanical and statistical techniques. Reaction probabilities, integral cross sections, and differential cross sections have been obtained with a statistical quantum method and the mean potential phase space theory. The statistical predictions are compared to exact results calculated by means of time dependent wave packet methods and a previously reported time independent exact quantum mechanical approach using the double many-body expansion (DMBE IV) potential energy surface (PES) [Pastrana et al.

A comparative study of the Si+O(2)-->SiO+O reaction dynamics from quasiclassical trajectory and statistical based methods.

The dynamics of the singlet channel of the Si+O(2)-->SiO+O reaction is investigated by means of quasiclassical trajectory (QCT) calculations and two statistical based methods, the statistical quantum method (SQM) and a semiclassical version of phase space theory (PST). The dynamics calculations have been performed on the ground (1)A(') potential energy surface of Dayou and Spielfiedel [J. Chem.

Time dependent wave packet and statistical calculations on the H + O(2) reaction.

The H + O(2)--> OH + O reaction has been theoretically investigated by means of an exact time dependent wave packet method and two statistical approaches: a recently developed statistical quantum model and phase-space theory. The exhaustive analysis of reaction probabilities at a zero total angular momentum would, in principle, reveal the existence of a complex-forming mechanism at low collision energies (E(c) = 1.15 eV), whereas deviations from a statistical behaviour at higher energies may be interpreted as the onset of a direct abstraction pathway which favours the production of highly excited rotational states of the OH fragment in its ground vibrational state.

Cross sections and low temperature rate coefficients for the H + CH+ reaction: a quasiclassical trajectory study.

The H + CH(+) reaction is studied by quasiclassical trajectory (QCT) calculations, along with phase space theory (PST) and quantum rigid rotor calculations, employing a global single-valued potential energy surface recently derived by our group. We report QCT total cross sections for each of the three channels, for low collision energies and different reactant rotational quantum numbers. At the lowest collision energies, all cross sections exhibit a capture-like behaviour, as expected from a barrierless reaction.