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The ring polymer molecular dynamics (RPMD) calculations are performed to calculate rate constants for the title reaction on the recently constructed potential energy surface based on permutation invariant polynomial (PIP) neural-network (NN) fitting [J. Li et al., J. Chem. Phys. 142, 204302 (2015)]. By inspecting convergence, 16 beads are used in computing free-energy barriers at 300 K ≤ T ≤ 1000 K, while different numbers of beads are used for transmission coefficients. The present RPMD rates are in excellent agreement with quantum rates computed on the same potential energy surface, as well as with the experimental measurements, demonstrating further that the RPMDis capable of producing accurate rates for polyatomic chemical reactions even at rather low temperatures. [ABSTRACT FROM AUTHOR]

We have constructed a potential energy surface (PES) for H-atoms interacting with fcc Au(111) based on fitting the analytic form of the energy from Effective Medium Theory (EMT) to ab initio energy values calculated with density functional theory. The fit used input from configurations of the H–Au system with Au atoms at their lattice positions as well as configurations with the Au atoms displaced from their lattice positions. It reproduces the energy, in full dimension, not only for the configurations used as input but also for a large number of additional configurations derived from ab initio molecular dynamics (AIMD) trajectories at finite temperature. Adiabatic molecular dynamics simulations on this PES reproduce the energy loss behavior of AIMD. EMT also provides expressions for the embedding electron density, which enabled us to develop a self-consistent approach to simulate nonadiabatic electron-hole pair excitation and their effect on the motion of the incident H-atoms. For H atoms with an energy of 2.7 eV colliding with Au, electron-hole pair excitation is by far the most important energy loss pathway, giving an average energy loss ≈3 times that of the adiabatic case. This increased energy loss enhances the probability of the H-atom remaining on or in the Au slab by a factor of 2. The most likely outcome for H-atoms that are not scattered also depends prodigiously on the energy transfer mechanism; for the nonadiabatic case, more than 50% of the H-atoms which do not scatter are adsorbed on the surface, while for the adiabatic case more than 50% pass entirely through the 4 layer simulation slab. [ABSTRACT FROM AUTHOR]

The infrared spectrum of the Al+–H2 complex is recorded in the H–H stretch region (4075–4110 cm-1) by monitoring Al+ photofragments. The H–H stretch band is centered at 4095.2 cm-1, a shift of -66.0 cm-1 from the Q1(0) transition of the free H2 molecule. Altogether, 47 rovibrational transitions belonging to the parallel Ka=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al+–H2 has a T-shaped equilibrium configuration with the Al+ ion attached to a slightly perturbed H2 molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 Å, decreasing by 0.03 Å when the H2 subunit is vibrationally excited. A three-dimensional potential energy surface for Al+–H2 is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al+–H2(para) and Al+–H2(ortho) are predicted, respectively, to be 469.4 and 506.4 cm-1, in good agreement with previous measurements. The calculations reproduce the experimental H–H stretch frequency to within 3.75 cm-1, and the calculated B and C rotational constants to within ∼2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum. [ABSTRACT FROM AUTHOR]

In a recent paper, we have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom-diatom reactions. In this paper, we show that the same methodology can readily be used to treat more complex polyatomic reactions in their full dimensionality, such as the hydrogen abstraction reaction from methane, H + CH4→ H2 + CH3. The present calculations were carried out using a modified and recalibrated version of the Jordan-Gilbert potential energy surface. The thermal rate coefficients obtained between 200 and 2000 K are presented and compared with previous results for the same potential energy surface. Throughout the temperature range that is available for comparison, the RPMD approximation gives better agreement with accurate quantum mechanical (multiconfigurational time-dependent Hartree) calculations than do either the centroid density version of quantum transition state theory (QTST) or the quantum instanton (QI) model. The RPMD rate coefficients are within a factor of 2 of the exact quantum mechanical rate coefficients at temperatures in the deep tunneling regime. These results indicate that our previous assessment of the accuracy of the RPMD approximation for atom-diatom reactions remains valid for more complex polyatomic reactions. They also suggest that the sensitivity of the QTST and QI rate coefficients to the choice of the transition state dividing surface becomes more of an issue as the dimensionality of the reaction increases. [ABSTRACT FROM AUTHOR]

The energetics of the reaction of HOCO with HO2 have been studied using the quadratic configuration interaction with single and double excitations (QCISD(T)) method and a large basis set on the singlet and triplet potential energy surfaces of the system. The results show that the ground-state O2+HOC(O)H products can be produced by a direct hydrogen abstraction via a transition state with a small barrier (1.66 kcal/mol) on the lowest triplet surface. A similar hydrogen abstraction can occur on the singlet electronic surface, but it leads to the singlet O2(a1Δ) and HOC(O)H. On the singlet surface, a new stable intermediate, HOC(O)OOH, hydroperoxyformic acid, has been found. This intermediate is formed by the direct addition of the terminal oxygen atom in HO2 onto the carbon atom in HOCO in a barrierless reaction. The HOC(O)OOH intermediate may dissociate into either the CO2+H2O2 or CO3+H2O products through elimination reactions with four-center transition states, or into HOC(O)O+OH through an O–O bond cleavage. The heat of formation of HOC(O)OOH is predicted to be -118.9±1.0 kcal/mol. In addition, the dynamics of the HO2+HOCO reaction have been investigated using a scaling-all correlation couple cluster method with single and double excitation terms (CCSD) on the singlet potential energy surface. Reaction mechanisms have been studied in detail. It was found that the direct and addition reaction mechanisms coexist. For the addition mechanism, the lifetime of the HOC(O)OOH intermediate is predicted to be 880±27 fs. At room temperature, the calculated thermal rate coefficient is (6.52±0.44)×10-11 cm3 molecule-1 s-1 with the product branching fractions: 0.77 (CO2+H2O2), 0.15 (HOC(O)O+OH), 0.056 (CO3+H2O), 0.019 (O2(a1Δ)+HOC(O)H), and 0.01 (O2(X 3Σ)+HOC(O)H). [ABSTRACT FROM AUTHOR]

To fast and accurately compute rate coefficients of the H/D + CH4→H2/HD + CH3 reactions, we propose a segmented strategy for fitting suitable potential energy surface (PES), on which ring-polymer molecular dynamics (RPMD) simulations are performed. On the basis of recently developed permutation invariant polynomial neural-network approach [J. Li et al., J. Chem. Phys. 142, 204302 (2015)], PESs in local configuration spaces are constructed. In this strategy, global PES is divided into three parts, including asymptotic, intermediate, and interaction parts, along the reaction coordinate. Since less fitting parameters are involved in the local PESs, the computational efficiency for operating the PES routine is largely enhanced by a factor of ~20, comparing with that for global PES. On interaction part, the RPMD computational time for the transmission coefficient can be further efficiently reduced by cutting off the redundant part of the child trajectories. For H + CH4, good agreements among the present RPMD rates and those from previous simulations as well as experimental results are found. For D + CH4, on the other hand, qualitative agreement between present RPMD and experimental results is predicted. [ABSTRACT FROM AUTHOR]

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H2 and D2 on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (Ts = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found. [ABSTRACT FROM AUTHOR]

Despite its relative simplicity, the role of tunneling in the reaction OH + CO → H + CO2 has eluded the quantitative predictive powers of theoretical reaction dynamics. In this study a one-dimensional effective barrier to the formation of H + CO2 from the HOCO intermediate is directly extracted from dissociative photodetachment experiments on HOCO and DOCO. Comparison of this barrier to a computed minimum-energy barrier shows that tunneling deviates significantly from the calculated minimum-energy pathway, predicting product internal energy distributions that match those found in the experiment and tunneling lifetimes short enough to contribute significantly to the overall reaction. This barrier can be of direct use in kinetic and statistical models and aid in the further refinement of the potential energy surface and reaction dynamics calculations for this system. [ABSTRACT FROM AUTHOR]

The room temperature rate constants for quenching of the fluorescence of H2, HD, and D2 B1Σ+u by 4He have been measured as a function of the initially excited rotational and vibrational levels of the hydrogen molecule. The effective quenching cross sections increase with increasing vibrational energy from about 1 Å2 up to a maximum of about 6 Å2. The effective cross sections for D2 (B, v’ = 0) were independent of the rotational level excited for 0 < J’ ≤ 7, and the cross sections for (v’ = 0, J’ = 0) were about 80% of the values for (v’ = 0, J’ >= 0) for all three isotopes studied. Quenching occurs via formation of an electronically excited (H2He)* collision complex followed by crossing to the repulsive H2(X)–He potential energy surface. The vibrational state dependence of the quenching cross sections fits a vibrationally adiabatic model for complex formation. From the vibrational state dependence of the quenching cross section, the barrier height for the quenching reaction is found to be 250±40 cm-1, and the difference in the H–H stretching frequencies between H2(B) and the H2–He complex at the barrier to reaction is 140±80 cm-1. Both values are substantially smaller than results from ab initio calculations. The rotational state dependence of the quenching cross sections suggests that quenching occurs with H2 rotating in a plane perpendicular to the relative velocity vector, in qualitative agreement with the rotational anisotropy of the H2(B)–He ab initio electronic potential energy surface. [ABSTRACT FROM AUTHOR]

We have measured rotational distributions for H2 and D2 thermally desorbed from Si(100) surfaces using resonance-enhanced multiphoton ionization (REMPI) for detection. These distributions are described by average rotational energies that are significantly lower than kTs (Ts=surface temperature) and exhibit slight, if any, isotopic dependence, i.e, =368±67 K and =348±65 K. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high-impact-parameter collisions. The rotational distributions indicate that some dynamical constraint causes very little torque to be applied to molecular hydrogen during recombination. Our data may be interpreted as resulting from an exaggerated preference for reactive trajectories that are characterized by low-impact parameters and/or a high degree of symmetry of the bond axis relative to the potential energy surface, followed by prompt desorption of the newly formed molecular hydrogen from Si(100). [ABSTRACT FROM AUTHOR]

The room temperature rate constants for molecular angular momentum reorientation of H2, HD, and D2 (B 1∑+u, v’=0, J’=1, MJ’=0) in collisions with He, Ne, Ar and H2 (X 1∑+g) have been measured. The effective cross sections for changing MJ’ in collisions of H2, HD, D2 with He and Ne were found to be about 30 Å2 and were nearly the same for each isotope and with He and Ne as collision partners. The measured He–H2(B) reorientation cross section is about 50% larger than the cross section calculated with a simple semiclassical model using a potential that approximates the ab initio data for the H2(B)–He potential energy surface. The cross sections for reorientation of HD and D2 in collisions with Ar were found to be 10.6±2.0 and 13.9±3.0 Å2, respectively. The smaller cross section is due to the dominant role played by quenching of the electronic energy of molecular hydrogen in collisions with Ar. The reorientation of D2(B) in collisions with room temperature H2(X) occurs with a 7.6±3.4 Å2 cross section. The small cross section for reorientation of the angular momentum is again due to the dominance of quenching in the collision dynamics. [ABSTRACT FROM AUTHOR]

The near-infrared spectrum of ArHF prepared in a slit supersonic expansion is recorded with a difference frequency infrared laser spectrometer. By virtue of the high sensitivity of the technique, and the lack of appreciable spectral congestion at the 10 K jet temperature, we observe 9 of the 11 vibrational states with energies below the Ar+HF(v=1, j=0) dissociation limit. These include (1000), the lowest bound HF (v=1) state, the singly, doubly, and quadruply van der Waals stretch excited states (1001) (1002), and (1004), both the Σ bend (1200) and Π bend (111e,f 0), and the multiply excited, Π bend plus van der Waals stretch (111e,f 1). Two Ar+HF(v=0) states, (0000) and (0001), are also characterized. This spectroscopic information is quite sensitive to the Ar+HF potential energy surface away from the equilibrium configuration, and thus provides a rigorous test of trial potential energy surfaces. Excellent agreement is obtained between experiment and the predictions of a recently reported Ar+HF(v=1) potential. [ABSTRACT FROM AUTHOR]

The dynamics of activated associative desorption is discussed with particular reference to the system H2–Cu and to the partitioning of the energy released among the various product degrees of freedom. It is argued that a simple theory based on transition-state concepts should hold for this system because the potential energy surface (PES) divides naturally into reactant and product regions, separated by a ‘‘seam’’ or ‘‘ridge’’ at which it is reasonable to assume a thermal distribution of desorbing trajectories. Using a PES constructed in accordance with available electronic structure calculations we consider the angular distributions and translational, vibrational, and rotational energy distributions of the desorbing molecules. It is shown that, whereas the rotational energy reflects the surface temperature, the vibrational energy is markedly enhanced because the energetically low-lying regions of the ridge in the PES correspond to an H–H bond distance that is distended as compared with the gas-phase equilibrium separation. The enhancement is found to be a strong function of the surface temperature. The translational energy, however, is found to be only very weakly dependent on the temperature. These results are discussed in connection with available data. [ABSTRACT FROM AUTHOR]

The unimolecular decomposition dynamics of Si2H4 have been investigated using classical trajectory methods on a global potential-energy surface fitted to the results of ab initio calculations and the available experimental data. The required phase-space averages are computed using Metropolis sampling techniques. It is found that unless the parameters of the Markov walk are adjusted for each different type of atom present, extremely long Markov walks are required to adequately cover the phase space of the system. Microcanonical rate coefficients for the decomposition of Si2H4 into all open channels are reported at energies in the range 5.0

The recombination dynamics for the SiH2+SiH2→H2Si=SiH2 reaction are studied by quasiclassical trajectory methods using a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 18 many-body terms whose functional forms are motivated by chemical and physical considerations. The surface contains 41 parameters which are fitted to calculated geometries, fundamental vibrational frequencies, and energies for H2Si=SiH2, H2Si=SiH, H2Si=Si, HSi=Si, Si2, H2, and SiH2, and to various calculated and/or measured reaction barrier heights and activation energies. In general, the equilibrium bond lengths and angles given by the global surface are in agreement with abinitio results to within 0.03 Å and 0.5°, respectively. The calculated exothermicities for various reactions involving silicon and hydrogen atoms are in excellent agreement with previous MP4 calculations and with experimental data. The average absolute error is 1.90 kcal/mol. The average absolute deviation of the predicted fundamental vibrational frequencies for H2Si=SiH2, H2Si=SiH, H2Si=Si, and SiH2 from the results reported by Ho et al. is 52.9 cm-1. The calculated barrier height for molecular hydrogen elimination from SiH2 is 34.27 kcal/mol with a backreaction barrier of 0.63 kcal/mol. The barrier for 1,2 elimination of H2 from H2Si=SiH2 is 115.3 kcal/mol with a backreaction barrier of 30.7 kcal/mol. The formation cross sections for H2Si=SiH2 decrease with both relative translational energy and internal SiH2 energy with translational energy being the more effective in reducing the cross sections. Thermally averaged formation cross sections vary from 66.3 Å2 at 300 K to 28.7 Å2 at 1500 K. The corresponding thermal rate coefficients lie in the range 2–4×1014 cm3/mol s over this temperature range and exhibit a maximum at an intermediate temperature. The... [ABSTRACT FROM AUTHOR]

We perform a detailed analysis of the dynamics of the dissociative adsorption of H[sub 2] molecules on a Pd(111) surface using ab initio data for the molecule-surface interaction and classical trajectory methods. We show that the reaction probability is completely determined by the molecule-surface interaction in the approach toward the surface before it reaches a critical distance of 1.5 Å. The corresponding dynamics can be reduced to a 2D one, involving only the translational and rotational degrees of freedom, except in the lower energy range where an important role is played by dynamic trapping. We establish the relation between the dissociation probability and the shape of 2D cuts of the potential energy surface using a simple model of the evolution of orientational forces as the molecule approaches the surface. Whereas above 1.5 Å the molecule evolves "as a whole," below 1.5 Å the dynamics has the character of independent atom-surface interactions which explains why it dissociates with a probability close to one once it has reached the critical distance of 1.5 Å. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 [sup 2]P[sub J]) with a H[sub 2] reaction, which is endothermic by 1680 cm-1. The LiH (v=0) distribution yields a single rotational temperature at ∼770 K, but the population in the v=1 level is not detectable. According to the potential energy surface (PES) calculations, the insertion mechanism in (near) C[sub 2v] collision geometry is favored. The Li (2 [sup 2]P[sub J])-H[sub 2] collision is initially along the 2A[sup ′] surface in the entrance channel and then diabatically couples to the ground 1A[sup ′] surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280±46 cm[sup -1], indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 [sup 2]S or 3 [sup 2]P), we cannot detect any LiH product. From a theoretical point of view, the 4A[sup ′] surface, correlating with the Li 3 [sup 2]S state, may feasibly couple to a repulsive 3A[sup ′] surface, from which the collision complex will rapidly break apart into Li (2 [sup 2]P[sub J]) and H[sub 2]. The probability for further surface hopping to the 2A[sup ′] or 1A[sup ′] surfaces is negligible, since the 3A[sup ′] and 2A[sup ′] surfaces are too far separated to allow for an efficient coupling. The Li (3 [sup 2]P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vibrationally state-resolved differential cross sections (DCS) and product rotational distributions have been measured for the Cl+HD(v=1, J=1)→HCl(DCl)+D(H) reaction at a mean collision energy of 0.065 eV using a photoinitiated reaction ("photoloc") technique. The effect of HD reagent rotational alignment in the Cl+HD(v=1, J=2) reaction has also been investigated. The experimental results have been compared with exact quantum mechanical and quasiclassical trajectory calculations performed on the G3 potential energy surface of Allison et al. [J. Phys. Chem. 100, 13575 (1996)]. The experimental measurements reveal that the products are predominantly backward and sideways scattered for HCl(v[sup ′]=0) and HCl(v[sup ′]=1), with no forward scattering at the collision energies studied, in quantitative agreement with theoretical predictions. The experimental product rotational distribution for HCl(v[sup ′]=1) also shows excellent agreement with quantum-mechanical calculations, but the measured DCl+H to HCl+D branching ratio is near unity, which is at variance with the theoretical calculations that predict about 3 times larger yield of HCl+D at these collision energies. The reactivity shows a marked dependence on the direction of the HD(v=1, J=2) rotational angular momentum, and experimental measurements of this reagent alignment effect are in good agreement with theoretical predictions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We employ the on-the-fly surface hopping classical trajectory algorithm to study the electron detachment process in low-energy H- + H2 collisions. The ground-state and the first-excited-state Local Complex Potentials (LCPs) calculated by the generalized diatomics-in-molecule method are used for the full three-dimensional nonadiabatic nuclear dynamics. Two kinds of nonadiabatic effects are taken into account: discrete-discrete transitions and discrete-continuous transitions. Discrete-discrete nonadiabatic transition probabilities are calculated by means of the adiabatic-potential-based formula within the Landau-Zener model for each individual trajectory computed along real parts of the LCPs. Discrete-continuous (electron detachment) nonadiabatic transition probabilities are calculated via quasi-stationary widths which are related to the imaginary parts of the LCPs of both the electronic states of the H- 3 anion. Two mechanisms of the electron detachment process are treated and discussed: the direct mechanism based on quasi-stationarity of the ground state and the indirect mechanism based on both nonadiabatic transitions from the ground state to the first excited state and quasi-stationarity of the excited state. It is shown that the direct mechanism prevails at low collision energies, while the indirect mechanism makes a substantial contribution at relatively high collision energies, roughly higher than 5 eV. At collision energies higher than 2 eV, the electron detachment probability has rather high values and this affects noticeably other inelastic processes in these collisions. The electron detachment cross section in H- collisions is calculated for the collision energy range from 1 to 100 eV and a reasonable agreement with available experimental data is obtained. [ABSTRACT FROM AUTHOR]2 collisions is calculated for the collision energy range from 1 to 100 eV and a reasonable agreement with available experimental data is obtained. [ABSTRACT FROM AUTHOR]

The hydrogens in protonated acetylene are very mobile and can easily migrate around the C2 core by moving between classical and non-classical structures of the cation. The lowest energy structure is the T-shaped, non-classical cation with a hydrogen bridging the two carbons. Conversion to the classical H2CCH+ ion requires only 4 kcal/mol. The effect of circularly polarized light on the migration of hydrogens in oriented C2H3+ has been simulated by Born-Oppenheimer molecular dynamics. Classical trajectory calculations were carried out with the M062X/6-311+G(3df,2pd) level of theory using linearly and circularly polarized 32 cycle 7 µm cosine squared pulses with peak intensity of 5.6 × 1013 W/cm² and 3.15 × 1013 W/cm², respectively. These linearly and circularly polarized pulses transfer similar amounts of energy and total angular momentum to C2H3+. The average angular momentum vectors of the three hydrogens show opposite directions of rotation for right and left circularly polarized light, but no directional preference for linearly polarized light. This difference results in an appreciable amount of angular displacement of the three hydrogens relative to the C2 core for circularly polarized light, but only an insignificant amount for linearly polarized light. Over the course of the simulation with circularly polarized light, this corresponds to a propeller-like motion of the three hydrogens around the C2 core of protonated acetylene. [ABSTRACT FROM AUTHOR]

The aqueous hydrogen molecule is studied with molecular dynamics simulations at ambient temperature and pressure conditions, using a newly developed flexible and polarizable H2 molecule model. The design and implementation of this model, compatible with an existing flexible and polarizable force field for water, is presented in detail. The structure of the hydration layer suggests that first-shell water molecules accommodate the H2 molecule without major structural distortions and two-dimensional, radial-angular distribution functions indicate that as opposed to strictly tangential, the orientation of these water molecules is such that the solute is solvated with one of the free electron pairs of H2O. The calculated self-diffusion coefficient of H2(aq) agrees very well with experimental results and the time dependence of mean square displacement suggests the presence of caging on a time scale corresponding to hydrogen bond network vibrations in liquid water. Orientational correlation function of H2 experiences an extremely short-scale decay, making the H2-H2O interaction potential essentially isotropic by virtue of rotational averaging. The inclusion of explicit polarizability in the model allows for the calculation of Raman spectra that agree very well with available experimental data on H2(aq) under differing pressure conditions, including accurate reproduction of the experimentally noted trends with solute pressure or concentration. [ABSTRACT FROM AUTHOR]

Methanol synthesis from CO and H2 over ZnO, which requires high temperatures and high pressures giving rise to a complex interplay of physical and chemical processes over this heterogeneous catalyst surface, is investigated using ab initio simulations. The redox properties of the surrounding gas phase are known to directly impact on the catalyst properties and thus, set the overall catalytic reactivity of this easily reducible oxide material. In Paper III of our series [J. Kiss, J. Frenzel, N. N. Nair, B. Meyer, and D. Marx, J. Chem. Phys. 134, 064710 (2011)] we have qualitatively shown that for the partially hydroxylated and defective ZnO(0001) surface there exists an intricate network of surface chemical reactions. In the present study, we employ advanced molecular dynamics techniques to resolve in detail this reaction network in terms of elementary steps on the defective surface, which is in stepwise equilibrium with the gas phase. The two individual reduction steps were investigated by ab initio metadynamics sampling of free energy landscapes in three-dimensional reaction subspaces. By also sampling adsorption and desorption processes and thus molecular species that are in the gas phase but close to the surface, our approach successfully generated several alternative pathways of methanol synthesis. The obtained results suggest an Eley-Rideal mechanism for both reduction steps, thus involving "near-surface" molecules from the gas phase, to give methanol preferentially over a strongly reduced catalyst surface, while important side reactions are of Langmuir-Hinshelwood type. Catalyst re-reduction by H2 stemming from the gas phase is a crucial process after each reduction step in order to maintain the catalyst's activity toward methanol formation and to close the catalytic cycle in some reaction channels. Furthermore, the role of oxygen vacancies, side reactions, and spectator species is investigated and mechanistic details are discussed based on extensive electronic structure analysis. [ABSTRACT FROM AUTHOR]

The Alm (-) ∙ (H2O)n systems are known to undergo water splitting processes in the gas phase giving HkAlm(OH)k (-) ∙ (H2. The migration of H atoms from one Al atom to another on the cluster's surface is of critical importance to the mechanism of the complete Hn-k production process. We have applied a combination of Molecular Dynamics and Rice-Ramsperger- Kassel-Marcus theory including tunneling effects to study the gas-phase evolution of HAl2. The migration of H atoms from one Al atom to another on the cluster's surface is of critical importance to the mechanism of the complete H2 production process. We have applied a combination of Molecular Dynamics and Rice-Ramsperger- Kassel-Marcus theory including tunneling effects to study the gas-phase evolution of HAl17(OH)(-), which can be considered a model system. First, we have performed an extensive search for local minima and the connecting saddle points using a density functional theory method. It is found that in the water-splitting process Al17 (-), the H atom which bonds to the Al cluster losses rather quickly its excess energy, which is easily "absorbed" by the cluster because of its flexibility. This fact ultimately determines that long-range hydrogen migration is not a very fast process and that, probably, tunneling only plays a secondary role in the migration dynamics, at least for moderate energies. Reduction of the total energy results in the process being very much slowed down. The consequences on the possible mechanisms of H2O) → HAl17(OH)(-), the H atom which bonds to the Al cluster losses rather quickly its excess energy, which is easily "absorbed" by the cluster because of its flexibility. This fact ultimately determines that long-range hydrogen migration is not a very fast process and that, probably, tunneling only plays a secondary role in the migration dynamics, at least for moderate energies. Reduction of the total energy results in the process being very much slowed down. The consequences on the possible mechanisms of H2 generation from the interaction of Al clusters and water molecules are discussed. [ABSTRACT FROM AUTHOR]

We derive equations for nonadiabatic Ehrenfest molecular dynamics within the projector augmented-wave (PAW) formalism. The discretization of the electrons is time-dependent as the augmentation functions depend on the positions of the nuclei. We describe the implementation of the Ehrenfest molecular dynamics equations within the real-space PAW method. We demonstrate the applicability of our method by studying the vibration of NaCl, the torsional rotation of H2C=NH2+ in both the adiabatic and the nonadiabatic regimes, and the hydrogen bombardment of C40H16. [ABSTRACT FROM AUTHOR]

The interaction of hydrogen with palladium surfaces represents one of the model systems for the study of the adsorption and absorption at metal surfaces. Theoretical gas-surface dynamics studies have usually concentrated on the adsorption dynamics on clean surfaces. Only recently it has become possible, based on advances in the electronic structure codes and improvements in the computer power, to address the much more complex problem of the adsorption dynamics on precovered surfaces. Here, I present ab initio molecular dynamics (AIMD) simulations based on periodic density functional theory (DFT) calculations of the adsorption of H2 on hydrogen-precovered Pd(100) for a broad variety of different hydrogen coverage structures. The stability of the adsorbate structures and the adsorption dynamics are analyzed in detail. Calculated sticking probabilities are larger than expected for pure site-blocking consistent with experimental results. It turns out that the adsorption dynamics on the strongly corrugated surfaces depends sensitively on the dynamic response of the substrate atoms upon the impact of the impinging H2 molecules. In addition, for some structures the adsorption probability was evaluated as a function of the kinetic energy. Adsorbate structures corresponding to the same coverage but with different arrangements of the adsorbed atoms can lead to a qualitatively different dependence of the adsorption probability on the kinetic energy changing also the order of the preferred structures, as far as the adsorption is concerned, as a function of the kinetic energy. This indicates that dynamical effects such as steering and dynamical trapping play an important role in the adsorption on these precovered substrates. [ABSTRACT FROM AUTHOR]

Higher order factorization schemes are developed for path integral molecular dynamics in order to improve the convergence of estimators for physical observables as a function of the Trotter number. The methods are based on the Takahashi-Imada and Susuki decompositions of the Boltzmann operator. The methods introduced improve the averages of the estimators by using the classical forces needed to carry out the dynamics to construct a posteriori weighting factors for standard path integral molecular dynamics. The new approaches are straightforward to implement in existing path integral codes and carry no significant overhead. The Suzuki higher order factorization was also used to improve the end-to-end distance estimator in open path integral molecular dynamics. The new schemes are tested in various model systems, including an ab initio path integral molecular dynamics calculation on the hydrogen molecule and a quantum water model. The proposed algorithms have potential utility for reducing the cost of path integral molecular dynamics calculations of bulk systems. [ABSTRACT FROM AUTHOR]

Collision induced dissociation (CID), four center reaction (4C), and single exchange reaction (SE) in H2 (v1 = high) + D2 (v2 = low) were studied by means of time-dependent wave packet approach within a full-dimensional model. Initial state-selected total reaction probabilities for the three competitive processes have been computed on two realistic global potential energy surfaces of Aguado-Suárez-Paniagua and Boothroyd-Martin-Keogh-Peterson (BMKP) with the total angular momentum J = 0. The role of both vibrationally excited and rotationally excited reagents was examined by varying the initial vibrational and rotational states. The vibrational excitation of the hot diatom gives an enhancement effect on the CID process, while the vibrational excitation of the cold diatom gives an inhibition effect. The rotational excitation of both reagents has a significant effect on the reaction process. The 4C and SE probabilities are at least one order of magnitude smaller than the CID probabilities over the energy range considered. Isotope substitution effects were also studied by substituting the collider D2 by H2 and HD on the BMKP potential energy surfaces. The CID process is most efficient for the H2 + D2 combination and least efficient for the H2 + H2 combination and is different for the 4C and SE processes. [ABSTRACT FROM AUTHOR]

A new method, here called thermal Gaussian molecular dynamics (TGMD), for simulating the dynamics of quantum many-body systems has recently been introduced [I. Georgescu and V. A. Mandelshtam, Phys. Rev. B 82, 094305 (2010)]. As in the centroid molecular dynamics (CMD), in TGMD the N-body quantum system is mapped to an N-body classical system. The associated both effective Hamiltonian and effective force are computed within the variational Gaussian wave-packet approximation. The TGMD is exact for the high-temperature limit, accurate for short times, and preserves the quantum canonical distribution. For a harmonic potential and any form of operator A⁁, it provides exact time correlation functions CAB(t) at least for the case of B⁁, a linear combination of the position, x⁁, and momentum, p⁁, operators. While conceptually similar to CMD and other quantum molecular dynamics approaches, the great advantage of TGMD is its computational efficiency. We introduce the many-body implementation and demonstrate it on the benchmark problem of calculating the velocity time auto-correlation function for liquid para-hydrogen, using a system of up to N = 2592 particles. [ABSTRACT FROM AUTHOR]

All the channels of the reaction dynamics of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Cu(111) surface have been studied by quasiclassical constant energy molecular dynamics simulations. The surface is flexible and is prepared at different temperature values, such as 30 K, 94 K, and 160 K. The adsorbates were distributed randomly on the surface to create 0.18 ML, 0.28 ML, and 0.50 ML of coverages. The multi-layer slab is mimicked by a many-body embedded-atom potential energy function. The slab atoms can move according to the exerted external forces. Treating the slab atoms non-rigid has an important effect on the dynamics of the projectile atom and adsorbates. Significant energy transfer from the projectile atom to the surface lattice atoms takes place especially during the first impact that modifies significantly the details of the dynamics of the collisions. Effects of the different temperatures of the slab are investigated in this study. Interaction between the surface atoms and the adsorbates is modeled by a modified London-Eyring-Polanyi-Sato (LEPS) function. The LEPS parameters are determined by using the total energy values which were calculated by a density functional theory and a generalized gradient approximation for an exchange-correlation energy for many different orientations, and locations of one- and two-hydrogen atoms on the Cu(111) surface. The rms value of the fitting procedure is about 0.16 eV. Many different channels of the processes on the surface have been examined, such as inelastic reflection of the incident hydrogen, subsurface penetration of the incident projectile and adsorbates, sticking of the incident atom on the surface. In addition, hot-atom and Eley-Rideal direct processes are investigated. The hot-atom process is found to be more significant than the Eley-Rideal process. Furthermore, the rate of subsurface penetration is larger than the sticking rate on the surface. In addition, these results are compared and analyzed as a function of the surface temperatures. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations with a polarizable multistate empirical valence-bond model were carried out to investigate NaOH dissociation and pairing in water bulk and at the air-water interface. It was found that NaOH readily dissociates in the bulk and the effect of the air-water interface on NaOH dissociation is fairly minor. Also, NaOH complexes were found to be strongly repelled from the air-water interface, which is consistent with surface tension measurements. At the same time, a very strong preference for the hydroxide anion to be oriented toward the air was found that persisted a few angstroms toward the liquid from the Gibbs dividing surface of the air-water interface. This was due to a preference for the hydroxide anion to have its hydrogen pointing toward the air and the fact that the sodium ion was more likely to be found near the hydroxide oxygen than hydrogen. As a consequence, the simulation results show that surfaces of NaOH solutions should be negatively charged, in agreement with experimental observations, but also that the hydroxide has little surface affinity. This provides the possibility that the surface of water can be devoid of hydroxide anions, but still have a strong negative charge. [ABSTRACT FROM AUTHOR]

We investigate and analyze the vibrational properties, including hydrogen/deuterium isotope effects, in a fundamental organic hydrogen bonded system using multiple experimental (infrared multiple photon dissociation and argon-tagged action spectroscopy) and computational techniques. We note a qualitative difference between the two experimental results discussed here and employ ab initio molecular dynamics simulations to explain these results. A deeper understanding of the differences between the isotopically labeled systems arises from an analysis of the simulated cluster spectroscopy and leads to a system-bath coupling interpretation. Specifically, when a few active modes, involving the shared hydrogen/deuterium stretch, are identified and labeled as “system,” with all other molecular vibrational modes being identified as “bath” modes, we find critical differences in the coupling between the system modes for the shared proton and shared deuteron cases. These differences affect the energy repartitioning between these modes resulting in a complex spectral evolution as a function of temperature. Furthermore, intensity borrowing across modes that are widely distributed in the frequency domain plays an important role on the simulated spectra. [ABSTRACT FROM AUTHOR]

This paper presents comparisons between molecular dynamics simulations (MDSs) and the Keilson and Storer (KS) model for collision-induced translational velocity changes in pure H2 at room temperature from four different points of view. The first considers various autocorrelation functions associated with the velocity. The second and third comparisons are made for the collision kernels and for the time evolutions of some conditional probabilities for changes in the velocity modulus and orientation. Finally, the evolutions, with density, of the half widths of the Q(1) lines of the isotropic Raman (1–0) fundamental band and of the (2–0) overtone quadrupole band are investigated. The results demonstrate that, while the KS approach gives a poor description of detailed velocity-to-velocity changes, it leads to accurate results for the correlation functions and spectral shapes, quantities resulting from large averages over the velocity. On the opposite, collision kernels derived from MDS lead to accurate predictions of all considered quantities. The results open promising perspectives for modeling of the spectral shapes of other systems. They also stress the value of direct calculations of speed-dependent broadening and shifting parameters from the intermolecular potential to avoid their determination from measured spectra and permit fully meaningful tests of the models. [ABSTRACT FROM AUTHOR]

The reaction of HOCO with O atoms has been studied using a direct ab initio dynamics approach based on the scaling all correlation UCCD/D95(d,p) method. Ab initio calculations point to two possible reaction mechanisms for the O+HOCO→OH+CO2 reaction. They are a direct hydrogen abstraction and an oxygen addition reaction through a short-lived HOC(O)O intermediate. The dynamics results show that only the addition mechanism is important under the conditions considered here. The lifetime of the HOC(O)O complex is predicted to be 172±15 fs. This is typical of a direct and fast radical-radical reaction. At room temperature, the calculated thermal rate coefficient is 1.44×10-11 cm3 mol-1 s-1 and its temperature dependence is rather weak. The two kinds of reactive trajectories are illustrated in detail. [ABSTRACT FROM AUTHOR]

Free energy calculations were carried out for water addition coupled reduction of aqueous ruthenate, RuO4-+H2O+e-→[RuO3(OH)2]2-, using Car-Parrinello molecular dynamics simulations. The full reaction is divided into the reduction of the tetrahedral monoanion, RuO4-+e-→RuO42-, followed by water addition, RuO42-+H2O→[RuO3(OH)2]2-. The free energy of reduction is computed from the fluctuations of the vertical energy gap using the MnO4-+e-→MnO42- reaction as reference. The free energy for water addition is estimated using constrained molecular dynamics methods. While the description of this complex reaction, in principle, involves multiple reaction coordinates, we found that reversible transformation of the reactant into the product can be achieved by control of a single reaction coordinate consisting of a suitable linear combination of atomic distances. The free energy difference of the full reaction is computed to be -0.62 eV relative to the normal hydrogen electrode. This is in good agreement with the experimental value of -0.59 eV, lending further support to the hypothesis that, contrary to the ruthenate monoanion, the dianion is not tetrahedral but forms a trigonal-bipyramidal dihydroxo complex in aqueous solution. We construct an approximate two-dimensional free energy surface using the coupling parameter for reduction and the mechanical constraint for water addition as variables. Analyzing this surface we find that in the most favorable reaction pathway the reduction reaction precedes water addition. The latter takes place via the protonated complex [RuO3(OH)]- and subsequent transport of the created hydroxide ion to the fifth coordination site of Ru. [ABSTRACT FROM AUTHOR]

A hybrid quantum/classical path integral Monte Carlo (QC-PIMC) method for calculating the quantum free energy barrier for hydrogen transfer reactions in condensed phases is presented. In this approach, the classical potential of mean force along a collective reaction coordinate is calculated using umbrella sampling techniques in conjunction with molecular dynamics trajectories propagated according to a mapping potential. The quantum contribution is determined for each configuration along the classical trajectory with path integral Monte Carlo calculations in which the beads move according to an effective mapping potential. This type of path integral calculation does not utilize the centroid constraint and can lead to more efficient sampling of the relevant region of conformational space than free-particle path integral sampling. The QC-PIMC method is computationally practical for large systems because the path integral sampling for the quantum nuclei is performed separately from the classical molecular dynamics sampling of the entire system. The utility of the QC-PIMC method is illustrated by an application to hydride transfer in the enzyme dihydrofolate reductase. A comparison of this method to the quantized classical path and grid-based methods for this system is presented. [ABSTRACT FROM AUTHOR]

A fast centroid molecular dynamics (CMD) methodology is proposed in which the effective centroid forces are predetermined through a force-matching algorithm applied to a standard path integral molecular dynamics simulation. The resulting method greatly reduces the computational cost of generating centroid trajectories, thus extending the applicability of CMD. The method is applied to the study of liquid para-hydrogen at two state points and liquid ortho-deuterium at one state point. The static and dynamical results are compared to those obtained from full adiabatic CMD simulations and found to be in excellent agreement for all three systems; the transport properties are also compared to experiment and found to have a similar level of agreement. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The three-dimensional hydrogen cyanide/isocyanide isomerization problem is taken as an example to present a general theory for computing the phase space structures which govern classical reaction dynamics in systems with an arbitrary (finite) number of degrees of freedom. The theory, which is algorithmic in nature, comprises the construction of a dividing surface of minimal flux which is locally a ''surface of no return.'' The theory also allows for the computation of the global phase space transition pathways that trajectories must follow in order to react. The latter are enclosed by the stable and unstable manifolds of a so-called normally hyperbolic invariant manifold (NHIM). A detailed description of the geometrical structures and the resulting constraints on reaction dynamics is given, with particular emphasis on the three degrees of freedom case. A procedure is given which uses these structures to compute orbits homoclinic to, and heteroclinic between, NHIMs. The role of homoclinic and heteroclinic orbits in global recrossings of dividing surfaces and transport in complex systems is explained. The complete description provided here is inherently one within phase space; it cannot be inferred from a configuration space picture. A complexification of the classical phase space structures to incorporate quantum effects is also discussed. The results presented here call into question certain assumptions routinely made on the global dynamics; this paper provides methods that enable one to understand and quantify the phase space dynamics of reactions without making such assumptions. [ABSTRACT FROM AUTHOR]

This work presents a study of reactions between neutral and negatively charged Au[sub n] clusters (n=2,3) and molecular hydrogen. The binding energies of the first and second hydrogen molecule to the gold clusters were determined using density functional theory (DFT), second order perturbation theory (MP2) and coupled cluster (CCSD(T)) methods. It is found that molecular hydrogen easily binds to neutral Au[sub 2] and Au[sub 3] clusters with binding energies of 0.55 eV and 0.71 eV, respectively. The barriers to H[sub 2] dissociation on these clusters with respect to Au[sub n]H[sub 2] complexes are 1.10 eV and 0.59 eV for n=2 and 3. Although negatively charged Au[sub n][sup -] clusters do not bind molecular hydrogen, H[sub 2] dissociation can occur with energy barriers of 0.93 eV for Au[sub 2][sup -] and 1.39 eV for Au[sub 3][sup -]. The energies of the Au[sub 2]H[sub 2][sup -] and Au[sub 3]H[sub 2][sup -] complexes with dissociated hydrogen molecules are lower than the energies of Au[sub 2][sup -]+H[sub 2] and Au[sub 3][sup -]+H[sub 2] by 0.49 eV and 0.96 eV, respectively. There is satisfactory agreement between the DFT and CCSD(T) results for binding energies, but the agreement is not as good for barrier heights. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An empirical correction to amide group vacuum force fields is proposed in order to account for the influence of the aqueous environment on the C==O stretching vibration (amide I). The dependence of the vibrational absorption spectral intensities on the geometry is studied with density functional theory methods at the BPW91/6-31G[sup **] level for N-methyl acetamide interacting with a variety of of water molecule clusters hydrogen bonded to it. These cluster results are then generalized to form an empirical correction for the force field and dipole intensity of the amide I (C==O stretch) mode. As an example of its extension, the method is applied to a larger (β-turn model) peptide molecule and its IR spectrum is simulated. The method provides realistic bandwidths for the amide I bands if the spectra are generated from the ab initio force field corrected by perturbation from an ensemble of solvent geometries obtained using molecular dynamic simulations. © 2003 American Institute of Physics.figref [ABSTRACT FROM AUTHOR]

Density functional theory has been used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the H[sub 2] molecule outside the nanotube are in the range 0.04-0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the tube and weakens the C-C bond. This effect can catalyze the shattering and scission of the tube by incoming hydrogen molecules with sufficient kinetic energy. [ABSTRACT FROM AUTHOR]

We have analyzed the molecular-dynamics (MD) trajectories for the oxygen and hydrogen atoms of liquid water, at six temperatures (from hot, T=361 K, to supercooled water, T=242 K); in the MD simulations the Nieser-Corongiu-Clementi ab initio potential has been used, since it yields reliable x-ray and neutron-diffraction data as well as infrared, Raman, and neutron-scattering spectra. Our analysis leads to two complementary models where we can consider each water as a solvated molecule (placed at the center of a solvation shell) or as a component of a cyclic polymer, a substructure of the hydrogen-bonded network. In the first solvation shell all water molecules are solvated with coordination values in the range 2–8. The most probable solvation number is four, at low temperature, and five at high temperature considering oxygen–oxygen pairs; however, the coordination number is four at all the temperatures if we consider oxygen–hydrogen pairs. The lifetime of the tetra coordinated complexes is the largest one and increases as temperature decreases. The computed population of cyclic polymers is highest for the pentameters in the studied temperature range, the second most probable cyclic structure is for hexamers. The average O–O distances in the liquid are temperature dependent and shorter than those in the gas phase, approaching ice values at low temperature (except for cyclic trimers, for which the O–O distance is nearly temperature independent). As a preliminary result, the lifetime of the polygons is estimated to be around 0.01 ps. [ABSTRACT FROM AUTHOR]

Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm-1–23 000 cm-1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser-induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well-resolved transition lines measured by high-resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specificity in the dissociation rates is observed. For states with approximately the same total energy those with higher excitation in the out-of-plane bending mode (ν6) dissociate more slowly than others. For rotationless levels in the tunneling region, the A’ states decay much more rapidly than the A‘ states indicating symmetry-induced mode specificity.The dissociation rates in HFCO almost surely exhibit mode specificity because the ν6 mode is weakly coupled to the reaction coordinate. The enhancement of this coupling by Coriolis forces appears to produce the unprecedentedly large rotational level dependence of the rates. However, the spectroscopically observed coupling of ν6 to the background of vibrationally mixed levels is not significantly increased by rotation. Thus states with nearly all of the excitation energy in the reaction coordinate appear not to be strongly mixed into the background states. The large effect of... [ABSTRACT FROM AUTHOR]

We report measurement of the HD product quantum state distributions and absolute cross section for the H+CD4→HD(v’,J’)+CD3 reaction at a collision energy of 1.5 eV. The total reaction cross section is small, 0.14±0.03 Å2, making the experimental measurements difficult. The HD quantum state distribution peaks at low J’ in both v’=0 and v’=1, the only vibrational states in which product is observed. Very little of the 1.5 eV available energy appears as internal excitation of the HD product molecule, 7% in HD product vibration and 9% in rotation. However, linear surprisal analysis shows that this limited internal energy disposal in the HD product in some ways exceeds that expected statistically, since two of the best-fit surprisal parameters (Θr=2.9±0.6 for v’=0, Θr=-1.9±0.5 for v’=1, λv=-2.2±0.6 ) are negative. The HD rovibrational state distribution shows an anomalous positive correlation of product vibrational and rotational excitation. Those molecules formed in the vibrationally excited state, v’=1, have significantly more rotational energy (=0.17 eV) than those molecules formed in the vibrational ground state, v’=0 (=0.13 eV). This behavior runs counter to the otherwise universal behavior for direct bimolecular reactions—a negative correlation of product vibrational and rotational excitation. We speculate as to the source of this anomalous energy disposal. [ABSTRACT FROM AUTHOR]

The velocity and angular distributions of N2 produced from the reduction of NO by H2 on Rh(111) have been measured in the low nitrogen coverage limit as a function of surface temperature. Both the angular and velocity distributions are well fit by bimodal forms. The high energy channel has average translational energies about six times that expected for molecules accommodated at the surface temperature, an unusually sharp angular distribution, and angle dependent velocity distributions. The low energy channel is also hyperthermal, with average translational energies about twice thermal, a cosine angular distribution, and velocity distributions which are independent of angle. Application of surprisal analysis to the data shows that the high energy channel may be characterized by constraints on the normal velocity and the total energy; the low energy channel may be characterized by a single constraint on the velocity. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibrational dynamics of hydrogen on the H/Si(111)1×1 surface are studied using the first principles Car–Parrinello molecular dynamics method. The calculated frequencies of the surface vibrations are found to be in good agreement with experimental results. The combination band spectra indicate there are anharmonic couplings between the Si–H stretching mode and other lower frequency silicon surface phonon modes. [ABSTRACT FROM AUTHOR]

Studies of the reaction of H atoms with Cl chemisorbed on Au(111) reveal two dynamically distinct mechanisms. Some reactions occur on essentially a single gas-surface collision, by way of a so-called Eley–Rideal (ER) mechanism. Others occur between accommodated H and Cl atoms, by way of a so-called Langmuir–Hinschelwood (LH) mechanism. The dynamics of these processes has been characterized by combining molecular beam techniques with quantum-state-specific detection. Specifically, I have used molecular beam time-of-flight (TOF) techniques to determine angular and velocity distributions of the HCl product, and I have used resonance-enhanced multiphoton ionization to determine rotational and vibrational state distributions. The TOF and angular distributions provide clear evidence for the ER mechanism. This mechanism yields a fast (early) peak in the TOF distributions and a narrow angular distribution that is asymmetric with respect to the surface normal. Moreover, the peak in the angular distribution moves further away from the normal in the direction of the specular angle as the energy of the incident H atom is increased from 0.07 to 0.3 eV.The mean energy of the ER product is about 0.6 eV, but increases slightly with increasing incidence energy. In contrast, the LH mechanism yields a relatively slow (late) TOF component that approximately follows a Boltzmann distribution at the surface temperature (Ts) and disappears at Ts<170 K. The form of the angular distribution of this LH component is close to a cosine function. The fast (ER) TOF component is itself found to be composed of at least two contributions, assigned to HCl product formed in v=0 and v=1 (with some contribution from v=2). The rotational state distribution for the HCl(v=0) product of the ER mechanism is found to be distinctly non-Boltzmann, with a mean rotational energy of about 0.11 eV, or about 5% of the available energy. The rotational distributions obtained for v=1 and v=2 are... [ABSTRACT FROM AUTHOR]

Non-adiabatic dynamics of the acetylene cation is investigated using mixed quantum-classical dynamics based on trajectory surface hopping. To describe the non-adiabatic effects, two surface hopping methods are used, namely, Tully's fewest switches and Landau-Zener surface hopping. Similarities and differences between the results based on those two methods are discussed. We find that the photoionization of acetylene into the first excited state A2Σg+ drives the molecule from the linear structure to a trans-bent structure. Through a conical intersection the acetylene cation can relax back to either the ground state of acetylene or vinylidene. We conclude that hydrogen migration always takes place after non-radiative electronic relaxation to the ground state of the monocation. Based on the analysis of correlation functions we identify coherent oscillations between acetylene and vinylidene with a period of about 70 fs after the electronic relaxation. [ABSTRACT FROM AUTHOR]

The rovibrational state distributions for the H[sub 2] product of the H+c-C[sub 6]H[sub 12]→H[sub 2]+c-C[sub 6]H[sub 11] abstraction reaction have been measured at 1.6 eV collision energy under single-collision conditions. The state distributions are compared to those for the H[sub 2] product of the reaction of H atoms with CD[sub 3](CH[sub 2])[sub 4]CD[sub 3] and n-C[sub 6]H[sub 14] that we have measured previously. Abstraction of secondary H atoms from the cyclic C[sub 6] alkane produces H[sub 2](v[sup ′]=0,j[sup ′]) and H[sub 2](v[sup ′]=1,j[sup ′]) rotational state distributions that are significantly colder than those from the reaction of the linear C[sub 6] alkane. These results are interpreted within a simple phenomenological local reaction model that we have developed for these atom+polyatom reactions. The substantial differences between the state distributions for the reactions of the cyclic and linear alkanes indicate that reactions of cyclohexane are not representative of reactions at all secondary C–H. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dissociative adsorption of H[sub 2] and D[sub 2] on Pt(533) (Pt{4(111)x(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D[sub 2] from (100) step sites is observed at lowest exposures in TPD (assigned β[sub 3]) at 375 K. Saturation of this peak at Θ[sub H]=0.14 corresponds to the filling of half of the available four-fold sites at the (100) step edge. At higher coverages, additional desorption takes place from the (111) terraces in a broad peak below 300 K similar to that observed (assigned β[sub 1] and β[sub 2]) for the Pt(111) surface. The incident kinetic energy (E[sub i]), surface temperature (T[sub s]), coverage (Θ[sub D]), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S[sub 0]) first decreases with increasing kinetic energy over the range 0i(meV)<150 (low energy component), and subsequently increases (high energy component). Comparison with D[sub 2] dissociation on Pt(111), where (S[sub 0]) increases linearly with E[sub i], leads to the conclusion that it is the step sites that are responsible for the low energy component to dissociation on Pt(533). The high energy component is a result of a direct dissociation channel on (111) terraces of the Pt(533) surface. The probability of dissociation through the direct channel on the (111) terraces is found to be independent of T[sub s]. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of E[sub i] where it contributes, is also shown to be independent of T[sub s]. Only at the very lowest value (6.6 meV) of E[sub i] investigated does S[sub 0] exhibit a (negative) temperature dependence. A (0.8-Θ[sub D])[sup 2] dependence (where 0.8 is the measured saturation coverage) of S with Θ[sub D] is observed at E[sub i]=18... [ABSTRACT FROM AUTHOR]