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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]

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]

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]

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 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]

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]