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The dissociation of methane is not only an important reaction step in catalytic processes, but also of fundamental interest. Dynamical effects during the dissociative chemisorption of methane on metal surfaces cause significant differences in computed reaction rates, compared to what is predicted by typical transition state theory (TST) models. It is clear that for a good understanding of the catalytic activation of methane dynamical simulations are required. In this paper, a general blueprint is provided for performing dynamical simulations of the dissociative chemisorption of methane on metal surfaces, by employing either the quasi-classical trajectory or ring polymer molecular dynamics approach. If the computational setup is constructed with great care-since results can be affected considerably by the setup - chemically accurate predictions are achievable. Although this paper concerns methane dissociation, the provided blueprint is, so far, applicable to the dissociative chemisorption of most molecules. [ABSTRACT FROM AUTHOR]

We present a comparison of ab initio molecular dynamics calculations for CHD3 dissociation on Pt(111) using the Perdew, Burke and Ernzerhof (PBE) functional and a specific reaction parameter (SRP) functional. Despite the two functionals predicting approximately the same activation barrier for the reaction, the calculations using the PBE functional consistently overestimate the experimentally determined dissociation probability, whereas the SRP functional reproduces the experimental values within a chemical accuracy (4.2 kJ/mol). We present evidence that suggests that this difference in reactivity can at least in part be attributed to the presence of a van derWaals well in the potential of the SRP functional which is absent from the PBE description. This leads to the CHD3 molecules being accelerated and spending less time near the surface for the trajectories run with the SRP functional, as well as more energy being transferred to the surface atoms.We suggest that both these factors reduce the reactivity observed in the SRP calculations compared to the PBE calculations. [ABSTRACT FROM AUTHOR]

Specific features of diffractive scattering of H2 from metal surfaces can serve as fingerprints of the reactivity of the metal towards H2, and in principle theory-experiment comparisons for molecular diffraction can help with the validation of semi-empirical functionals fitted to experiments of sticking of H2 on metals. However, a recent comparison of calculated and Debye-Waller (DW) extrapolated experimental diffraction probabilities, in which the theory was done on the basis of a potential energy surface (PES) accurately describing sticking to Ru(0001), showed substantial discrepancies, with theoretical and experimental probabilities differing by factors of 2 and 3.We demonstrate that assuming a particular amount of random static disorder to be present in the positions of the surface atoms, which can be characterized through a single parameter, removes most of the discrepancies between experiment and theory. Further improvement might be achievable by improving the accuracy of the DW extrapolation, the model of the H2 rotational state distribution in the experimental beams, and by fine-tuning the PES. However, the question of whether the DW model is applicable to attenuation of diffractive scattering in the presence of a sizable van der Waals well (depth ≈ 50 meV) should also receive attention, in addition to the question of whether the amount of static surface disorder effectively assumed in the modeling by us could have been present in the experiments. [ABSTRACT FROM AUTHOR]

Two-dimensional, three-dimensional, and four-dimensional quantum dynamic calculations are performed on the dissociative chemisorption of CH4 on Ni(111) using the multiconfiguration time-dependent Hartree (MCTDH) method. The potential energy surface used for these calculations is 15-dimensional (15D) and was obtained with density functional theory for points which are concentrated in the region that is dynamically relevant to reaction. Many reduced dimensionality calculations were already performed on this system, but the molecule was generally treated as pseudodiatomic. The main improvement of our model is that we try to describe CH4 as a polyatomic molecule by including a degree of freedom describing a bending vibration in our three-dimensional and four-dimensional models. Using a polyspherical coordinate system, a general expression for the 15D kinetic energy operator is derived, which discards all the singularities in the operator and includes rotational and Coriolis coupling. We use seven rigid constraints to fix the CH3 umbrella of the molecule to its gas phase equilibrium geometry and to derive two-dimensional, three-dimensional, and four-dimensional Hamiltonians, which were used in the MCTDH method. Only four degrees of freedom evolve strongly along the 15D minimum energy path: the distance of the center of mass of the molecule to the surface, the dissociative C[Single_Bond]H bond distance, the polar orientation of the molecule, and the bending angle between the dissociative C[Single_Bond]H bond and the umbrella. A selection of these coordinates is included in each of our models. The polar rotation is found to be important in determining the mode selective behavior of the reaction. Furthermore, our calculations are in good agreement with the finding of Xiang et al. [J. Chem. Phys. 117, 7698 (2002)] in their reduced dimensional calculation that the helicopter motion of the umbrella symmetry axis is less efficient than its cartwheel motion for promoting the reaction. The effect of pre-exciting the bend modes is qualitatively incorrect at higher energies, suggesting the necessity of including additional rotational and vibrational degrees of freedom in the model. [ABSTRACT FROM AUTHOR]

Six-dimensional quantum dynamical calculations are presented for the reaction of (v,j) H[SUB2] on Cu(100), at normal incidence, for v=0-1 and j=0-5. The dynamical calculations employed a potential energy surface computed with density functional theory, using the generalized gradient approximation and a slab representation for the adsorbate-substrate system. The aim of the calculations was to establish signatures from which experiments could determine the dominant reaction site of H[SUB2] on the surface and the dependence of the reaction site on the initial rovibrational state of H[SUB2]. Two types of signatures were found. First, we predict that, at energies near threshold, the reaction of (v = 1)H[SUB2] is rotationally enhanced, because it takes place at the top site, which has an especially late barrier and a reaction path with a high curvature. On the other hand, we predict the reaction to be almost independent of j for (v = 0) H[SUB2], which reacts at the bridge site. Second, we predict that, at collision energies slightly above threshold for which the reaction probabilities of the (v = 0) and (v = 1 ) states are comparable, the rotational quadrupole alignment of (v = 1 ) reacting molecules should be larger than that of (v =0) reacting molecules, for j= 1, 4, and 5. For (j = 2) H[SUB2], the opposite should be true, and for (j = 3) H[SUB2], the rotational quadrupole alignment should be approximately equal for (v = 1) and (v = 0)H[SUB2]. These differences can all be explained by the difference in the predicted reaction site for (v = 1) and (v=0)H[SUB2] (top and bridge) and by the differences in the anisotropy of the potential at the reaction barrier geometries associated with these sites. Our predictions can be tested in associative desorption experiments, using currently available experimental techniques. [ABSTRACT FROM AUTHOR]

We present results of calculations on dissociative and rotationally (in)elastic diffractive scattering of H[sub 2] from Pt(11l), treating all six molecular degrees of freedom quantum mechanically. The six-dimensional (6D) potential energy surface was taken from density functional theory calculations using the generalized gradient approximation and a slab representation of the metal surface. The 6D calculations show that out-of-plane diffraction is very efficient, at the cost of in-plane diffraction, as was the case in previous four-dimensional (4D) calculations. This could explain why so little in-plane diffraction was found in scattering experiments, suggesting the surface to be flat, whereas experiments on reaction suggested a corrugated surface. Results of calculations for off-normal incidence of (υ=0,j=0) H[sub 2] show that initial parallel momentum inhibits dissociation at low normal translational energies, in agreement with experiment, but has little effect for higher energies. Reaction of initial (υ= 1,j = 0) H[sub 2] is predicted to be vibrationally enhanced with respect to (υ = 0,j = 0) H[sub 2], as was also found in three-dimensional (3D) and 4D calculations, even though H[sub 2] + Pt(111) is an early barrier system. [ABSTRACT FROM AUTHOR]

Results are reported of a four-dimensional dynamics study on the dissociation of H2 on Cu(100). The potential-energy surface was taken from density functional calculations, which employed the generalized gradient approximation and a slab representation for the surface. Reaction occurs preferentially in impacts near the bridge and hollow sites. Collisions near top sites promote vibrational excitation. The conclusion that vibrationally inelastic scattering and reaction occur preferentially on different sites can be generalized to other low index Cu surfaces. Resonances affect the reaction in the 4D model through a mechanism in which the molecule, trapped by excitation of the molecular bond which is weakened at the surface near top sites, is allowed more time to tunnel through the barrier to reaction. The calculated dependence of the diffraction probabilities on incidence energy suggests that a measurement of low-order diffraction would be able to determine whether the minimum barrier to reaction occurs for impacts on the bridge sites (as theory predicts) or on the hollow or top sites. The calculated reaction probabilities are in good agreement with experiment, further improvement being expected from including the rotational degrees of freedom in the model. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The specific reaction parameter (SRP) approach to density functional theory (DFT) has enabled a chemically accurate description of reactive scattering experiments for activated H2-metal systems (H2 + Cu(111) and Cu(100)), but its application has not yet resulted in a similarly accurate description of non-activated or weakly activated H2-metal systems. In this study, the effect of the choice of the exchange-correlation functional in DFT on the potential energy surface and dynamics of H2 dissociation on Ru(0001), a weakly activated system, is investigated. In total, full potential energy surfaces were calculated for over 20 different functionals. The functionals investigated include functionals incorporating an approximate description of the van der Waals dispersion in the correlation functional (vdW-DF and vdW-DF2 functionals), as well as the revTPSS meta-GGA.With two of the functionals investigated here, which include vdW-DF and vdW-DF2 correlation, it has been possible to accurately reproduce molecular beam experiments on sticking of H2 and D2, as these functionals yield a reaction probability curve with an appropriate energy width. Diffraction probabilities computed with these two functionals are however too high compared to experimental diffraction probabilities, which are extrapolated from surface temperatures (Ts) ⩾ 500 K to 0 K using a Debye-Waller model. Further research is needed to establish whether this constitutes a failure of the two candidate SRP functionals or a failure of the Debye-Waller model, the use of which can perhaps in future be avoided by performing calculations that include the effect of surface atom displacement or motion, and thereby of the experimental Ts. [ABSTRACT FROM AUTHOR]

Recently, an implementation of the specific reaction parameter (SRP) approach to density functional theory (DFT) was used to study several reactive scattering experiments of H2 on Cu(111). It was possible to obtain chemical accuracy (1 kcal/mol ≈ 4.2 kJ/mol), and therefore, accurately model this paradigmatic example of activated H2 dissociation on a metal surface. In this work, the SRP-DFT methodology is applied to the dissociation of hydrogen on a Pd(111) surface, in order to test whether the SRP-DFT approach is also applicable to non-activated H2-metal systems. In the calculations, the Born-Oppenheimer static surface approximations are used. A comparison to molecular beam sticking experiments, performed at incidence energies >=125 meV, on H2 + Pd(111) suggested the PBE-vdW [where the Perdew, Burke, and Ernzerhof (PBE) correlation is replaced by van der Waals correlation] functional as a candidate SRP density functional describing the reactive scattering of H2 on Pd(111). Unfortunately, quantum dynamics calculations are not able to reproduce the molecular beam sticking results for incidence energies <125 meV. From a comparison to initial state-resolved (degeneracy averaged) sticking probabilities it seems clear that for H2 + Pd(111) dynamic trapping and steering effects are important, and that these effects are not yet well modeled with the potential energy surfaces considered here. Applying the SRP-DFT method to systems where H2 dissociation is non-activated remains difficult. It is suggested that a density functional that yields a broader barrier distribution and has more non-activated pathways than PBE-vdW (i.e., non-activated dissociation at some sites but similarly high barriers at the high energy end of the spectrum) should allow a more accurate description of the available experiments. Finally, it is suggested that new and better characterized molecular beam sticking experiments be done on H2 + Pd(111), to facilitate the development of a more accurate theoretical description of this system. [ABSTRACT FROM AUTHOR]

To investigate the isotope effects on the photodesorption processes of X2O (X = H,D) ice, molecular dynamics calculations have been performed on the ultraviolet photodissociation of an H2O or a D2O molecule in an H2O or a D2O amorphous ice surface, and on HOD photodissociation in an H2O amorphous ice surface, where the photodissociated molecules were located in the top four or five monolayers at ice temperatures of 10, 20, 30, 60, and 90 K. Three photodesorption processes can occur upon X2O photodissociation: X atom photodesorption, OX radical photodesorption, and X2O (or HOD) molecule photodesorption. X2O (or HOD) photodesorption can occur after recombination of X and OX, or after an energetic X atom photofragment kicks a surrounding X2O molecule from the ice surface. Isotope effects are observed for the X atom and the OX radical photodesorption as well as for the kick-out photodesorption. However, no isotope effects were noticeable for the photodesorption of recombined X2O molecules. The average D atom photodesorption probabilities are about a factor 0.9 smaller than those for the H atom, regardless of the isotope of the surrounding ice system. Also, the kick-out mechanism is more likely to occur if a D photofragment is created upon dissociation than if an H atom is created. These observations can be explained by more efficient energy transfer from the D atom to water molecules than from the H atom. Reasoning based on the X2O phonon frequencies associated with the librational modes and energy transfer efficiencies explain why the OX radical photodesorption probabilities are noticeably larger if the OX radical desorbs from a D2O ice system than from an H2O ice system. Also, the OX radical photodesorption is more probable upon dissociation of DOX (X = H,D) than upon dissociation of HOX (X = H,D), because the initial kinetic energy of the OX radical is larger if the dissociation products are D + OX than H + OX. The branching ratio of ODOH desorption following photodissociation of an HOD molecule in ice (about 1.0) is much lower than the ODOH branching ratio in gas-phase HOD photodissociation. This may lead to differences in isotope fractionation in OH(g) formation in dense and diffuse clouds in the interstellar medium. [ABSTRACT FROM AUTHOR]

We have studied survival and rotational excitation probabilities of H2(vi = 1, Ji = 1) and D2(vi = 1, Ji = 2) upon scattering from Cu(111) using six-dimensional (6D) adiabatic (quantum and quasi-classical) and non-adiabatic (quasi-classical) dynamics. Non-adiabatic dynamics, based on a friction model, has been used to analyze the role of electron-hole pair excitations. Comparison between adiabatic and non-adiabatic calculations reveals a smaller influence of non-adiabatic effects on the energy dependence of the vibrational deexcitation mechanism than previously suggested by low-dimensional dynamics calculations. Specifically, we show that 6D adiabatic dynamics can account for the increase of vibrational deexcitation as a function of the incidence energy, as well as for the isotope effect observed experimentally in the energy dependence for H2(D2)/Cu(100). Furthermore, a detailed analysis, based on classical trajectories, reveals that in trajectories leading to vibrational deexcitation, the minimum classical turning point is close to the top site, reflecting the multidimensionally of this mechanism. On this site, the reaction path curvature favors vibrational inelastic scattering. Finally, we show that the probability for a molecule to get close to the top site is higher for H2 than for D2, which explains the isotope effect found experimentally. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D2O in an amorphous D2O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H2O, the main processes after UV photodissociation are trapping and desorption of either fragments or D2O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D2O molecule photodesorption. D2O desorption takes places either by direct desorption of a recombined D2O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D2O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D2O in the top four monolayers and are compared quantitatively with those for H2O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D2O photodesorption probability is larger than that of H2O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D2O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D2O ice, because the D atom formed after D2O photodissociation has a larger momentum than photogenerated H atoms from H2O, and D transfers momentum more easily to D2O than H to H2O. The total (OD + D2O) yield has been compared with experiments and the total (OH + H2O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D2O ice than when we compare with calculated yields for H2O ice. [ABSTRACT FROM AUTHOR]

Transition states and reaction paths for a hydrogen molecule dissociating on small aluminum clusters have been calculated using density functional theory. The two lowest spin states have been taken into account for all the Aln clusters considered, with n=2-6. The aluminum dimer, which shows a 3Πu electronic ground state, has also been studied at the coupled cluster and configuration interaction level for comparison and to check the accuracy of single determinant calculations in this special case, where two degenerate configurations should be taken into account. The calculated reaction barriers give an explanation of the experimentally observed reactivity of hydrogen on Al clusters of different size [Cox et al., J. Chem. Phys. 84, 4651 (1986)] and reproduce the high observed reactivity of the Al6 cluster. The electronic structure of the Aln-H2 systems was also systematically investigated in order to determine the role played by interactions of specific molecular orbitals for different nuclear arrangements. Singlet Aln clusters (with n even) exhibit the lowest barriers to H2 dissociation because their highest doubly occupied molecular orbitals allow for a more favorable interaction with the antibonding σu molecular orbital of H2. [ABSTRACT FROM AUTHOR]

The ultraviolet (UV) photodissociation of amorphous water ice at different ice temperatures is investigated using molecular dynamics (MD) simulations and analytical potentials. Previous MD calculations of UV photodissociation of amorphous and crystalline water ice at 10 K [S. Andersson et al., J. Chem. Phys. 124, 064715 (2006)] revealed—for both types of ice—that H atom, OH, and H2O desorption are the most important processes after photoexcitation in the uppermost layers of the ice. Water desorption takes place either by direct desorption of recombined water, or when, after dissociation, an H atom transfers part of its kinetic energy to one of the surrounding water molecules which is thereby kicked out from the ice. We present results of MD simulations of UV photodissociation of amorphous ice at 10, 20, 30, and 90 K in order to analyze the effect of ice temperature on UV photodissociation processes. Desorption and trapping probabilities are calculated for photoexcitation of H2O in the top four monolayers and the main conclusions are in agreement with the 10 K results: desorption dominates in the top layers, while trapping occurs deeper in the ice. The hydrogen atom photodesorption probability does not depend on ice temperature, but OH and H2O photodesorption probabilities tend to increase slightly (∼30%) with ice temperature. We have compared the total photodesorption probability (OH+H2O) with the experimental total photodesorption yield, and in both cases the probabilities rise smoothly with ice temperature. The experimental yield is on average 3.8 times larger than our theoretical results, which can be explained by the different time scales studied and the approximations in our model. [ABSTRACT FROM AUTHOR]

We have studied the influence of preadsorbed CO on the dissociative adsorption of H2 on Ru(0001) with density functional theory calculations. For a coverage of 1/3 ML CO, we investigated different possible reaction paths for hydrogen dissociation using nudged elastic band and adaptive nudged elastic band calculations. One reaction path was studied in detail through an energy decomposition and molecular orbital type of analysis. The minimum barrier for H2 dissociation is found to be 0.29 eV. At the barrier the H–H bond is hardly stretched. Behind this barrier a molecular chemisorption minimum is present. Next, the molecule overcomes a second barrier, with a second local chemisorption minimum behind it. To finally dissociate to chemisorbed atoms, the molecule has to overcome a third barrier. To move along the reaction path from reactants to products, the hydrogen molecule needs to rotate, and to significantly change its center-of-mass position. The procedure of mapping out reaction paths for H2 reacting on low-index surfaces of bare metals (computing two-dimensional elbow plots for fixed impact high-symmetry sites and H2 orientations parallel to the surface) does not work for H2+CO/Ru. The first barrier in the path is recovered, but the features of the subsequent stretch to the dissociative chemisorption minimum are not captured, because the molecule is not allowed to change its center-of-mass position or to rotate. The dissociative chemisorption of H2 on CO/Ru(0001) is endoergic, in contrast to the case of H2 on bare Ru(0001). The zero-point energy corrected energies of molecularly and dissociatively chemisorbed H2 are very close, suggesting that it may be possible to detect molecularly chemisorbed H2 on (3×3)R30°CO/Ru(0001). The presence of CO on the surface increases the barrier height to dissociation compared with bare Ru(0001). Based on an energy decomposition and molecular orbital analysis we attribute the increase in the barrier height mainly to an occupied-occupied interaction between the bonding H2 σg orbital and the (surface-hybridized) CO 1π orbitals, i.e., to site blocking. There is a small repulsive contribution to the barrier from the interaction between the H2 molecule and the Ru part of the CO covered Ru surface, but it is smaller than one might expect based on the calculations of H2 interacting with a clean Ru surface, and on calculations of H2 interacting with the CO overlayer only. Actually, the analysis suggests that the Ru surface as a subsystem is (slightly) more reactive for the reaction path studied with CO preadsorbed on it than without it. Thus, the results indicate that the influence of CO on H2 dissociation on Ru is not only a simple site-blocking effect, the electronic structure of the underlying Ru is changed. [ABSTRACT FROM AUTHOR]

The photo-oxidation of water on the monoclinic P21/nWO3 (200, 020, and 002) surfaces is investigated using density functional theory calculations, employing the PW91-generalized gradient approximation, and the method developed by No\rskov et al. [J. Phys. Chem. B 108, 17886 (2004)] based on the free energy differences between the reaction intermediates. We first relax the bulk material unit cell and then investigate the relative stability of different surface terminations of WO3 and analyze the overpotential needed for the photoelectrolysis of water. We found that the rate limiting step is the transfer of a proton from the surface adsorbed OH to the electrolyte, and that the computed overpotential for O2 evolution (1.04 V) is available upon illumination of the surface with visible light. [ABSTRACT FROM AUTHOR]

The photo-oxidation of water on the monoclinic P21/nWO3 (200, 020, and 002) surfaces is investigated using density functional theory calculations, employing the PW91-generalized gradient approximation, and the method developed by No\rskov et al. [J. Phys. Chem. B 108, 17886 (2004)] based on the free energy differences between the reaction intermediates. We first relax the bulk material unit cell and then investigate the relative stability of different surface terminations of WO3 and analyze the overpotential needed for the photoelectrolysis of water. We found that the rate limiting step is the transfer of a proton from the surface adsorbed OH to the electrolyte, and that the computed overpotential for O2 evolution (1.04 V) is available upon illumination of the surface with visible light. [ABSTRACT FROM AUTHOR]

The quasiclassical absorption spectrum of the water dimer in the A band was calculated taking into account motion in all degrees of freedom of the system. The ab initio excited state potentials employed were interpolated by the modified Shepard interpolation method using QMRCI energies and state-averaged MCSCF gradients and Hessians. The ground state vibrational wavefunction was variationally calculated using an adiabatic separation between the high and low frequency normal modes of the system. The calculated spectrum of water dimer shows a clear blueshift with respect to the monomer, but also a small red tail, in agreement with the prediction by Harvey et al. [J. Chem. Phys. 109, 8747 (1998)]. Previous three-dimensional model studies of the photodissociation of the water dimer by Valenzano et al. [J. Chem. Phys. 123, 034303 (2005)] did not show this red tail. A thorough analysis of the dependence of the spectrum on the modes coupled explicitly in the calculation of the spectrum shows that the red tail is due to coupling between the intramolecular stretch vibrations on different monomers. [ABSTRACT FROM AUTHOR]

Six-dimensional quantum dynamical and quasiclassical trajectory (QCT) calculations are reported for the reaction and vibrationally inelastic scattering of (v=0,1,j=0) H2 scattering from Cu(110), and for the reaction and rovibrationally elastic and inelastic scattering of (v=1,j=1) H2 scattering from Cu(110). The dynamics results were obtained using a potential energy surface obtained with density functional theory using the PW91 functional. The reaction probabilities computed with quantum dynamics for (v=0,1,j=0) were in excellent agreement with the QCT results obtained earlier for these states, thereby validating the QCT approach to sticking of hydrogen on Cu(110). The vibrational de-excitation probability P(v=1,j=0→v=0) computed with the QCT method is in remarkably good agreement with the quantum dynamical results for normal incidence energies En between 0.2 and 0.6 eV. The QCT result for the vibrational excitation probability P(v=0,j=0→v=1) is likewise accurate for En between 0.8 and 1 eV, but the QCT method overestimates vibrational excitation for lower En. The QCT method gives probabilities for rovibrationally (in)elastic scattering, P(v=1,j=1→v′,j′), which are in remarkably good agreement with quantum dynamical results. The rotationally averaged, initial vibrational state-selective reaction probability obtained with QCT agrees well with the initial vibrational state-selective reaction probability extracted from molecular beam experiments for v=1, for the range of collision energies for which the v=1 contribution to the measured total sticking probability dominates. The quantum dynamical probabilities for rovibrationally elastic scattering of (v=1,j=1) H2 from Cu(110) are in good agreement with experiment for En between 0.08 and 0.25 eV. [ABSTRACT FROM AUTHOR]

We have studied the dissociative chemisorption and scattering of N2 on and from Ru(0001), using a six-dimensional quasiclassical trajectory method. The potential energy surface, which depends on all the molecular degrees of freedom, has been built applying a modified Shepard interpolation method to a data set of results from density functional theory, employing the RPBE generalized gradient approximation. The frozen surface and Born-Oppenheimer [Ann. Phys. (Leipzig) 84, 457 (1927)] approximations were used, neglecting phonons and electron-hole pair excitations. Dissociative chemisorption probabilities are found to be very small even for translational energies much higher than the minimum reaction barrier, in good agreement with experiment. A comparison to previous low dimensional calculations shows the importance of taking into account the multidimensional effects of N2 rotation and translation parallel to the surface. The new calculations strongly suggest a much smaller role of nonadiabatic effects than previously assumed on the basis of a comparison between low dimensional results and experiments [J. Chem. Phys. 115, 9028 (2001)]. Also in agreement with experiment, our theoretical results show a strong dependence of reaction on the initial vibrational state. Computed angular scattering distributions and parallel translation energy distributions are in good agreement with experiments on scattering, but the theory overestimates vibrational and rotational excitations in scattering. [ABSTRACT FROM AUTHOR]

Four-dimensional quantum dynamics calculations are performed on the dissociative chemisorption of H2 on Pt(111) using the multiconfiguration time-dependent Hartree method. The aim of this work is to study the performance of the multiconfiguration time-dependent Hartree method for a gas-surface reaction by comparison with the standard time-dependent wave-packet propagation method. The initial-state resolved dissociation probability of H2 is calculated within two four-dimensional models. The first four-dimensional model treats explicitly the rotational motion of the molecule and the H2 dissociation is studied above two different sites of the Pt(111). For this model, only a potential-energy surface of general form was available. This potential was refitted to a sum of product form to allow efficient calculations with the multiconfiguration time-dependent Hartree method. The second model focuses on the description of the center-of-mass motion parallel to the surface, the rotational motion of the molecule being frozen. These four-dimensional quantum dynamics calculations yield important insights which can help with performing full six-dimensional calculations on H2 dissociating on Pt(111). The multiconfiguration time-dependent Hartree method is shown to be particularly efficient for computing initial-state selective dissociation probabilities for the system studied, with a good accuracy and a reduced amount of memory and computational time when compared to the standard time-dependent wave-packet method. [ABSTRACT FROM AUTHOR]

We present a discretization method that allows one to interpret measurements on diffraction of diatomic molecules from solid surfaces using six-dimensional (6D) classical trajectory calculations. It has been applied to the D2/NiAl(110) and H2/Pd(111) systems (which are models for activated and nonactivated dissociative chemisorption, respectively) using realistic potential energy surfaces obtained from first principles. Comparisons with experimental results and 6D quantum dynamical calculations show that, in general, the method is able to predict the relative intensity of the most important diffraction peaks. We therefore conclude that classical mechanics can be an efficient guide for experimentalists in the search for the most significant diffraction channels. [ABSTRACT FROM AUTHOR]

Six-dimensional quantum dynamical calculations are reported for the dissociative chemisorption of (v=0, 1, j=0) H2 on Cu(100), and for rovibrationally inelastic scattering of (v=1, j=1) H2 from Cu(100). The dynamics results were obtained using a new potential-energy surface (PES5), which was based on density-functional calculations using a slab representation of the adsorbate-substrate system and a generalized gradient approximation to the exchange-correlation energy. A very accurate method (the corrugation reducing procedure) was used to represent the density-functional theory data in a global potential-energy surface. With the new, more accurately fitted PES5, the agreement between the dynamics results and experimental results for reaction and rovibrationally elastic scattering is not as good as was obtained with a previous potential-energy surface (PES4), which was based on a subset of the density-functional theory data not yet including the results for the low-symmetry Cu sites. Preliminary density-functional theory results suggest that the agreement between theory and experiment will improve over that obtained with PES5 if the density-functional calculations are repeated using a larger basis set and using more copper layers than employed in PES4 and PES5. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Within the harmonic approximation to transition state theory, the biggest challenge involved in finding the mechanism or rate of transitions is the location of the relevant saddle points on the multidimensional potential energy surface. The saddle point search is particularly challenging when the final state of the transition is not specified. In this article we report on a comparison of several methods for locating saddle points under these conditions and compare, in particular, the well-established rational function optimization (RFO) methods using either exact or approximate Hessians with the more recently proposed minimum mode following methods where only the minimum eigenvalue mode is found, either by the dimer or the Lanczos method. A test problem involving transitions in a seven-atom Pt island on a Pt(111) surface using a simple Morse pairwise potential function is used and the number of degrees of freedom varied by varying the number of movable atoms. In the full system, 175 atoms can move so 525 degrees of freedom need to be optimized to find the saddle points. For testing purposes, we have also restricted the number of movable atoms to 7 and 1. Our results indicate that if attempting to make a map of all relevant saddle points for a large system (as would be necessary when simulating the long time scale evolution of a thermal system) the minimum mode following methods are preferred. The minimum mode following methods are also more efficient when searching for the lowest saddle points in a large system, and if the force can be obtained cheaply. However, if only the lowest saddle points are sought and the calculation of the force is expensive but a good approximation for the Hessian at the starting position of the search can be obtained at low cost, then the RFO approaches employing an approximate Hessian represent the preferred choice. For small and medium sized systems where the force is expensive to calculate, the RFO approaches employing an approximate Hessian is also the more efficient, but when the force and Hessian can be obtained cheaply and only the lowest saddle points are sought the RFO approach using an exact Hessian is the better choice. These conclusions have been reached based on a comparison of the total computational effort needed to find the saddle points and the number of saddle points found for each of the methods. The RFO methods do not perform very well with respect to the latter aspect, but starting the searches further away from the initial minimum or using the hybrid RFO version presented here improves this behavior considerably in most cases.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have used a modified Shepard (MS) interpolation method, initially developed for gas phase reactions, to build a potential energy surface (PES) for studying the dissociative chemisorption of H[sub 2] on Pt(111). The aim was to study the efficiency and the accuracy of this interpolation method for an activated multidimensional molecule–surface reactive problem. The strategy used is based on previous applications of the MS method to gas phase reactions, but modified to take into account special features of molecule–surface reactions, like the presence of many similar reaction pathways which vary only slightly with surface site. The efficiency of the interpolation method was tested by using an already existing PES to provide the input data required for the construction of the new PES. The construction of the new PES required half as many ab initio data points as the construction of the old PES, and the comparison of the two PESs shows that the method is able to reproduce with good accuracy the most important features of the H[sub 2]+Pt(111) interaction potential. Finally, accuracy tests were done by comparing the results of dynamics simulations using the two different PESs. The good agreement obtained for reaction probabilities and probabilities for rotationally and diffractionally inelastic scattering shows clearly that the MS interpolation method can be used efficiently to yield accurate PESs for activated molecule–surface reactions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study rotational effects in dissociation of H[sub 2] on Pd(111) through six-dimensional quantum dynamical and classical trajectory calculations. The potential energy surface was obtained from density functional theory. Quantum dissociative adsorption and rotational excitation probabilities are compared with initial-rotational-state-selective measurements. At low energies, dynamic trapping plays an important role, promoting reaction. For low values of the rotational quantum number J, the trapping is mainly due to translation to rotation energy transfer. The decreasing role of trapping when J increases contributes to the decrease of the dissociation probability. For larger values of J trapping is the result of energy transfer to parallel translational motion. Because trapping due to energy transfer to parallel translational motion is only effective at very low energies, the change in trapping mechanism with J causes the minimum of the reaction probability versus collision energy curve to shift to lower energies with increasing J, as previously observed in experiments. Together with dynamic trapping, rotational hindering (for small values of J) and an adiabatic energy transfer from rotation to translation (for high values of J) produce the nonmonotonous dependence of P[sub diss] on J that is observed in our calculations and experiments at low energies. Finally, we predict a nonmonotonous dependence of the quadrupole alignment A[sub 0][sup (2)] on J as observed in associative desorption experiments on H[sub 2]/Pd(100). It is due to rotational hindering for small J and adiabatic energy transfer from rotation to translation for large J. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Following earlier three-dimensional (3D) calculations, we present results of four-dimensional (4D) calculations on dissociative and diffractive scattering of H[sub 2] from Pt(111) by extending the 3D model with a second degree of freedom parallel to the surface. A 4D potential energy surface (PES) is constructed by interpolating four 2D PESs obtained from density-functional theory calculations using the generalized gradient approximation and a slab representation of the metal surface. The 4D calculations show that out-of-plane diffraction is much more efficient than in-plane diffraction, providing a partial explanation for the paradox that diffraction experiments measure little in-plane diffraction, whereas experiments on reaction suggest the surface to be corrugated. Calculations for off-normal incidence of υ=0 H[sub 2] show that, in agreement with experiment, initial parallel momentum inhibits dissociation at low normal translational energies, and enhances reaction for higher energies. Our 4D calculations also show that the reaction of initial υ = 1 H[sub 2] is vibrationally enhanced with respect to υ = 0 H[sub 2], as was found in the 3D model, even though H[sub 2]+Pt(111) is an early barrier system. [ABSTRACT FROM AUTHOR]

By applying a corrugation-reducing procedure we have interpolated the six-dimensional (6D) potential energy surfaces for the H[sub 2]/Pt(111) and H[sub 2]/Cu(100) systems from data obtained by density functional theory (DFT) calculations. We have compared interpolated values with a large number of DFT results not used in the basis for the interpolation and we have obtained an average error below 20 meV and a maximum error of about 30 meV in the regions important for dissociative adsorption. Near the surface the corrugation-reducing procedure gives excellent results using only data from high-symmetry sites. However, we show that to reach the above mentioned accuracy level far from the surface, it is necessary to include information from at least one low-symmetry site. Care has been taken to demonstrate the quality of the interpolation along all degrees of freedom in different regions of the configuration space. The strengths of the method are shown together with the aspects requiring careful handling. A comparison with a direct interpolation method is also made. [ABSTRACT FROM AUTHOR]

Context. The nondissociative ultraviolet photodesorption of water ice is a nonthermal desorption mechanism required to account for detected abundances of gas-phase water toward cold regions within the interstellar medium. Previous experimental and theoretical studies provide a range of photodesorption rates for H2O ice and point to a convoluted competition with other molecular processes following the absorption of a UV photon in the ice. Ultraviolet irradiation also induces photodissociation, resulting in the formation of radicals that may directly desorb triggering gas-phase reactions or recombine in surface reactions. Aims. In this work, we aim to quantify the effects of photodesorption and investigate photoconversion upon UV photolysis of an H2O ice. Methods. We irradiated a porous amorphous H2O ice at 20 K with UV photons in the 7–10.2 eV range and compared the measurements to a nearly identical experiment that included a layer of argon coating on top of the water ice. The purpose of the argon coating is to quench any type of photon-triggered desorption. To trace ice composition and thickness, laser desorption post ionization time-of-flight mass spectrometry was utilized. This method is independent of the (non)dissociative character of a process and provides a diagnostic tool different from earlier studies that allows an independent check. Results. The total photodesorption rate for a porous amorphous H2O ice at 20 K we derive is (1.0 ± 0.2) × 10−3 per incident UV photon (7–10.2 eV), which is in agreement with the available literature. This rate is based on the elemental balance of oxygen-bearing species. As a result, we placed an upper limit on the intact (H2O) and dissociative (OH) desorption rates equal to 1.0 × 10−3 per incident UV photon, while for the reactive desorption (O2), this limit is equal to 0.5 × 10−3 per incident UV photon. Photoconversion depletes the H2O ice at a rate of (2.3 ± 0.2) × 10−3 per incident UV photon. At low UV fluence (9.0 × 1017 photons cm−2), the loss of H2O is balanced by photoproduct formation (O2 and H2O2). At high UV fluence (4.5 × 1018 photons cm−2), about 50% of the initial H2O molecules are depleted. This amount is not matched by the registered O-bearing products, which points to an additional, unaccounted loss channel of H2O. [ABSTRACT FROM AUTHOR]

The influence of water molecules attached to napthol on the spectroscopic properties of the neutral excited-state S(sub 1) and the ionic state were investigated. The comparison of ionic spectra measured through different intermediate states allows us to assign various vibration modes.

Photophysical properties are measured and compared for the two known isomers ([6,6]-closed and [5,6]-open of C61H2, that are the parent C60 methano adduct. It is concluded that isomeric forms of simple C60 monoadducts can show major differences in excited-state relaxation.

The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-one (2CP) and deuterated derivatives were recorded near 385 nm. The ring-bending (v' (sub 30)) levels for the T(sub 1) state were determined to be at 36.5, 118.9, 213.7, 324.5, and 446.4 cm (super -1) for the undeuterated molecule.

Density functional calculations were carried out on the S(sub 0),S(sub 1)(n,pi*), and T(sub 1)(n,pi*) states of 2-cyclopenten-1-one (2CP) to complement the experimental study of the triplet state. The structural information was used for the analysis of triplet-state spectral data.

Three pulse infrared nonlinear studies of the N-H mode of pyrrole in carbon tetrachloride were carried out. Essential features of the N-H dynamics that include the population relaxation time, the orientational diffusion coefficients and the N-H vibrational frequency correlation function were obtained.

The kinetics of the reaction of the OH radical with hydrogen peroxide, H2O2, is studied over a temperature range of 96-296 K. The combined low- and high-temperature kinetic data suggests that the reaction mechanism changes from direct abstraction at high temperature to a mechanism involving formation of a hydrogen-bonded complex at low temperatures.

The predictive reliability of calculations of the excitation energies, geometries, and vibration frequencies for the lower singlet states of NCCCCN is tested. Their performance encourages confidence in the predictions for the so far unanalyzed first singlet transition of NCCCCN.

The 2800 angstrom forbidden transition of dicyanoacetylene shows conventional vibronic origins. It is reported that the rotational constants are not uniformly linear in the quantum numbers of the bending vibrations.

The rate constant curves k(E) for the formation of C(sub 7)H(sub 7)(super +) ions by methyl loss from energy selected ethylbenzene and o-xylene ions were determined over a range from 10(super 3) s (super -1) to 5 x 10(super 6)s(super -1). The C(sub 7)H(sub 7)(super +1) ions formed at the threshold have the structure of benzylium ions for ethylbenzene and that of tropylium ions for the o-xylene.

The results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of ice I(sub h) are presented. The molecular dynamics calculations predict that the average binding energy of the absorbed CO is 4.33 kJ/mol with a maximum value of 10.4 kJ/mol.