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We present a comparison of ab initio molecular dynamics calculations for CHD

Specific features of diffractive scattering of H

We have constructed first principle based six-dimensional (6D) potential energy surfaces (PESs) describing the interaction of H2 with Cu(111) obtained by interpolation of a set of density functional theory (DFT) total energy data. The DFT calculations have been performed within the generalized gradient approximation (GGA) framework. In applying the GGA we have tested the two exchange−correlation (XC) functionals most popular in surface science, i.e., the PW91 and RPBE functionals. The interpolation of the PW91 PES has been performed using two different methods, the corrugation reducing procedure (CRP), which has been proven to be one of the most successful interpolation methods to build 6D PESs, and the modified Shepard (MS) interpolation method, a very promising method to build high dimensional (nD) PESs, which is computationally cheaper than the CRP. We show that, in spite of the difference between the CRP-PES and the MS-PES and the inaccuracies found in the latter, quantum and classical reaction and sc...

A structural study of the surface of an ultrathin ice film grown on a Pt(111) substrate was performed using dynamical low-energy electron diffraction (LEED) at 90 K, together with total-energy calculations and molecular dynamics (MD) simulations. This ice film exhibits the common hexagonal phase Ih and exposes the (0001) surface without reconstruction. The surface is terminated as a full-bilayer that maximizes the number of surface hydrogen bonds as confirmed by our total-energy calculations. Both LEED and MD simulations find that the outermost water molecules have enhanced vibrational amplitudes making them practically undetectable by LEED even at 90 K. MD simulations of the half-bilayer terminated surface yield results inconsistent with the LEED findings, thus excluding this model.

A four-dimensional dynamics study was performed on vibrational excitation and dissociation of ${\mathrm{H}}_{2}$ in collisions with Cu(100). The potential-energy surface was taken from density-functional calculations. Large probabilities for vibrational excitation (\ensuremath{\gtrsim}10%) are obtained. Two-dimensional fixed-site calculations show that the vibrational excitation is due to impacts on the top site. Impacts on the bridge and hollow sites are more efficient in causing dissociation. \textcopyright{} 1996 The American Physical Society.

We investigate the usefulness of density functional theory (DFT) for calculating excited state potential energy surfaces. In the DFT calculations, the generalized gradient approximation (GGA) is used. As a test case, the photodissociation of H2O through the first excited A 1B1 state was considered. Two‐dimensional potential energy surfaces were obtained for both the X 1A1 ground state and the first excited state. Wave packet calculations employing these surfaces were used to obtain both the absorption spectrum and partial photodissociation cross sections, which are resolved with respect to the final vibrational state of the OH fragment. Comparisons are made with a previously calculated high level ab initio potential energy surface, with dynamics calculations using that surface, and with experiment. The vertical excitation energy for the (X 1A1→A 1B1) transition calculated using DFT is in good agreement with the previous ab initio calculations. The absorption spectrum and the partial cross sections obt...

The H2+LiF(001) system was used to investigate the performance of the hybrid close‐coupling wave packet (CCWP) method and of a symmetry adapted, fully close‐coupled wave packet (SAWP) method for a molecule–surface problem characterized by fairly high corrugation. In the calculations, a realistic, φ‐dependent model potential was used. The calculations were performed for a collision energy of 0.2 eV, with H2 initially in its j=0 rotational state at normal incidence to the surface. Large increases in the computational efficiencies of both wave packet methods were achieved by taking advantage of the potential coupling matrices associated with both methods becoming sparser with increasing molecule–surface distance. For the present model problem and employing this increased sparseness at longer range, the SAWP method is faster than the CCWP method by a factor of 2. The potential usefulness of the SAWP method for dissociative chemisorption problems is discussed.

The close‐coupling wave packet (CCWP) method has been adapted for performing calculations on molecule‐surface scattering with arbitrary angles of incidence. The method used involves a slight modification of the fast Fourier transform (FFT) technique for evaluating the action of the translational kinetic energy operator on the wave function, employing the shifting theorem of Fourier analysis. We present and compare results of CCWP and close‐coupling (CC) calculations on the He+LiF and H2+LiF systems using simple model potentials. The results presented establish the validity of the proposed technique and may be useful as benchmarks.

Results of quantum calculations on rotationally and vibrationally inelastic scattering of rotationally excited 1Au(S1) trans‐glyoxal (in collisions with H2, at a collision energy of 80 meV) are reported. These results are used to assess the influence of sequential collisions on a recent experiment on glyoxal +H2. For ‖ΔK‖>3, the cross sections for rotationally inelastic scattering σ(00,K=k→00,K’=K+ΔK) depend exponentially on (ΔK)2 (and not on the energy difference between the initial and final states), which shows that the rotationally inelastic scattering is dominated by angular momentum transfer rather than by energy transfer. Taking into account sequential collisions leads to a substantial improvement in the agreement between the calculated and experimental values of the rotationally resolved cross sections for excitation of the lowest frequency ν7 torsional mode.

We have studied the influence of preadsorbed CO on the dissociative adsorption of H(2) 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 H(2) 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 H(2) reacting on low-index surfaces of bare metals (computing two-dimensional elbow plots for fixed impact high-symmetry sites and H(2) orientations parallel to the surface) does not work for H(2)+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 H(2) on CO/Ru(0001) is endoergic, in contrast to the case of H(2) on bare Ru(0001). The zero-point energy corrected energies of molecularly and dissociatively chemisorbed H(2) are very close, suggesting that it may be possible to detect molecularly chemisorbed H(2) on (sq.rt(3) x sq.rt(3))R30 degrees 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 H(2) sigma(g) orbital and the (surface-hybridized) CO 1pi orbitals, i.e., to site blocking. There is a small repulsive contribution to the barrier from the interaction between the H(2) molecule and the Ru part of the CO covered Ru surface, but it is smaller than one might expect based on the calculations of H(2) interacting with a clean Ru surface, and on calculations of H(2) 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 H(2) dissociation on Ru is not only a simple site-blocking effect, the electronic structure of the underlying Ru is changed.

Using the AVCC-1OS (azimuthal and vibrational close-coupled, infinite-order sudden) method, we have calculated efficiencies for vibrational relaxation of glyoxal in collisions with He and Ar at room temperature. For glyoxal- d 2 (deuterated glyoxal) + He the calculated efficiency P ( 0°, k = 0→7 1 ) is in reasonable agreement with experiment, but for glyoxal- d 2 + Ar the calculated v of P(0°, k =0→7 1 ) is much lower than the experimental value. While part of the discrepancy may be due to the sudden approximation (to the sl nevertheless suggest that, especially for heavier collision partners, the experimental values of P (0°→7 1 ) should be much too high because the question of how rotational relaxation should affect the measurement of vibrationally relaxed and unrelaxed emission was not considered in the experiment. For glyoxal- d 2 + He, the calculated efficiencies P (8 1 →6 1 ) and P (8 1 →8 1 7 1 ) agreement with experiment. Because the frequency difference between the ν 6 and the ν 8 modes is much smaller in glyoxal- h 2 (“normal” or protonated glyoxal), for glyoxal- h 2 + He the calculated efficiency P (8 1 →6 1 ) is much higher than for glyoxal- d 2 + He.

Results of quantum calculations on rotationally and vibrationally inelastic scattering of rotationally excited 1Au(S1) trans‐glyoxal (in collisions with H2, at a collision energy of 80 meV) are reported. These results are used to assess the influence of sequential collisions on a recent experiment on glyoxal +H2. For ‖ΔK‖>3, the cross sections for rotationally inelastic scattering σ(00,K=k→00,K’=K+ΔK) depend exponentially on (ΔK)2 (and not on the energy difference between the initial and final states), which shows that the rotationally inelastic scattering is dominated by angular momentum transfer rather than by energy transfer. Taking into account sequential collisions leads to a substantial improvement in the agreement between the calculated and experimental values of the rotationally resolved cross sections for excitation of the lowest frequency ν7 torsional mode.

Results of quantum calculations on rotationally and vibrationally inelastic scattering from four vibrational levels of 1 A u (S 1 ) trans-glyoxal (in collisions with helium, at a collision energy of 96 meV) are reported. The results can serve as predictions to be borne out by experiments that have already been carried out, but have not been analyzed yet. The calculations predict that rotationally inelastic scattering should be similar for scattering from the 0 0 , 5 1 and 8 1 levels, but different for scattering from the 7 2 level. A comparison of the glyoxal+He results with results for scattering of vibrationless deuterated glyoxal also presented here shows that, at a collision energy of 96 meV, rotational excitation of glyoxal by helium is governed by angular momentum transfer rather than by energy transfer. Vibrationally inelastic scattering is dominated by the lowest frequency ν 7 torsional mode, but the calculations predict that transitions involving other vibrational modes should also be observable.

Results of quantum calculations on rotationally and vibrationally inelastic scattering of 1Au(S1) trans‐glyoxal by H2,He, and Ar(E=80 meV) are presented. For scattering of 00 glyoxal and 72 glyoxal by H2, a quantitative comparison with a recent crossed beam experiment is made. The calculated cross sections for rotationally inelastic scattering are in good agreement with the experimental values, and the trend that σ(72, k=0→72, k’) falls off faster with k’ than σ(00, k=0→00, k’) is reproduced. Our calculations do not reproduce the experimental finding that, for the initial 72 level, the cross sections for the Δv7=−1 and Δv7=+1 transitions are nearly identical. The calculations on 72 glyoxal + He and Ar show important differences with 72 glyoxal + H2 in the competition of rotational excitation with vibrational excitation. Model calculations also presented here yield some interesting trends in the k’ dependence of the cross sections σ(00, k=0→x1, k’) on the vibrational mode x excited.

Using the Monte Carlo classical trajectory (CT) method and the azimuthal close‐coupled, infinite‐order sudden (ACC‐IOS) method, we have calculated cross sections for rotational excitation of S1 trans‐glyoxal by H2 at E=80 meV. The cross sections σ(k=0, j→k’) calculated with the CT method are nearly independent of j. The classical values of σ(k=0, j=5→k’) are in good agreement with the quantum values of σ(k=0→k’) for 2≤k’≤12, although the quantum calculations show a slight preference for odd Δk transitions which is not found in the CT calculations. Both the CT results and the ACC‐IOS results are in good agreement with results obtained in a recent crossed beam experiment. Rotational excitation to high k’ (k’=11,12) occurs by collisions of H2 with one of the H atoms of glyoxal, and the initial value of the orbital angular momentum approximately equals the final value of k in such collisions. Since backward scattering is dominant in collisions leading to high k’, angular momentum constraints alone cannot expl...

We have calculated rate constants for vibrational relaxation of the ν 7 torsional mode of trans-(S 1 ) glyoxal in collisions with helium at low temperatures (0.6–10.0 K). At T =3 K, orbiting resonances contribute as much as 34% to the rate constant for vibrational relaxation. The calculated probability per hard sphere collision of the transition (ν 7 =1, k =0→ν 7 =0) has been fitted to a power law ( P if = aT −b ) as a function of temperature. The values obtained for a and b agree quite well with experimental results for the transition (ν 7 =2, k =1→ν 7 =1).

An ice film with thickness greater than 10 A was crystallized on a clean Pt(111) surface. Its external surface structure was investigated at 90 K by dynamical low-energy electron diffraction (LEED), followed by molecular dynamics simulations and ab initio quantum chemical calculations. The results favor the common hexagonal ice 1h structure over other forms of ice, with (0001) termination. A full-bilayer termination is found, but with much enhanced amplitudes of motion of the O atoms in the outermost layer of H{sub 2}O molecules, even at 90 K, so that these molecules were undetected experimentally by LEED. 14 refs., 2 figs.

An improved method for performing asymptotic analysis developed by Balint‐Kurti et al. [J. Chem. Soc. Faraday Trans. 86, 1741 (1990)] was used with the close‐coupling wave packet (CCWP) method. S‐matrix elements are computed from the time dependence of the wave packet amplitude at a dividing surface in the asymptotic region. The analysis technique can be combined in a natural way with the use of an optical potential to absorb the scattered wave function beyond the dividing surface and with a technique in which the initial wave function is brought in on a separate, one‐dimensional grid, thereby allowing the use of a smaller grid. The use of the method in conjunction with the Chebyshev and short‐iterative Lanczos propagation techniques is demonstrated for a model problem in which H2 is scattered from LiF(001). Computed S‐matrix elements are in good agreement with those obtained using a time‐independent close‐coupling method.

The H2+LiF(001) system was used to investigate the performance of the hybrid close‐coupling wave packet (CCWP) method and of a symmetry adapted, fully close‐coupled wave packet (SAWP) method for a molecule–surface problem characterized by fairly high corrugation. In the calculations, a realistic, φ‐dependent model potential was used. The calculations were performed for a collision energy of 0.2 eV, with H2 initially in its j=0 rotational state at normal incidence to the surface. Large increases in the computational efficiencies of both wave packet methods were achieved by taking advantage of the potential coupling matrices associated with both methods becoming sparser with increasing molecule–surface distance. For the present model problem and employing this increased sparseness at longer range, the SAWP method is faster than the CCWP method by a factor of 2. The potential usefulness of the SAWP method for dissociative chemisorption problems is discussed.

Results of quantum calculations on rotationally and vibrationally inelastic scattering of 1Au(S1) trans‐glyoxal by H2,He, and Ar(E=80 meV) are presented. For scattering of 00 glyoxal and 72 glyoxal by H2, a quantitative comparison with a recent crossed beam experiment is made. The calculated cross sections for rotationally inelastic scattering are in good agreement with the experimental values, and the trend that σ(72, k=0→72, k’) falls off faster with k’ than σ(00, k=0→00, k’) is reproduced. Our calculations do not reproduce the experimental finding that, for the initial 72 level, the cross sections for the Δv7=−1 and Δv7=+1 transitions are nearly identical. The calculations on 72 glyoxal + He and Ar show important differences with 72 glyoxal + H2 in the competition of rotational excitation with vibrational excitation. Model calculations also presented here yield some interesting trends in the k’ dependence of the cross sections σ(00, k=0→x1, k’) on the vibrational mode x excited.

The classical trajectory methodology for studying scattering of ions, atoms, or molecules from ice surfaces, and photodissociation of water at or in the surface of ice, is pre- sented. The forces between the collider and the water molecules, or between the fragments of a dissociating molecule and the surrounding water molecules, are based on pair potentials taken either from ab initio calculations or derived empirically. Dynamical observables like sticking probabilities and kinetic energy distributions of desorbing photo-fragments are computed by solving Newton's equations of motion, starting from representative initial conditions. Four stud- ies with relevance to astrochemistry are considered: the sticking of H atoms to ice Ih, the sticking of CO to ice Ih and ice Ia , the sticking of protons to ice Ih, and the photodissociation of water in ice Ih and ice Ia , also with a view to the subsequent chemistry of the H- and OH-products with co-adsorbed molecules. Where possible, the theoretical results are compared with experiments.

Using the Monte Carlo classical trajectory method, we have calculated cross sections for complex formation and complex lifetimes for glyoxal–helium collisions at low relative translational energies (