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Dynamics of the Eley-Rideal (ER) abstraction of H2 from W(110) is analyzed by means of quasiclassical trajectory calculations. Simulations are based on two different molecule-surface potential energy surfaces (PES) constructed from Density Functional Theory results. One PES is obtained by fitting, using a Flexible Periodic London-Eyring-Polanyi-Sato (FPLEPS) functional form, and the other by interpolation through the corrugation reducing procedure (CRP). Then, the present study allows us to elucidate the ER dynamics sensitivity on the PES representation. Despite some sizable discrepancies between both H+H/W(110) PESs, the obtained projectile-energy dependence of the total ER cross sections are qualitatively very similar ensuring that the main physical ingredients are captured in both PES models. The obtained distributions of the final energy among the different molecular degrees of freedom barely depend on the PES model, being most likely determined by the reaction exothermicity. Therefore, a reasonably good agreement with the measured final vibrational state distribution is observed in spite of the pressure and material gaps between theoretical and experimental conditions. [ABSTRACT FROM AUTHOR]

The dissociative adsorption of molecular oxygen on metal surfaces has long been controversial, mostly due to the spin-triplet nature of its ground state, to possible non-adiabatic effects, such as an abrupt charge transfer from the metal to the molecule, or even to the role played by the surface electronic state. Here, we have studied the dissociative adsorption of O2 on CuML/Ru(0001) at normal and off-normal incidence, from thermal to super-thermal energies, using quasi-classical dynamics, in the framework of the generalized Langevin oscillator model, and density functional theory based on a multidimensional potential energy surface. Our simulations reveal a rather intriguing behavior of dissociative adsorption probabilities, which exhibit normal energy scaling at incidence energies below the reaction barriers and total energy scaling above, irrespective of the reaction channel, either direct dissociation, trapping dissociation, or molecular adsorption. We directly compare our results with existing scanning tunneling spectroscopy and microscopy measurements. From this comparison, we infer that the observed experimental behavior at thermal energies may be due to ligand and strain effects, as already found for super-thermal incidence energies. [ABSTRACT FROM AUTHOR]

An approach based on reactive force fields is applied to the parametrization of potential energy surface (PES) for chemical reactions on surfaces with a benchmark system, H2/Pd(111). We show that a simple reactive force field based on the second moment approximation does not allow for obtaining reliable results of reaction dynamics for the considered system. With a more elaborate reactive force field, i.e., reactive bond order (REBO) force field, we succeeded in obtaining a reliable PES for H2/Pd(111). The accuracy of the constructed REBO force field is carefully checked through various tests including the comparison not only between energies calculated with density functional theory and those with REBO force field but also between the available results of ab initio molecular dynamics simulations and those with our force field. Moreover, our REBO force field is endowed with some transferability since the force field constructed with a database containing only information on H2/Pd(111) allows for obtaining also accurate results for H2/Pd(100) and qualitatively correct results for H2/Pd(110) without any refitting. With the help of our reactive force field, the molecular dynamics simulation for the dissociation of H2 on the considered Pd surfaces is speeded up by five orders of magnitude compared to ab initio molecular dynamics method. The demonstrated reliability and the very high computational efficiency of reactive force fields open extremely attractive perspectives for studying large-scale complex reacting systems. [ABSTRACT FROM AUTHOR]

We study the dissociative dynamics of O2 molecules on the Ag(100) surface. Initially, the impinging molecules are either in the spin-triplet ground state or in the spin-singlet excited state. The molecule-surface interaction is obtained in each case by constructing the six-dimensional potential energy surface (PES) from the interpolation of the energies calculated with spin-polarized and non-spin-polarized density functional theories, respectively. Classical trajectory calculations performed in both PESs show that O2 molecules initially in the spin-triplet ground state only dissociate for incidence energies above 1.05 eV. This result is consistent with molecular beam experiments performed in this system. Interestingly, our results also suggest that for the spin-singlet O2 dissociation occurs even for incidence energies as low as 50 meV. We propose the use of spin-singlet excited O2 molecules to improve the otherwise low dissociative reactivity of O2 at clean Ag(100). [ABSTRACT FROM AUTHOR]

We study the dissociative adsorption of N2 on W(100) and W(110) by means of density functional theory and classical dynamics. Working with a full six-dimensional adiabatic potential energy surface (PES), we find that the theoretical results of the dynamical problem strongly depend on the choice of approximate exchange-correlation functional for the determination of the PES. We consider the Perdew-Wang-91 [Perdew et al., Phys. Rev. B 46, 6671 (1992)] and Perdew-Burke-Ernzerhof (RPBE) [Hammer et al., Phys. Rev. B 59, 7413 (1999)] functionals and carry out a systematic comparison between the dynamics determined by the respective PESs. Even though it has been shown in earlier works that the RPBE may provide better values for the chemisorption energies, our study brings evidence that it gives rise to a PES with excessive repulsion far from the surface. [ABSTRACT FROM AUTHOR]

The six-dimensional potential energy surface for the dissociation of N2 molecules on the W(110) surface has been determined by density functional calculations and interpolated using the corrugation reducing procedure. Examination of the resulting six-dimensional potential energy surface shows that nonactivated paths are available for dissociation. In spite of this, the dissociation probability goes to a very small value when the impact energy goes to zero and increases with increasing energy, a behavior usually associated with activated systems. Statistics on the dynamics indicate that this unconventional result is a consequence of the characteristics of the potential energy surface at long distances. Furthermore, two distinct channels are identified in the dissociation process, namely, a direct one and an indirect one. The former is responsible for dissociation at high energies. The latter, which includes long-lasting dynamic trapping in the vicinity of a potential well above the W top position, is the leading mechanism at low and intermediate energies. [ABSTRACT FROM AUTHOR]

We have performed H2(D2) diffraction experiments on a Pd(110) surface using two different high-sensitivity set-ups. We have found that, although the total reflectivity of Pd(110) is comparable to that observed in other reactive systems, the corresponding H2(D2) diffraction patterns are quite different: no diffraction peak, including the specular one, is observed on Pd(110). This unexpected result is the consequence of dynamic trapping. Such interpretation is supported by classical dynamics calculations based on accurate ab initio potential energy surfaces. [ABSTRACT FROM AUTHOR]

We present quasiclassical dynamics calculations of H2 and D2 scattering by the NiAl(110) surface using a recently proposed six-dimensional potential-energy surface (PES) obtained from density-functional theory calculations. The results for dissociative adsorption confirm several experimental predictions using (rotationally hot) D2 beams, namely, the existence of a dissociation barrier, the small isotopic effect, the importance of vibrational enhancement, and the existence of normal energy scaling. The latter conclusion shows that normal energy scaling is not necessarily associated with weak corrugated surfaces. The results for rotationally elastic and inelastic diffractions are also in reasonable agreement with experiment, but they show that many more diffractive transitions are responsible for the observed structures than previously assumed. This points to the validity of the PES recently proposed [P. Rivière, H. F. Busnengo, and F. Martín, J. Chem. Phys. 121, 751 (2004)] to describe dissociative adsorption as well as rotationally elastic and inelastic diffractions in the H2/NiAl(110) system. [ABSTRACT FROM AUTHOR]

We study the relaxation of hot H atoms produced by dissociation of H2 molecules on the Pd(111) surface. Ab initio density-functional theory calculations and the “corrugation reducing procedure” are used to determine the interaction potential for a H atom in front of a rigid surface as well as its modification under surface-atom vibrations. A slab of 80 Pd atoms is used to model the surface together with “generalized Langevin oscillators” to account for energy dissipation to the bulk. We show that the energy relaxation is fast, about 75% of the available energy being lost by the hot atoms after 0.5 ps. As a consequence, the hot atoms do not travel more than a few angstroms along the surface before being trapped into the potential well located over the hollow site. [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]

We present results of extensive density functional theory (DFT) calculations for H and H2 interacting with NiAl(110). Continuous representations of the full dimensional potential energy surface (PES) for the H/NiAl(110) and H2/NiAl(110) systems are obtained by interpolation of the DFT results using the corrugation reducing procedure. We find a minimum activation energy barrier of ∼300 meV for dissociative adsorption of H2, which is consistent with the energy threshold obtained in molecular beam experiments for H2 (ν=0). We explain vibrational enhancement observed in experiments as the consequence of vibrational softening in the entrance channel over the most reactive surface site. The H2/NiAl(110) PES shows a high surface site selectivity: for energies up to 0.1 eV above threshold, H2 adsorption can only take place around top-Ni sites (within a circle of radius ∼0.3 Å). A strong energetic corrugation is observed: energy barriers for dissociation vary by more than 1 eV between the most and the least reactive sites. In contrast, geometric corrugation is much less pronounced and comparable to that of low index single metal surfaces like Cu or Pt. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study the scattering of H[sub 2] (v=0, J=0) molecules by the Pd(110) surface using classical trajectory methods. We show that the dissociative adsorption probability barely depends on incidence angle (total energy scaling) up to an impact energy of 200 meV. This is the consequence of a “loss of memory” of the initial incidence angle, mostly due to dynamic trapping, which also reflects itself in a cosinelike angular distribution of reflected molecules. Consequently, a cosinelike distribution can be the result of a subpicosecond process that involves neither energy dissipation to the surface nor transient thermal accommodation.© 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]

(We present results of classical trajectory calculations on dissociative adsorption and reflection of H[SUB2] impinging on a frozen Pd(110) surface. The six-dimensional potential energy surface is obtained by interpolation of density functional theory (DFT) calculations using the corrugation reducing (procedure. The comparison with earlier work for H[SUB2]/Pd(111) shows that DFT calculations predict that the Pd(110) surface, though more open, is less reactive than the Pd(111) one. This lower reactivity is associated with a more prominent role of dynamic trapping, at low energies, and with the fact that trapping may end up in reflection as well as in dissociation. [ABSTRACT FROM AUTHOR]

We study the scattering of H[sub 2] (υ=0, J=0) molecules by the Pd(111) surface using classical trajectory methods. We show that the variation of the reflectivity with incidence angle can be explained with the combination of two processes: "dynamic trapping" and "direct dissociation" that verify total and normal energy scaling, respectively. The presence of the dissociation channel barely affects the angular distribution of scattered molecules. In the patterns of final angular distributions, the main difference, with respect to atom scattering, is a strong momentum transfer from motion normal to the surface toward molecular rotation. [ABSTRACT FROM AUTHOR]

We present dissociative adsorption probabilities of H[sub 2] on Pd(111) computed with the classical trajectory method. We perform both classical (C) and quasiclassical (QC) calculations, the latter including, by contrast with the former, the initial zero point energy (ZPE) of H[sub 2]. We analyze in detail the role played by the ZPE and demonstrate the strong and weak points of both C and QC calculations. We show that ZPE is crucial in accelerating the molecules toward the surface through vibrational softening. However, at low energies, dynamic trapping is quenched in QC calculations by processes of vibration to rotation energy transfer that would be associated with closed channels in a quantum approach. In this study we use a new representation of the H[sub 2]/Pd(111) potential energy surface (obtained by interpolation of ab initio data) with a significantly better accuracy in the entrance channel region which plays a decisive role in the dissociation dynamics. © 2002 American Institute of Physics. [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]

The role of spin non-adiabatic effects in the reactivity of O2 on metal surfaces has been a matter of debate for several years. By means of density functional theory with a semi-local exchange–correlation functional, and classical dynamics calculations, we show that the recently observed activated character of O2 adsorption on Cu(111) and CuML/Ru(0001), as well as the delicate interplay between dissociative and non-dissociative O2 sticking on Cu(111) at different surface temperatures, can be explained by assuming an adiabatic evolution of the molecular spin. This suggests that spin adiabaticity during O2 adsorption on metal surfaces could be a more general scenario than anticipated. [ABSTRACT FROM AUTHOR]