A. Salin

Representation of the 6D potential energy surface for a diatomic molecule near a solid surface

An efficient method is proposed to construct the six-dimensional Potential Energy Surface (PES) for diatomic molecule-surface interactions from low dimensional cuts obtained in ab initio calculations. The efficiency of our method results from a corrugation-reducing procedure based on the observation that most of the corrugation in a molecule-surface PES is already embedded in the atom-surface interactions. Hence, substraction of the latter leads to a much smoother function which makes accurate interpolations possible. The proposed method is a general one and can be implemented in a systematic way for any system. Its efficiency is illustrated for the case of H2/Pd(111) by using recent ab initio data. We report also the results of very stringent checks against ab initio calculations not used in the interpolation. These checks show the high accuracy of our method.

Constructing accurate potential energy surfaces for a diatomic molecule interacting with a solid surface: H2+Pt(111) and H2+Cu(100)

By applying a corrugation-reducing procedure we have interpolated the six-dimensional (6D) potential energy surfaces for the H2/Pt(111) and H2/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 wit...

Analysis of H2 dissociation dynamics on the Pd(111) surface

We perform a detailed analysis of the dynamics of the dissociative adsorption of H2 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 A. 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 A the molecule evolves “as a whole,” below 1.5 A the dynamics has the character of independent atom–surface interactions which explains why it dissociates with a probability close ...

Classical dynamics of dissociative adsorption for a nonactivated system: The role of zero point energy

We present dissociative adsorption probabilities of H2 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 H2. 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 H2/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.

Role of dynamic trapping in H2 dissociation and reflection on Pd surfaces

We present results of classical trajectory calculations on dissociative adsorption and reflection of H2 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 H2/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.

Six-dimensional classical dynamics of H2 dissociative adsorption on Pd(111)

Six-dimensional classical dynamics simulations are carried out to study the dissociative adsorption of H2 on a Pd(111) surface. The 6D potential energy surface (PES) used in the simulations is constructed by interpolation of ab initio results. The high accuracy of the PES construction is ascertained by comparison with a set of ab initio data which are not used in the interpolation. We obtain a non monotonic variation of the dissociative adsorption probability with normal incident energy as found experimentally. Our results provide further support to the importance of dynamical steering.

CDW-EIS theory of ionization by ion impact with Hartree-Fock description of the target

We have carried out a generalization of the continuum-distorted-wave-eikonal-initial-state (CDW-EIS) approximation for ion-impact single ionization where the interaction of the active electron with the target is represented by a Hartree-Fock potential. We apply this model to the ionization of He, Ne and Ar by proton and multiply-charged bare-ion impact. Doubly differential and total cross sections are calculated from each subshell. These cross sections summed over all subshells show a much better agreement with experimental data than those obtained from the previous formulations of the CDW-EIS approximation which use hydrogenic wavefunctions with effective charges.

Rotational effects in dissociation of H2 on Pd(111): Quantum and classical study

We study rotational effects in dissociation of H2 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 energie...

Angular distribution of H2 molecules scattered from the Pd(111) surface

We study the scattering of H2 (v=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.

Evidence for Two-Centre Effects in the Electron Emission from 25 MeV/u Mo40+ + He Collisions: Theory and Experiment

Absolute doubly differential cross-sections for electron emission in 25 MeV/u Mo40+ + He collisions were measured as a function of the electron energy and emission angle. The data show enhancement at forward angles and reduction at backward angles with respect to results derived using the Born approximation. Comparison with a continuum distorted-wave approximation suggests that this observation can be explained by the combined influence of the projectile and target Coulomb fields on the ejected electron. This two-centre effect is found to be particularly important for electron emission at backward angles.