Scattering of Nitrogen Atoms off Ag(111) Surfaces: A Theoretical Study (original) (raw)
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Physical Review Letters, 2012
We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab initio potential energy surface. The model is applied to study energy dissipation in the scattering of N 2 on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel, whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.
Diffraction of swift atoms after grazing scattering from metal surfaces: N/Ag(111) system
Physical Review A, 2010
Diffraction patterns produced by grazing scattering of fast N atoms from a Ag(111) surface are investigated by employing the surface eikonal approximation. This method is a distorted-wave theory that takes into account the coherent addition of contributions coming from different projectile paths. In the model the projectile-surface potential is obtained from an accurate density-functional theory calculation. The dependence of the scattered projectile spectra on impact energy and incidence channel is analyzed, and possible incident direction and energy range for the observation of the interference patterns are predicted. In addition, it is found that as a result of the high reactivity of N atoms, asymmetries of the surface potential might be detected through their effects on diffraction patterns.
Direct inelastic scattering of N2 from Ag(111). III. Normal incident N2
The Journal of Chemical Physics, 1988
The orientation of the angular momentum of N2 scattered from clean Ag(111) is determined by resonance-enhanced multiphoton ionization. The orientation is the net helicity or handedness of the sense of rotation, i.e., clockwise vs counterclockwise. The orientation of the scattered N2 is measured along a direction perpendicular to the scattering plane. The degree and sign of the orientation is found to depend strongly on the final rotational quantum number J and on the final scattering angle. The results require that there are forces acting in the plane of the surface during the scattering. The observed behavior can be reproduced qualitatively by a conventional hard-cube, hard-ellipsoid model to which a tangential friction has been introduced to account for the in-plane forces. This produces a splitting of the rotational rainbow peak which leads to changes of sign of the orientation as a function of rotational quantum number. Thus, orientation measurements provide a unique probe of in-plane gassurface forces.
Calculation of the Debye-Waller factor for atom-surface scattering: He on Ag(111)
Physical Review B, 1987
By a consistent application of the distorted-wave Born approximation, the Debye-Wailer exponent for the scattering of He atoms from the Ag(111) surface is directly computed. When diffraction is negligible, as in this case, the decrease in specular intensity is simply obtained by summing the scattering due to all possible one-phonon processes. Using one-phonon computed intensities that agree with experiment along high-symmetry directions of the surface Brillouin zone, we find that the computed Debye-Wailer exponent agrees with experiment and corresponds to an effective Debye temperature of 241 K. All corrections to the eikonal-type formula, 2 8' =4p, (tt,), are automatically included. In particular, the contribution of phonons with high parallel momentum is sharply cut off.
Direct inelastic scattering of N2 from Ag(111). I. Rotational populations and alignment
The Journal of Chemical Physics, 1988
The rotational state populations and the quadrupole and hexadecapole alignment moments of N2 scattered off clean Ag(111) are determined by resonance enhanced multiphoton ionization (REMPI). The scattered N2 is found to be highly aligned with its rotational angular momentum vector J parallel to the surface. The degree of alignment is found to increase with increasing rotational excitation. We see less than perfect alignment at intermediate J values indicating that the surface is not completely flat. The alignment is relatively insensitive to incident energy, incident angle, or surface temperature Ts. However, the rotational state population distributions show pronounced rainbows for higher incident energy and/or more grazing exit angle. The rotational state distributions are found to depend strongly on the final scattering angle at low Ts; this effect is markedly reduced at higher Ts. Time-of-flight measurements are used to determine the average velocity of the scattered N2 as a function of rotational level. It is found that higher rotational excitation correlates with lower average velocity and that the incident molecules lose 20%-30% of their translational energy to the solid. No correlation is found between velocity and alignment. A comparison is made with published results for the NO/ Ag(111) system and a variety of theoretical models found in the literature.
Energy dissipation channels in the adsorption of N on Ag(111)
Computational and Theoretical Chemistry, 2012
We theoretically study the competition between different energy dissipation channels in the adsorption of N atoms on Ag(1 1 1) surfaces. The three-dimensional potential energy surface that describes the interaction between the N atoms and the metal surface is built from density functional theory calculations. Classical dynamics simulations are subsequently performed to evaluate the adsorption probabilities. The contribution of electron-hole pairs excited in the surface during the adsorption process is included in the simulation by an electronic friction coefficient. Phonon excitations are also considered through the Generalized Langevin Oscillator model. We show that the role of the two channels during the adsorption dynamics is very different: phonons are responsible for determining the adsorption probability but electronic excitations are relevant at a later stage to fix the N atoms to the adsorption positions. We conclude that a theoretical model that intrinsically combines both energy dissipation channels is necessary to properly describe the full dynamics of the process.
Out-of-plane scattering of Ar from Ag(111)
Surface Science, 1996
We present the resulting out-of-plane scattering data for the collisions of Ar with an Ag(111) surface, using a classical molecular dynamics simulation with a potential energy surface based on the local density approximation. The incidence energies for Ar vary from 0.03 eV up to 22 eV with an incidence angle of 40 °. Results are presented for a static surface and for a surface with a finite temperature of 600 K. For the low incidence energies, the peak of the angular scattering distributions lies in the plane of incidence and the in-plane width is larger than the out-of-plane one. In the high energy range, the peaks of the scattering distributions appear not in the plane of incidence and for the static surface, clear out-of-plane surface rainbow peaks appear. For a surface temperature of 600 K, the surface rainbow peaks diminish and the angular distributions smooth. The total angular scattering distributions yield the relative amount of in-plane scattering and that ranges from 10 to 40%, depending on the incidence energy. A maximum of the in-plane scattering occurs for an incidence energy of 1 eV, where the effective surface corrugation has a minimum.
Van der Waals contribution to the inelastic atom-surface scattering
Journal of Electron Spectroscopy and Related Phenomena, 2003
A calculation of the inelastic scattering rate of Xe atoms on Cu(111) is presented. We focus in the regimes of low and intermediate velocities, where the energy loss is mainly associated to the excitation electron-hole pairs in the substrate. We consider trajectories parallel to the surface and restrict ourselves to the Van der Waals contribution. The decay rate is calculated within a self-energy formulation. The effect of the response function of the substrate is studied by comparing the results obtained with two different approaches: the Specular Reflection Model and the Random Phase Approximation. In the latter, the surface is described by a finite slab and the wave functions are obtained from a one-dimensional model potential that describes the main features of the surface electronic structure while correctly retains the imagelike asymptotic behaviour. We have also studied the influence of the surface state on the calculation, finding that it represents around 50% of the total probability of electron-hole pairs excitation.
The Journal of Chemical Physics, 2012
We describe a method to obtain absolute vibrational excitation probabilities of molecules scattering from a surface based on measurements of the rotational state, scattering angle, and temporal distributions of the scattered molecules and apply this method to the vibrational excitation of NO scattering from Au(111). We report the absolute excitation probabilities to the v = 1 and v = 2 vibrational states, rotational excitation distributions, and final scattering angle distributions for a wide range of incidence energies and surface temperatures. In addition to demonstrating the methodology for obtaining absolute scattering probabilities, these results provide an excellent benchmark for theoretical calculations of molecule-surface scattering.
Scattering of low-energy atoms by metallic surfaces
Surface Science, 1978
The problem of a neutral low-energy atom impinging on a well-defined metallic surface is approached from first principles. The solid and its potential energy of interaction with the incident atom is treated in the most general way, but under the following assumptions: (a) the conduction electrons interact adiabatically with the lattice ions and the gas atom; (b) no chemical reactions occur; (c) the one-phonon approximation is valid. The scattering amplitudes for surface and bulk mode excitations are obtained in terms of the dynamical properties of the metaltic surface. Direct collisions of the incident atoms with the lattice ions are shown to give a negiigible contribution to the scattering. The most important contribution comes from the interaction of the gas atom with the surface conduction electrons; the excitation of lattice vibrational modes occurs through the electron-phonon term of the Hamiltonian. The general expressions for the scattering amplitudes obtained show that the scattering is incoherent. With further assumptions one obtains a separation of the scattering amplitude into a coherent and incoherent part.