Sharani Roy

Dynamical Steering and Electronic Excitation in NO Scattering from a Gold Surface

Simulating Surfaces Although modern computational chemistry can often match or even exceed experimental accuracy in modeling gas phase reactions, the surface-bound processes involved in most practical catalysis pose a substantially greater challenge to theory (see the Perspective by Hasselbrink). Díaz et al. (p. 832) show that a modification to standard density functional methods can predict reaction barrier heights to within 1 kilocalorie per mole for the widely studied dissociative adsorption of dihydrogen on copper. In a complementary study, Shenvi et al. (p. 829) apply an efficient algorithmic framework to model transitions among multiple electronic states at a metal surface and successfully account for the complex dependence of nitric oxide scattering on the small molecules vibrations and rotations. Theory accounts for the complex ways in which vibrations and rotations of nitric oxide molecules affect scattering from a surface. Nonadiabatic coupling of nuclear motion to electronic excitations at metal surfaces is believed to influence a host of important chemical processes and has generated a great deal of experimental and theoretical interest. We applied a recently developed theoretical framework to examine the nature and importance of nonadiabatic behavior in a system that has been extensively studied experimentally: the scattering of vibrationally excited nitric oxide molecules from a Au(111) surface. We conclude that the nonadiabatic transition rate depends strongly on both the N-O internuclear separation and the molecular orientation and, furthermore, that molecule-surface forces can steer the molecule into strong-coupling configurations. This mechanism elucidates key features of the experiments and provides several testable predictions regarding the dependence of vibrational energy transfer on the initial vibrational energy, molecular orientation, and incident angle.

Nonadiabatic dynamics at metal surfaces: independent-electron surface hopping.

Recent experiments have shown convincing evidence for nonadiabatic energy transfer from adsorbate degrees of freedom to surface electrons during the interaction of molecules with metal surfaces. In this paper, we propose an independent-electron surface hopping algorithm for the simulation of nonadiabatic gas-surface dynamics. The transfer of energy to electron-hole pair excitations of the metal is successfully captured by hops between electronic adiabats. The algorithm is able to account for the creation of multiple electron-hole pairs in the metal due to nonadiabatic transitions. Detailed simulations of the vibrational relaxation of nitric oxide on a gold surface, employing a multistate potential energy surface fit to density functional theory calculations, confirm that our algorithm can capture the underlying physics of the inelastic scattering process.

Model Hamiltonian for the interaction of NO with the Au(111) surface

We have constructed a model Hamiltonian to describe the interaction of a nitric oxide (NO) molecule with a Au(111) surface. The diagonal elements of the 2x2 Hamiltonian matrix represent the diabatic potential energy surfaces corresponding to the neutral and negative-ion states of the molecule. A position-dependent off-diagonal element controls the extent of mixing of the two diabatic states. The parameters of the Hamiltonian matrix were determined from ground-state density functional theory calculations, both in the absence and presence of a small applied electric field to perturb the extent of charge transfer to the molecule. The resulting model Hamiltonian satisfactorily reproduces the ab initio results, and scattering simulations of the incident translational energy dependence of trapping probability and final rotational energy of NO agree quite well with experiment. The explicit incorporation of neutral and ionic configurations should serve as a realistic and practical platform for elucidating the importance of charge transfer and nonadiabatic effects at metal surfaces, as well as provide a useful testing ground for the development of theories of nonadiabatic dynamics.

Vibrational relaxation of NO on Au(111) via electron-hole pair generation

Recent experiments have demonstrated the breakdown of the Born-Oppenheimer approximation when NO undergoes inelastic scattering from a Au(111) surface. In this paper, we provide a simple theoretical model for understanding this phenomenon. Our model predicts multiquanta vibrational relaxation through the creation of high-energy electron-hole pair excitations in the metal. Using experimentally determined parameters, our model gives qualitatively accurate predictions for the final vibrational state populations of the scattered molecule and predicts efficient conversion of vibrational energy into electronic energy.

Melting of 55-atom Morse clusters

Canonical ensemble Monte Carlo simulations of 55-atom Morse clusters are used to study the effect of the range of the pair interaction on the cluster melting transition. Several different structural indicators are employed to monitor the solid-liquid transition and to locate the melting and freezing temperatures. The behavior of Landau free energy curves in the solid-liquid phase coexistence regime is correlated with the distribution of inherent minima sampled by the system. The melting transition temperatures, the width of the phase coexistence regime, and the internal energy change on melting are shown to increase with decreasing range of the pair interaction, which parallels the behavior seen in bulk Morse systems. Unlike in the case of bulk melting, cluster melting falls into three distinct categories based on the range of the pair interaction: (i) a rigidity transition in long-range systems with a low density of metastable states, (ii) the cluster analogue of bulk melting where the system transits from the basin of an ordered global minima into a set of metastable, amorphous packing minima, and (iii) transition from a set of defected solid-like minima into a set of amorphous packing minima.

Structure of oxidised silver (1 1 1) and (1 1 0) surfaces

Abstract An exhaustive suite of classical molecular dynamics simulations is performed to investigate the stability of oxygen on silver (1 1 1) and (1 1 0) surfaces as a function of surface/subsurface location, binding site, fractional occupancy, and temperature. The ReaxFF potential is used to allow charge transfer between the oxygen and silver components. Comparison of the binding energies at various sites from ReaxFF and ab initio calculations reveals partial agreement between the two approaches. For many of the conditions sampled in the current study, we observe an initial state gives rise to a more disordered reconstruction, which is energetically more favourable. The driving force behind this reconstruction is largely an increase in the coordination of O by Ag, resulting in a more favourable binding site. The extent of reconstruction and atomic motion that initiates the reconstructive process is highly dependent on surface type, fractional occupancy, initially occupied site and temperature. For example, in the temperature range studied (77–500 K), on Ag(1 1 1) it is fractional occupancy that predominantly dictates the type and extent of reconstruction. However, on Ag(1 1 0) it is temperature rather than fractional coverage that is seen to have a more influential effect on the extent of surface reconstruction. These simulations clearly show that O atoms move from surface to subsurface sites, as has been observed experimentally.