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A method is presented for calculating electron-hole pair excitation due to an incident atom or molecule interacting with a metal surface. Energy loss is described using an ab initio approach that obtains a position-dependent friction coefficient for an adsorbate moving near a metal surface from a total energy pseudopotential calculation. A semi-classical forced oscillator model is constructed, using the same friction coefficient description of the energy loss, to describe excitation of the electron gas due to the incident molecule. This approach is applied to H and D atoms incident on a Cu(111) surface, and we obtain theoretical estimates of the 'chemicurrents' measured by Nienhaus et al [Phys. Rev. Lett. 82, 446 (1999)] for these atoms incident on the surface of a Schottky diode.PACS numbers: 73.20.Hb, 34.50.Dy, Considerable progress has been made in recent years in the theory of gas-surface interactions. This has been due to the parallel developments of large-scale electronic structure calculations based on density functional theory, combined with multi-dimensional quantum and classical analysis of the dynamics [1,2]. Despite these advances there remains one key area that is still largely unexplored and poorly understood; the process of energy dissipation into substrate degrees of freedom. Although this is known to be of central importance in many situations [2], there exist few 'real' calculations to date for the energy loss to either phonons or electrons in the surface [3].In particular there have been a number of recent experiments that provide convincing evidence that energy dissipation by the creation of electron-hole (e-h) pairs is a significant effect in gas-surface dynamics [4,5]. Of particular interest for this Letter are the results of Nienhaus and co-workers [6,7] who directly measured the hot electrons and holes created at Ag and Cu surfaces by the adsorption of thermal hydrogen and deuterium atoms in the form of a 'chemicurrent' in a Schottky diode. Here we report ab initio calculations of e-h pair creation for H/Cu(111) and the resulting chemicurrents.The calculation proceeds in three stages. First, a standard Kohn-Sham (KS) total energy calculation is carried out for a range of positions of the incident atom. Second, the resulting KS states and potentials are used to obtain a friction coefficient for the motion of the atom via Time Dependent Density Functional Theory (TDDFT). Finally, a classical trajectory for the incident atom is calculated and a forced oscillator model (FOM) is used to obtain a semi-classical (classical atomic motion coupled to quantum metallic electrons) description of e-h pair creation. It is important to note that out theory refers to a nearly adiabatic process with multiple low energy excitations. This is in contrast to the truly nonadiabatic charge transfer models used to describe, for example, exoemission of electrons [8].The initial total energy calculation provides the ground state properties of an interacting surface/atom system using a plane-wave basis, pseudopotent...

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A method is described to compute the modes propagating at a given frequency in dielectric systems that are periodic in two dimensions and uniform in the third dimension, using a plane-wave basis expressed in a system of generalized curvilinear coordinates. The coordinates are adapted to the structure under consideration by increasing the effective plane-wave cutoff in the vicinity of the interfaces between dielectrics, where the electromagnetic fields vary most rapidly. The favorable efficiency and convergence properties of the method are shown by comparison with the conventional plane-wave formulation of Maxwell's equations. Although the method is developed to study propagation in photonic crystal fibers, it is also applicable more generally to plane-wave modal solutions of structured dielectrics.

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