Electronically non-adiabatic effects in the adsorption dynamics

In all dynamical simulations presented so far, it has been assumed that the electrons stay in their ground state throughout the whole process, i.e. the simulations have been based on the Born-Oppenheimer approximation. Still, at metal surfaces with their continuous spectrum of electronic states at the Fermi energy electron-hole (e-h) pair excitations with arbitrarily small energies are possible. However, the incorporation of electronically nonadiabatic effects in the dynamical simulation of the interaction dynamics of molecules with surface is rather difficult [2, 109, 110]. Hence the role of electron-hole pairs in the adsorption dynamics as an additional dissipation channel is still unclear [4], 

Recent experiments determining the so-called chemicurrent [111] have provided some information on the importance of electron-hole pair excitation in adsorption processes. Using thin films deposited on n-type Si(l 1 1) as a Schottky diode device, the nonadiabatically generated electron-hole pairs upon both atomic and molecular chemisorption create the chemicurrent which can be measured [111, 112]. It has been estimated that for example in the NO adsorption on Ag one quarter of the adsorption energy is dissipated to electron-hole pairs. Adsorption-induced electron-hole pair creation has also been found for other metal substrates, such as Au, Pt, Pd, Cu, Ni and Fe, and even for semiconductors such as GaAs and Ge [112, 113]. 

Since DFT calculations are in principle only applicable for the electronic ground state, they cannot be used in order to describe electronic excitations. Still it is possible to treat electronic exciations from first principles by either using quantum chemistry methods [114] or time-dependent density-functional theory (TDDFT) [115,116], First attempts have been done in order to calculate the chemicurrent created by an atom incident on a metal surface based on time-dependent density functional theory [117, 118]. In this approach, three independent steps are preformed. First, a conventional Kohn-Sham DFT calculation is performed in order to evaluate the ground state potential energy surface. Then, the resulting Kohn-Sham states are used in the framework of time-dependent DFT in order to obtain a position dependent friction coefficient. Finally, this friction coefficient is used in a forced oscillator model in which the probability density of electron-hole pair excitations caused by the classical motion of the incident atom is estimated. 

This formalism has been employed [ 118] to address the chemicurrent measured in experiments of the adsorption of hydrogen atoms on copper surfaces [119]. Satisfactory agreement with the experiment has been obtained. However, only one single trajectory of a hydrogen atom impinging on the top site has entered the forced oscillator description so that the effect of corrugation has been entirely neglected. 

Electron-hole pairs have already been treated on the Hartree-Fock level in otherwise classical high-dimensional molecular dynamics simulation using the molecular dynamics with electronic friction method [120]. In this approach, the energy transfer between nuclear degrees of freedom and the electron bath of the surface is also modelled with a position-dependent friction term, but additionally temperature-dependent fluctuating forces are included. 

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