Electron-hole spectra created by adsorption on metals from density-functional theory

TL;DR

This paper develops a first-principles theoretical framework using density-functional theory to predict electron-hole excitation spectra during metal surface adsorption, successfully matching experimental data and providing insights into isotope effects.

Contribution

It introduces a perturbative DFT-based method to calculate excitation spectra during adsorption, offering a simpler alternative to full time-dependent DFT calculations.

Findings

Method accurately predicts isotope effects in H/D adsorption.

Results align with experimental data for noble metal surfaces.

Qualitative agreement with full TDDFT calculations, except for spin effects.

Abstract

Non-adiabaticity in adsorption on metal surfaces gives rise to a number of measurable effects, such as chemicurrents and exo-electron emission. Here we present a quantitative theory of chemicurrents on the basis of ground-state density-functional theory (DFT) calculations of the effective electronic potential and the Kohn-Sham band structure. Excitation probabilities are calculated both for electron-hole pairs and for electrons and holes separately from first-order time-dependent perturbation theory. This is accomplished by evaluating the matrix elements (between Kohn-Sham states) of the rate of change of the effective electronic potential between subsequent (static) DFT calculations. Our approach is related to the theory of electronic friction, but allows for direct access to the excitation spectra. The method is applied to adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are compatible with the available experimental data (for noble metal surfaces); in particular, the observed isotope effect in H versus D adsorption is described by the present theory. Moreover, the results are in qualitative agreement with computationally elaborate calculations of the full dynamics within time-dependent density-functional theory, with the notable exception of effects due to the spin dynamics. Being a perturbational approach, the method proposed here is simple enough to be applied to a wide class of adsorbates and surfaces, while at the same time allowing us to extract system-specific information.