Ab initio Studies of Molecular Adsorbates on Surfaces
Our main goal is the understanding of the adsorption of molecules on surfaces and of the resulting electronic properties, as observed in electronic and optical spectroscopy. From the point of view of a theoretician, these issues constitute a big challenge due to the strong influence of electronic many-body effects (exchange and correlation). While density-functional theory (DFT) provides an excellent tool for the realistic description of many systems, it fails dramatically in two respects. On the one hand, Van-der-Waals interaction is badly described by DFT. On the other hand, excited electronic states do not result from DFT. Both problems require to go beyond DFT. Excited states, for instance, can be addressed by many-body perturbation theory (MBPT), which is one of our main topics. In here, exchange and correlation effects and their influence on the electronic and optical spectra are treated in terms of the electron self-energy operator, as evaluated in the GW approximation (GWA), followed by solving the Dyson equation for quasiparticle excitations (electrons or holes), and by solving the Bethe-Salpeter equation (BSE) for optically excited electron-hole pairs. More recent developments concern the dynamics of excited electronic states, i.e. the femtosecond dynamics of the electronic degrees of freedom and the (usually much slower) molecular dynamics of the atoms, leading to spectral broadening, Stokes shifts, desorption and dissociation.
Current topics of our work consider the adsorption of PTCDA on the Ag(111) surface (see figure), including an extensive study of geometrical features and scanning-tunnelling images, as well as the laser-induced desorption of hydrogen from the H:S(001)-(2x1) surface, laser dissociation of potassium iodide (KI), image states on the Si(001)-c(4x2) surface, and the optical spectrum of calcium fluoride (CaF2).
