We investigate by first-principles simulations the resonant electron-transfer lifetime from the excited state of an organic adsorbate to a semiconductor surface, namely isonicotinic acid on rutile TiO$_2$(110). The molecule-substrate interaction is d
escribed using density functional theory, while the effect of a truly semi-infinite substrate is taken into account by Greens function techniques. Excitonic effects due to the presence of core-excited atoms in the molecule are shown to be instrumental to understand the electron-transfer times measured using the so-called core-hole-clock technique. In particular, for the isonicotinic acid on TiO$_2$(110), we find that the charge injection from the LUMO is quenched since this state lies within the substrate band gap. We compute the resonant charge-transfer times from LUMO+1 and LUMO+2, and systematically investigate the dependence of the elastic lifetimes of these states on the alignment among adsorbate and substrate states.
We have investigated polyyne and cumulene prototypes based on the density-functional theory. Our independent-particle spectra show that the various carbynes can be distinguished by optical properties comparing the low-energy spectral structure as wel
l as using very general considerations. The latter conclusion is supported by results based on the random-phase approximation including local-field effects.
