Ultrafast charge transfer and atomic orbital polarization

The role of orbital polarization for ultrafast charge transfer between an atomic adsorbate and a substrate is explored. Core hole clock spectroscopy with linearly polarized x-ray radiation allows to selectively excite adsorbate resonance states with defined spatial orientation relative to the substrate surface. For $c(4×2)S∕Ru(0001)$ the charge transfer times between the sulfur $2s−13p*+1$ antibonding resonance and the ruthenium substrate have been studied, with the $2s$ electron excited into the $3p⊥*$ state along the surface normal and the $3p‖*$ state in the surface plane. The charge transfer times are determined as $0.18±0.07$ and $0.84±0.23fs$⁠, respectively. This variation is the direct consequence of the different adsorbate-substrate orbital overlap.