Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Charge transfer rates at metal/organic interfaces affect the efficiencies of devices for organicelectronics and photovoltaics. A quantitative study of electron transfer rates, which occur on the femtosecond timescale, is often difficult, especially since in most systems the molecular adsorption geometry is unknown. Here, we use x-ray resonant photoemission spectroscopy to measure ultrafast charge transfer rates across the pyridine/Au(111) interfaces while also controlling the molecular orientation on the metal. We demonstrate that a bi-directional charge transfer across the molecule/metal interface is enabled upon creation of a core-exciton on the molecule with a rate that has a strong dependence on the molecular adsorption angle. By density functional theory calculations, we show that the alignment of molecular levels relative to the metal Fermi level is modified when a core-hole is created on the molecule, allowing the lowest unoccupied molecular orbital to fall partially below the metal Fermi level. Within a description derived by electron transport in which a semi-infinite substrate is taken into account we calculate charge transfer rates as a function of molecular adsorption geometry and find a trend that agrees with the experiment. These findings thus give insight into the charge transfer dynamics of a photo-excited molecule on a metal surface.