title: Ultrafast electron injection into photo-excited organic molecules.
abstract: Charge transfer rates at metal/organic interfaces affect the efficiencies of devices for organic based electronics and photovoltaics. A quantitative study of electron transfer rates, which take place 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 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. Through density functional theory calculations, we show that the alignment of molecular levels relative to the metal Fermi level is dramatically altered when a core-hole is created on the molecule, allowing the lowest unoccupied molecular orbital to fall partially below the metal Fermi level. We also 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.
authors.affiliation: Faculty of Mathematics and Physics, University of Ljubljana, Slovenia. dean.cvetko@fmf.uni-lj.si and CNR-IOM, Laboratorio TASC, Basovizza, Trieste, Italy and J. Stefan Institute, Ljubljana, Slovenia.
authors.forename: Dean
authors.initials: D
authors.lastname: Cvetko
authors.affiliation: ETSF and Dipartimento di Fisica, Università degli Studi di Milano, Italy. guido.fratesi@unimi.it.
authors.forename: Guido
authors.initials: G
authors.lastname: Fratesi
authors.affiliation: Faculty of Mathematics and Physics, University of Ljubljana, Slovenia. dean.cvetko@fmf.uni-lj.si and CNR-IOM, Laboratorio TASC, Basovizza, Trieste, Italy.
authors.forename: Gregor
authors.initials: G
authors.lastname: Kladnik
authors.affiliation: CNR-IOM, Laboratorio TASC, Basovizza, Trieste, Italy.
authors.forename: Albano
authors.initials: A
authors.lastname: Cossaro
authors.affiliation: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Italy.
authors.forename: Gian Paolo
authors.initials: GP
authors.lastname: Brivio
authors.affiliation: Department of Applied Physics, Columbia University, New York, NY, USA. lv2117@columbia.edu and Department of Chemistry, Columbia University, New York, NY, USA.
authors.forename: Latha
authors.initials: L
authors.lastname: Venkataraman
authors.affiliation: CNR-IOM, Laboratorio TASC, Basovizza, Trieste, Italy and Dipartimento di Fisica, Università di Trieste, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Morgante