SYMPOSIUM ON SURFACE SCIENCE 2011 Baqueira Beret, Lleida ...

Ultrafast charge transfer at graphene surfaces
Silvano Lizzit 1 , Rosanna Larciprete 2 , Paolo Lacovig 1 , Krassimir L. Kostov 3 ,
and Dietrich Menzel 4
1 ELETTRA, Sincrotrone Trieste S.C.p.A, 34149 Trieste, Italy
2 CNR-Institute for Complex Systems, 00133 Roma, Italy
3 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
4 Physik-Dept. E20, Techn. Univ. München, 85748 Garching, Germany, and
Fritz-Haber-Institut der MPG, Dept. CP, 14195 Berlin, Germany
(corresponding author: dietrich.menzel@ph.tum.de)
The dynamics of charge transfer (CT) between adsorbates and surfaces are important for the
electronic response of surfaces and for photochemistry on surfaces [1]. The corresponding
times, for instance investigated by the lifetime of excess electrons on an adsorbate, are very
short – for weak to medium coupling they lie in the low femtosecond range, and for strong
coupling they can be in the sub-fs range [1]. One possibility for its measurement is the socalled
core hole clock (CHC) method, in which the transfer time of an excited electron
(created by a resonant core hole excitation) is determined by a quantitative analysis of the
core hole decay spectra which differ for the cases when the localized charge persists longer or
shorter than the core hole life time [2,3]. The method makes use of the so-called Auger
resonant Raman effect [4]. The rate of charge transfer depends on the overlap of the excited
electron’s orbital with empty surface states of proper symmetry [5]. Qualitative connections
appear to exist to the tunneling probability seen in STM and to conduction through atomic
chains and molecular bridges; these have not yet been pursued in detail quantitatively. While
it is likely that the main contribution to the transfer time of an initially localized excited
electron comes from the transfer to the first neighbor which is normally embedded in a threedimensional
conductor, it cannot be safely assumed that this is always so; the spreading of
charge after transfer may also contribute to the time scale. Crudely speaking this can be
expressed as imagining 3D vs. 2D final states of CT. Graphene (Gr) monolayers [6], which
can be produced on metal surfaces in various degrees of coupling and geometric adaptation
[7], as well as on insulators like SiC [8] (i.e. decoupled from the substrate), may offer an
access to these questions. Also, the contrast seen in STM scans of Gr on metal surfaces and
controversially discussed in terms of geometric and electronic contributions to tunneling [8,9]
should become visible in our CT times as well.
We have therefore measured CT times for resonantly core-excited Ar (2p3/2 >> 4s) atoms
adsorbed on various graphene layers on metal and SiC surfaces with the well-established
CHC method [1,2,5], using narrowband synchrotron light at the SuperESCA beam line of
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