AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS3-TuA

Paper SS3-TuA4
Atomic Scale Femtochemistry: Electron Induced Manipulation of Single CO Molecules on Cu(111) @footnote 1@

Tuesday, November 3, 1998, 3:00 pm, Room 314/315

Session: Photon- and Electron-Induced Chemistry
Presenter: L. Bartels, Freie Universität Berlin, Germany
Authors: L. Bartels, Freie Universität Berlin, Germany
G. Meyer, Freie Universität Berlin, Germany
K.-H. Rieder, Freie Universität Berlin, Germany
D. Velic, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
E. Knoesel, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
A. Hotzel, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
M. Wolf, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
G. Ertl, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Correspondent: Click to Email
Low Temperature STM experiments on the excitation dynamics of isolated CO molecules on Cu(111) were performed. CO molecules adsorb on-top on Cu(111). By injection of electrons at a minimum sample bias of 2.4V hops of an adsorbed CO molecule can be induced. These hops either lead to the transfer of the CO onto the STM tip or to its diffusion on the sample with a probability of approx. 1:3. It was found that at constant bias the hopping rate linearly depends on the rate of electron tunneling from tip to sample (i.e. the tunneling current), thus bearing witness of a one-electron excitation at the heart of this process. However, the corresponding quantum yield of approx. 10@super -11@ is ultra-low only. Increasing the bias from 2.4V to 3.0V this rate increases. Spectroscopy with the STM reveals in the given energy range an increased density of states of CO-Cu(111) with reference to bare Cu(111). This was confirmed by two photon photoemission spectroscopy (2PPE) to correspond to the 2@pi@* level of CO. Estimating the energy barrier for CO transfer to the tip from the known CO-Cu vibrational energy, the known gap-distance and the know desorption barrier it comes out to be only insignificantly lower than the desorption barrier itself. This allows to treat this STM-induced excitation process in terms of a stimulated desorption experiment. Then the MGR-model can be applied to deduce the electron attachment rate from the net quantum yield, if an isotope effect can be found. Carefully comparing the hopping rates on mixed coverages of @super 12@C@super 16@O and @super 13@C@super 18@O an isotope effect of 2.7 can be estimated. This allows to estimate that 0.5% of the tunneling current passes through 2@pi@*. Parametrization of the isotope effect in a semiclassical model with the lifetime of the excited state and its potential slope only yields correct results, if a lifetime of the excited state of less than 3.5fs is assumed. This is in good agreement with the rang of 0.8-5fs obtained by time-resolved 2PPE. @FootnoteText@ @footnote 1@ L. Bartels, G. Meyer, K.-H. Rieder, D. Velic, E. Knoesel, A. Hotzel, M. Wolf, G. Ertl, Phys. Rev. Lett. 80, 2004 (1998)