Lifetime of an adsorbate excitation modified by a tunable two-dimensional substrate

M. Wiesenmayer, M. Bauer, S. Mathias, M. Wessendorf, E. V. Chulkov, V. M. Silkin, A. G. Borisov, J.-P. Gauyacq, P. M. Echenique, and M. Aeschlimann

Phys. Rev. B 78, 245410 – Published 9 December 2008

Abstract

The coupling efficiency between an adsorbate and a two-dimensional substrate is probed by real-time monitoring of the ultrafast charge transfer between a Cs atom and an ultrathin silver film of varying thickness adsorbed on a Cu(111) surface. For the first two monolayers of the silver film, a reduction in the resonance lifetime of the cesium $6 s − 6 p$ hybrid state of approximately 35% is observed. When the silver coverage further increases, the resonance lifetime stays constant at a value close to the value for Cs adsorption on a bulk Ag(111) surface. Both the one-electron resonant and the multielectron inelastic contributions to the adsorbate-substrate charge transfer are theoretically evaluated based on wave-packet propagation and $G W$ approximation. The results support the experimental findings and allow us to assign the observed dependence of the lifetime change in the multielectron inelastic contribution to the electron transfer rate between the Cs resonance and the very top atomic layers of the substrate.

Received 30 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.245410

Authors & Affiliations

M. Wiesenmayer¹, M. Bauer^1,*, S. Mathias², M. Wessendorf², E. V. Chulkov^3,4, V. M. Silkin^3,4, A. G. Borisov^5,6, J.-P. Gauyacq^5,6, P. M. Echenique^3,4, and M. Aeschlimann²

¹Institut für Experimentelle und Angewandte Physik, Christian-Albrechts Universität zu Kiel, 24089 Kiel, Germany
²Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany
³Departamento de Física de Materiales, Facultad de Ciencias Químicas, UPV/EHU and Centro Mixto CSIC-UPV/EHU, Apartado1072, San Sebastián/Donostia, 20080 Basque Country, Spain
⁴Donostia International Physics Center (DIPC), Paseo de Manuel Lardizibal, 4 San Sebastián/Donostia, 20018 Basque Country, Spain
⁵Laboratoire des Collisions Atomiques et Moléculaires, CNRS, Laboratorie des Collisions Atomiques et Moléculaires, UMR8525, Bâtiment 351, 91405 Orsay Cedex, France
⁶Laboratoire des Collisions Atomiques et Moléculaires, Université Paris-Sud, Laboratorie des Collisions Atomiques et Moléculaires, UMR8525, Bâtiment 351, 91405 Orsay Cedex, France

^*Corresponding author: FAX: -49 (0)431 880 1685; bauer@physik.uni-kiel.de.

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Vol. 78, Iss. 24 — 15 December 2008

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Images

Figure 1
(a) Surface state binding energy of Ag/Cu(111) as function of silver film thickness (data taken from Ref. 43). (b) Change in the work function as function of silver film thickness determined from the onset of the photoemission spectra recorded for different coverages. (c) 1PPE $E (k_{∥})$ intensity maps recorded with the HeI line of the VUV-lamp for increasing silver coverage in the thickness regime between 0 and 1 ML. As the thickness increases, the Shockley surface state of the 1-ML-thick silver areas appear in the spectrum. For a closed 1 ML film (very right intensity map), the signal of the Cu(111) Shockley surface state has disappeared.Reuse & Permissions
Figure 2
(Color online) 2PPE $E (k_{∥})$ intensity maps for Cs adsorption on a silver film grown on Cu(111) at increasing coverage. (a) 2PPE map without Cs showing the dispersive Shockley SS of the silver overlayer. (b) Even for very small Cs coverages a clear nondispersive signature from the Cs resonance state (Cs Res) appears in the spectrum; still visible is some residual from the Shockley surface state. [(c) and (d)] As the Cs coverage increases, the Cs resonance shifts down in energy.Reuse & Permissions
Figure 3
(Color online) (a) 1PPE and 2PPE excitation schemes illustrating the appearance of the SS peaks and the Cs peak in the corresponding spectra. Note that the population of the ${Cs}^{*}$ resonance state in the 2PPE process does not arise from a resonant excitation from the surface states SS1 and SS2. (b) 1PPE and 2PPE spectra of an uncesiated (top) and a cesiated surface (middle and bottom spectra, coverage $\sim 0.05 ML$ ) of a 10-ML-thick silver layer grown on Cu(111) at room temperature after flashing to 600 K. The SS1 feature is the Shockley surface state of the surface areas covered with 2ML of silver, and the SS2 feature is the Shockley surface state of areas covered with 10 ML of silver. The energy scale refers to the binding energy with respect to the occupied density of states. Note that the Cs resonance state is an excited state located at $E − E_{F} = + 2.7 eV$ .Reuse & Permissions
Figure 4
Comparison of time-resolved 2PPE autocorrelation traces measured for the excitation of the ${Cs}^{*}$ resonance at two different silver film thicknesses (1 and 8 ML). For reference also the laser autocorrelation determined by 2PPE from the Cu(111) Shockley surface state is shown. The difference in the two Cs traces arises from the variation in the resonance lifetime as the silver film thickness increases. $T_{1, eff}$ are the effective population relaxation times deduced from the experimental autocorrelation traces (details see text).Reuse & Permissions
Figure 5
${Cs}^{*}$ resonance effective lifetime as function of silver film thickness: (a) $T_{1, eff}$ as measured in the TR-2PPE experiment and (b) $T_{1, eff}$ as determined from theory.Reuse & Permissions

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