Berlin 2005 – wissenschaftliches Programm

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O: Oberflächenphysik

O 5: Zeitaufgelöste Spektroskopie I

O 5.5: Vortrag

Freitag, 4. März 2005, 11:45–12:00, TU EB107

Two-Photon Photoemission Spectroscopy of Thiophenol Self-Assembled Monolayers on Gold(111) — •S. Dantscher, T. Schutzmeier, C. Kennerknecht, and W. Pfeiffer — Physikalisches Institut, Universität Würzburg, Am Hubland, 97074 Würzburg

The aromatic molecule thiophenol forms ordered self assembled monolayers (SAM) on gold surfaces. Using two-photon photoemission spectroscopy we investigate the influence of this surface modification on the occupied and unoccupied electronic structure of both the substrate and the adsorbate.

The samples consist of 600Åthick gold(111) films on mica that have been evaporated in UHV. The preparation of the SAMs from an ethanolic solution takes place in an inert nitrogen atmosphere. Femtosecond laser pulses with wavelengths of 400nm and 266nm are used for the photoemission measurements.

The adsorption of thiophenol results in a decrease of the work function of the sample from 4.8eV to 3.7eV. The photoemission spectra exhibit peaks related to the SAM formation that shift with applied laser intensity. These peaks are attributed to electronic states in a physisorbed molecular overlayer on the chemisorbed monolayer. This overlayer can easily be charged by injection of excited electrons.

Accompanying these experiments, density functional theory calculations of free and adsorbed thiophenol molecules were performed. The calculated static dipole moment of thiophenol on a small gold cluster is in agreement with the observed work function change.