Dresden 2006 – scientific programme

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O: Oberflächenphysik

O 7: Surface reactions I

O 7.1: Talk

Monday, March 27, 2006, 11:15–11:30, WIL C207

Comparison of chemically and photo induced electronic excitations on gold surfaces — •Beate Mildner, Peter Thissen, Eckart Hasselbrink, and Detlef Diesing — Institut für Physikalische Chemie, Universität Duisburg Essen

Within an adiabatic picture exothermic surface reactions dissipate their excess energy into adsorbate and substrate vibrations. Deviations from this behavior are manifested in internal electronic excitation of the reaction products or in electronic excitations of the metal. The latter ones can be detected by the emission of electrons from the surface. However, this is only possible, when the energy of the electrons is larger than the work function of the metal. Electronic excitations with smaller energies can be detected as internal currents in tunnel or Schottky devices, whose barriers are smaller than the metals work function. We use Ta–TaOx–Au tunnel junctions as detectors for electronic excitations. In the course of the recombination reaction of atomic hydrogen on the gold surface chemically induced tunnel currents are observed. The band structure of the tunnel devices can be modified by the application of a bias voltage between the Ta and Au layers allowing an energy selective detection of the chemically excited charge carriers. This experiment is compared with the energy selective detection of photo induced tunnel currents. We used laser diodes with h ν = 1.95, 1.53, 1.37, 1.27  eV. The spectrum of the chemically induced excitations agrees best with that observed for a photo excitation with h ν = 1.27  eV, a value which is close to the exothermicity of the Langmuir-Hinshelwood reaction between two adsorbed hydrogen atoms of approx. 1.4 eV.