Detektion elektronischer Anregungen bei chemischen Reaktionen an Metalloberflächen
An experimental setup for the detection of chemicurrents (CC) generated during the reaction of molecular and atomic gas species with metal surfaces is presented. Chemicurrents were detected with silver- and magnesium-silicon schottky-diodes with ultra thin metal films. The schottky-diodes were characterised by current-voltage (IV) measurements as well as internal photoemission (IPE). The resulting barrier heights are explained within the theory of metal induced gap states (MIGS). The reaction of nitric oxide (NO) with thin silver films was investigated with chemicurrent measurements, secondary electron emission microscopy (SEM) and X-Ray photoemission spectroscopy (XPS). The chemicurrent follows the reaction kinetic at the surface. Two reaction processes, the NO adsorption and the intermolecular reaction of NO leading to N2O and chemisorbed oxygen are monitored in real time. A kinetic modelling of the chemicurrent allows the extraction of kinetic rate constants of the surface reaction. Ultrathin silver films on Si(111)-surfaces are shown to be unstable under the oxidation with NO resulting in a disruption of the thin silver films and an oxidation of the underlying silicon substrate. Silver films on Si(100)-surfaces are not oxidised at 180 K and are stable under NO exposition. The oxidation of magnesium with molecular oxygen and nitric oxides was observed with chemicurrent measurements combined with Auger electron spectroscopy (AES). The chemicurrent is due to internal exoemission of hot holes as well as the detection of photons from surface chemiluminescence. It is shown that the chemicurrent is directly proportional to the reaction rate, the detected chemicurrent charge directly follows the oxygen uptake. Model calculations indicate that the oxidation process can be described by an island and nucleation growth model.
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