Monitoring the kinetics of surface reactions under live conditions with thin film electronic devices
Hermann Nienhaus, Faculty of Physics/University of Duisburg-Essen, Duisburg, GermanyUlrich Hagemann, ICAN/University of Duisburg-Essen, Duisburg, Germany
Chemical reactions on metal surfaces transfer energies of a few eV per particle to the metal substrate. Such events can perturb the electronic system during the gas-metal interaction and can cause significant electronic excitations with lifetimes on the femtosecond timescale and energies in the lower eV range . Experimental evidence of such non-adiabatic processes is gained by detecting chemiluminescence and hot hole or hot electron generation in the metal using an internal detection scheme. The method uses ultrathin-film electronic devices with internal potential barriers as high-pass energy filters. Metal-semiconductor (Schottky) diodes are the most prominent examples. When hot charge carriers are chemically excited at the surface they can travel ballistically to the interface and may surpass the Schottky barrier inducing a measurable reverse current in the diode, provided that the metal film is in the nm range. Such chemicurrents are proportional to the reaction rate. Therefore, the current transients reflect the chemical kinetics monitored under live conditions.
Examples of various reaction systems will be presented which exhibit typical reaction kinetics: spontaneous adsorption and abstraction is observed for interaction of H atoms on Ag; nucleation and growth kinetics with extremely small initial sticking coefficients is characterized with the low-temperature oxidation of Mg and alkali metal surfaces; electric field-assisted bulk reactions (Mott-Cabrera) are detected for the reaction of O2 with Al and of Cl2 with K surfaces. In addition, the kinetics of intermolecular reactions can be monitored as well, e.g., the reaction of adsorbed NO molecules to N2O and O on Ag surfaces. Modelling the different kinetics allows the extraction of reaction parameters and cross sections. The internal detection is a sensitive tool especially for low-power impact. Chemicurrents of less than 100 pA are easily detectable with particle fluxes in the 10e12 per s and cm2 range. This corresponds to an reactive energy flow of 100 nW/cm2.
 H. Nienhaus, Surf. Sci. Rep. 45 (2002) 3.