Excited States in Surface Photochemistry

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9417866 The objectives of the proposed research are to probe the energies and lifetimes of the adsorbate and substrate excited states and to understand the connection between the excited state relaxation processes and the observed photochemical products. Two-photon photoemission in the pump-probe configuration with a tunable laser will be used to measure the energies and lifetimes of the excited states. A mass spectrometer and a high-resolution electron energy loss spectrometer provide chemical information on the photochemical products. The uniquely low work function of Lanthanum hexaboride(100) serves as a model metal substrate on which molecules with different electron affinities are adsorbed. The effects of a band gap on the relaxation processes will be investigated on clean and K-preadsorbed gallium nitride thin films. Studies on the photochemistry of oxygen coadsorbed with carbon monoxide on platinum and the photodesorption of carbon monoxide from silicon will be pursued. %%%% Surface photochemistry is intimately connected to the relaxation of excited states at the surface. Information on the excited states at surfaces is very scarce, despite their importance in surface photochemistry. The subsequent microscopic steps following absorption of photons are largely unknown and at best conjectured from the distribution of end products. It has been firmly established that the two most important and widespread mechanisms of surface photochemistry are: 1. direct electronic excitation of the adsorbed molecules, and 2. substrate excitation followed by interactions of photogenerated hot carriers with the adsorbed molecules. The objectives of the proposed research are to probe the energies and lifetimes of the adsorbate and substrate excited states and to understand the connection between the excited state relaxation processes and the observed photochemical products.