State resolved desorption measurements as a probe of surface reactions

Surface reactions which lead directly to gas phase products can be investigated by using state resolved techniques to measure the energy released into the newly formed molecules. This technique has been used extensively to explore oxidation of CO and the dynamics of H-2 recombinative desorption at surfaces, but so far has been applied to few other reactions. Here we review the application of final state measurements and discuss the conditions under which dynamical information can be obtained for Langmuir-Hinshelwood type surface reactions. Combining resonance enhanced multiphoton ionisation with ion time of flight detection allows translational energy distributions to be measured for a wide range of products, with full quantum state resolution. The energy release reflects scattering from a thermally populated transition state, with the recombination dynamics determining how the product state distributions depart from a thermal distribution at the surface temperature. Using the principle of detailed balance the desorption dynamics can be related to the reverse process, dissociative chemisorption. Making the link between adsorption and desorption has two benefits. Firstly, it allows us to discuss quantitatively the influence of surface temperature on the product state distributions formed by surface reactions, allowing us to avoid naive models, which treat the transition state as having a unique, well-defined energy. Secondly, the desorption results can be used to obtain relative sticking probabilities with full quantum state and translational energy resolution, providing a way to determine how internal energy influences dissociation for both hydrogen and for heavy molecules, such as nitrogen. The conditions necessary to apply detailed balance successfully are discussed and the desorption distributions expected for different types of adsorption behaviour illustrated. The recombination/dissociation dynamics of hydrogen are summarised briefly and the energy partitioning into different coordinates described. Product state measurements for some 'heavy' molecule reactions, such as NH3 and NO reduction to form nitrogen and CO and H-2 oxidation are reviewed and compared to the behaviour seen for hydrogen, The desorption dynamics and the shape of the potential energy surfaces for nitrogen recombination at different metal surfaces are discussed and we suggest some future lines of development. (C) 2000 Elsevier Science Ltd. All rights reserved.