Coupled desorption and site conversion of co-adsorbates: A lattice gas analysis of thermal desorption and spectroscopic data of (NO+O)/Ru(001)

The (NO+O)/Ru(001) system constitutes a good example of complex desorption-site conversion with the advantage that the geometrical structures, the coverages, as well as detailed spectroscopic information on all participating species are available. We present a combined set of desorption data and vibrational spectra with in situ determination of partial coverages in desorption which show site-conversion for both increasing and decreasing temperatures. This system is understood in detail by modeling the (NO+O) coadsorbate layer on Ru(001) as a lattice gas that accounts for the adsorbate structure and site-conversion, and explains the TPD spectra. The unusual features of the TPD spectra result in part from the dramatic coverage dependence of the sticking coefficient induced by site-exclusion.