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Carbon Monoxide chemisorbed on Pt(111), plays a role in various catalytic processes and has been studied both experimentally and theoretically for decades. A recent comprehensive analysis of this system, has explained the adsorption/desorption properties using a model with separate terms for the CO-Pt interaction and the long range lateral interaction between the CO molecules[1].
In this work we present Helium Spin Echo measurements[2], of the microscopic diffusion of CO/Pt(111). The atomic scale sensitivity of this technique allows us to study both the Potential Energy Surface (PES), related to the static adsorbate-substrate interaction[3], and the lateral interaction between the adsorbates[4].
As seen previously in macroscopic measurements[5], the CO diffusion is strongly coverage dependent, with an activation energy which drops by ~ 50 meV when the coverage is increased to 0.3ML. However, our diffusion measurements show a clear lack of correlation between the motion of the different CO molecules. The results allow us to exclude the existence of the long range repulsive interactions suggested previously. Using MD simulations we show that the explanation for the observed coverage dependency can be related to a change in the CO-Pt interaction which occurs with increasing coverage. Thus, this adsorbate system can not be correctly modelled using a separate rigid adsorbate-substrate PES and a pairwise CO-CO interaction. The results of this study emphasize the importance of microscopic diffusion measurements in understanding adsorbate systems.
[1] JS McEwen et. al. Surf. Sci. 545, 47 (2003).
[2] AP Jardine et. al. Science 305, 1790 (2004).
[3] G Alexandrowicz et. al. Phys. Rev. Lett. 93, 156103(2004).
[4] G Alexandrowicz et. al. Phys. Rev. Lett. 97, 156103(2006).
[5] J Ma et. al. Phys. Rev. B, 58, 4977 (1998). |