We report x-ray emission and absorption spectroscopy studies of the electronic structure of the predissociative α(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage α(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of "lying down" CO in the hollow site is well described in terms of π to π∗ charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage α(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of "vertical" on-top adsorbed CO similar to other transition-metal surfaces.
The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C 2 H 2 island formation and adsorbed C 4 H 4 , which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C 2 H 2 diffusion is determined to be 43 ± 1 kJ/mol. The activation barrier for formation of the C 4 H 4 intermediate is found to be 84 ± 6 kJ/mol and the barrier for benzene formation 5 ± 3 kJ/mol lower. Barriers to diffusion and formation of C 4 H 4 and C 6 H 6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C 2 H 2 islands is included.
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