Abstract
Scanned-energy mode C 1s photoelectron diffraction has been used to investigate the localadsorption geometry of benzene on Si(001) at saturation coverage and room temperature.The results show that two different local bonding geometries coexist, namely the‘standard butterfly’ (SB) and ‘tilted bridge’ (TB) forms, with a composition of58 ± 29% of the SB species. Detailed structural parameter values are presented for both speciesincluding Si–C bond lengths. On the basis of published measurements of the rateof conversion of the SB to the TB form on this surface, we estimate that thetimescale of our experiment is sufficient for achieving equilibrium, and in this caseour results indicate that the difference in the Gibbs free energy of adsorption,ΔG(TB)−ΔG(SB), is inthe range −0.023 to +0.049 eV. We suggest, however, that the relative concentration of the two species may also beinfluenced by a combination of steric effects influencing the kinetics, and a sensitivity of theadsorption energies of the adsorbed SB and TB forms to the nature of the surroundingbenzene molecules.
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