Dependence on the deposition conditions in the adsorption of C₆H₈ molecules on a Si(100)-2×1 surface

We studied the molecular 1,4-cyclohexadiene (CHD) adsorption at room temperature on a Si(100)-2×1 surface, from the first stages up to saturation [about half a monolayer (ML)] at two different deposition rates. The chemical configuration of the adsorbed CHD on the silicon surface was obtained by valence band and C 1s core level photoemission analyses. Such a molecular adsorption, at the earlier stages and low rates, showed only weak features of chemical bonding with Si; at higher coverages (θ>0.3 ML) and higher deposition rates, a chemisorption occurred with an upright structure of the molecule, in which one out of the two unsaturated π orbitals reacted to form a bond with the silicon dimer. The whole investigation is based on an accurate determination of the strongly asymmetric C 1s core level components obtained by means of an angular resolved photoelectron diffraction analysis.