Adsorption state of 1,4-cyclohexadiene onSi(100)(2×1)

The adsorption state of 1,4-cyclohexadiene on a $\mathrm{Si}\left(100\right)(2\mathrm{}\times 1)$ surface is studied using low-energy electron diffraction (LEED) and photoelectron spectroscopy (PES). LEED shows a sharp $2\times 1$ pattern at saturation after the adsorption of 1,4-cyclohexadiene indicating that the Si dimer structure is maintained. Judging from the PES results, almost all the dangling bonds react on adsorption of 1,4-cyclohexadiene, and one of the two π bonds in 1,4-cyclohexadiene reacts with the Si dimer. Thus, the molecule adsorbs on a $\mathrm{Si}\left(100\right)(2\mathrm{}\times 1)$ dimer one to one with $di-\sigma$ bonding, and the ideal saturation coverage is expected to be 0.5 monolayer. It is concluded that the molecules are anisotropically aligned and the molecular plane is inclined from the surface normal, with the remaining π bond located at the vacuum side.