{\em Ab initio} study of the structure of 1,3-butadiene on a Si(001) surface

By employing {\em ab initio} total-energy calculations we have studied the adsorption of 1,3-butadiene molecule on a dimer-reconstructed Si(001) surface. We have found that the [4+2] products are thermodynamically more stable than the [2+2] products. There are basically two kinds of [4+2] products: (i) the well-known on-top di-$\sigma$ configuration over a single dimer and (ii) end-bridge configurations occupying two successive dimers in the same dimer row. In [EB] configuration at one ML coverage, 1,3-butadiene molecules at opposite end of dimer bond together to form cubane (C$_8$H$_8$)-like square geometry with one hydrogen atom at each apex, which is stabilized by the full compensation of dangling bonds and converting two $\pi$-bonds to two $\sigma$-bonds. The staggered end-bridge (s-[EB]) is also stable by strain release. By comparing the simulated STM images and experimental ones, we pointed out that the [EB] configuration shows similar feature to that attributed as [2+2] configuration in previous experiments. The calculated core-level shifts(CLS) of C $1s$ orbital show little difference between peak from interface C atom bonded directly to silicon and that from surface C atom with unsaturated double bond.