Violation of the symmetry rule for the [2+2] addition in the chemisorption of C2H4 on Si(100)

The activation barrier for a concerted and symmetry forbidden [2+2] addition of C2H4 to a Si-Si dimer on a reconstructed Si(100) surface is found to be only 0.1 eV by first-principles calculations. We present proof that the Woodward-Hoffmann rule is actually violated in such an extended system, because the transfer of electrons is facilitated by the crossing of energy bands through the Fermi-level. There are thus two mechanisms for the chemisorption of C2H4 on Si(100), an asymmetric path via a π complex and a concerted [2+2] path in violation of the Woodward-Hoffmann rule. Only the barrier for the concerted path increases considerably after the formation of a 0.5 monolayer. The experimentally observed preference of C2H4 molecules to adsorb on alternate dimer sites along a dimer row at low surface coverage and the substantial slowdown in the adsorption rate after the formation of a 0.5 monolayer indicates a significant role for the concerted path at room temperature.