Inhibition of the [2+2] cycloaddition: Maleic anhydride on Si(100)-2x1

The chemisorption of maleic anhydride on Si(100)-2x1 at room temperature ha s been studied with scanning tunneling microscopy (STM). We observe that, a t low coverages below 0.12 monolayer, the unsaturated organic compound adso rbs almost exclusively on inter-dimer sites and not through a direct [2+2] cycloaddition. The STM images provide evidence that the adsorption of a sec ond maleic anhydride molecule on an adjacent inter-dimer site results in th e cleavage of the silicon dimer bond located between the two molecules. The chemisorption of the organic molecules induces a local rearrangement of un saturated silicon dimers.