Metastable adsorption of benzene on the Si(001) surface

We have investigated the adsorption of benzene on the Si(001) surface using scanning tunneling microscopy. The central result is that benzene initially adsorbs in a metastable state and then converts to a lower-energy final state. The conversion to the final state occurs rather slowly at room temperature, with a time constant of 19 min, implying an activation energy barrier of about 1.0 eV. We study in detail the appearance of each state and its position relative to the underlying surface dimers in order to make a correspondence to existing proposed models for the structure of benzene adsorbed on Si(001). We measure a lower bound on the energy difference between the two states of 0.14 eV.