The frustrated motion of benzene on the surface of Si(111)

Benzene adsorption on Si(lll)-7X7 is studied with scanning tunneling microscopy. Benzene diffusion is found to be inhibited. Ordinarily surface diffusion is controlled by a substantially lower energy of activation than is desorption. In this case diffusion is frustrated by a barrier to diffusion that is comparable to that for desorption. Both desorption and diffusion are monitored. On average, for every two adsorbate disappearances, one readsorption is observed and one molecule desorbs. Site-specific activation barriers of 0.94+/-0.01 eV and 0.95+/-0.01. eV for center faulted and corner faulted adatom sites, respectively, are extracted. Residence times increase as coverage decreases, implying adsorbate crowding causes the strength of the surface-adsorbate bond to weaken. Diffusion is generally found to involve jumps to sites beyond nearest neighbors. It emerges that the adsorbate largely breaks its existing bond to a surface site before forming a substantial bonding interaction with a new site. We surmise that this ''break before make'' scheme leaves the adsorbate in an intermediate, essentially physisorbed state, where it is sufficiently mobile to make longer than nearest neighbor jumps, or from which it desorbs. (C) 1995 American Institute of Physics.