Benzene adsorption on Si(lll)-7X7 is studied with scanning tunneling m
icroscopy. Benzene diffusion is found to be inhibited. Ordinarily surf
ace diffusion is controlled by a substantially lower energy of activat
ion than is desorption. In this case diffusion is frustrated by a barr
ier to diffusion that is comparable to that for desorption. Both desor
ption 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, implyin
g adsorbate crowding causes the strength of the surface-adsorbate bond
to weaken. Diffusion is generally found to involve jumps to sites bey
ond nearest neighbors. It emerges that the adsorbate largely breaks it
s 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 physiso
rbed state, where it is sufficiently mobile to make longer than neares
t neighbor jumps, or from which it desorbs. (C) 1995 American Institut
e of Physics.