  Using the break junction (BJ) technique we show that Au(RS)2 units play a
significant role in thiol-terminated molecular junctions formed on gold. We
have studied a range of thiol-terminated compounds, either with the sulfur
atoms in direct conjugation with a phenyl core, or bonded to saturated
methylene groups. For all molecules we observe at least two distinct groups of
conductance plateaus. By a careful analysis of the length behavior of these
plateaus, comparing the behavior across the different cores and with methyl
sulfide anchor groups, we demonstrate that the lower conductance groups
correspond to the incorporation of Au(RS)2 oligomeric units at the contacts.
These structural motifs are found on the surface of gold nanoparticles but they
have not before been shown to exist in molecular-break junctions. The results,
while exemplifying the complex nature of thiol chemistry on gold, moreover
clarify the conductance of 1,4-benzenedithiol on gold. We show that true
Au-S-Ph-S-Au junctions have a relatively narrow conductance distribution.
