  The formation of ordered phases of dimethyl-disulfide on the Au(111) surface
has been investigated by means of Low-Energy Electron Diffraction (LEED), X-ray
Photoemission Spectroscopy (XPS), and state-of-the-art Density Functional
Theory (DFT) periodic supercell calculations. The LEED diffraction pattern,
obtained after a production method that includes a two-step dosing and
prolonged post-deposition annealing, unambiguously corresponds to a novel phase
that consists of (3x4) domains coexisting with the as-deposited (sqrt3xsqrt3
)R30 structure. XPS measurements indicate that the coverage of the new (3x4)
superstructure is the same as that of the (sqrt3xsqrt3)R30 phase. In both
phases, the binding energy of the S 2p3/2 core-level peak is found to be 162.2
eV, corresponding to the formation of a thiolate layer. The DFT calculations
allow us to identify a viable metastable (3x4) structure where the S headgroups
of the CH3S radicals select distinct adsorption sites: three quarters of them
adsorb at bridge sites and one quarter at top sites. The relative energetics of
the (3x4) and (sqrt3xsqrt3)R30 configurations suggest that the two structures
may coexist on the surface, in agreement with experimental data.
