  Chains of Na atoms and dicyanovinyl-quinquethiophene (DCV5T-Me2) molecules
with ionic bonds form a superlattice on Au(111). Through a detailed analysis of
the interchain distances obtained from scanning tunneling microscopy images at
various molecular coverages, we found that the chain arrangement substantially
deviates from a random distribution of noninteracting chains. Instead, the
distribution of chain spacings provides evidence for the existence of an
interchain long-range repulsive potential. Furthermore, the experimental
results can be modeled by a repulsive potential with a 1/d distance dependence,
characteristic of Coulomb interactions. Density functional theory calculations
of free-standing molecular chains reveal a charge depletion at the periphery of
the chains, which is attributed to the intrinsic polar property of the molecule
and is responsible for the long-range repulsion.
