  Microscopic insight into interactions is key for understanding the properties
of heterogenous interfaces. We analyze local attraction and repulsion in
non-covalently bonded Xe-Cs+ aggregates on Cu(111). By combining two-photon
photoemission spectroscopy, scanning tunneling microscopy, and coupled cluster
calculations we explain the intricate impact Xe has on Cs+/Cu(111). We find
that attraction between Cs+ and Xe counterbalances the screened Coulomb
repulsion between Cs+ on Cu(111). Furthermore, we observe that the Cs 6s
electron is repelled from Cu(111) due to xenon's electron density. Together,
this yields a dual, i.e. attractive or repulsive, response of Xe depending on
the positive or negative charge of the respective counterparticle, which
emphasizes the importance of the Coulomb interaction in these systems.
