  Near Edge X-ray Absorption, Valence and Core-level Photoemission and Density
Functional Theory calculations are used to study molecular levels of
tetracyano-2,3,5,6-tetrafluoroquinodimethane (F$_4$TCNQ) deposited on Ag(111)
and BiAg$_2$/Ag(111). The high electron affinity of F$_4$TCNQ triggers a large
static charge transfer from the substrate, and, more interestingly,
hybridization with the substrate leads to a radical change of symmetry, shape
and energy of frontier molecular orbitals. The Lowest Unoccupied Molecular
Orbital (LUMO) shifts below the Fermi energy, becoming the new Highest Occupied
Molecular Orbital ($n$-HOMO), whereas the $n$-LUMO is defined by a hybrid band
with mixed $\pi^*$ and $\sigma^*$ symmetries, localized at quinone rings and
cyano groups, respectively. The presence of Bi influences the way the molecule
contacts the substrate with the cyano group. The molecule/surface distance is
closer and the bond more extended over substrate atoms in F$_4$TCNQ/Ag(111),
whereas in F$_4$TCNQ/BiAg$_2$/Ag(111) the distance is larger and the contact
more localized on top of Bi. This does not significantly alter molecular
levels, but it causes the respective absence or presence of optical excitations
in F$_4$TCNQ core-level spectra.
