  The electronic excitation of a conjugated molecule-insulator interface, as
exemplified by the adsorption of benzoic acid and its phenolic derivative on
NaCl(001) surface, is addressed by many-body Green's function methods. By
solving the two-particle Bethe-Salpeter equation on top of the $GW$
quasiparticle energies, it turns out that instead of the intramolecular
$\pi-\pi^\ast$ transition of the adsorbate, the lowest singlet excited state of
the adsorbate system, being a charge transfer excitonic effect, is essentially
assigned to the transition from the surface valence band maximum to the
$\pi-\pi^\ast$ state of the molecule. An accurate description of this lowest
electronic excitations confined at the interface requires the knowledge of a
full excitonic Hamiltonian due to the sizable electron-hole exchange
interaction.
