  The electronic structure of molecules on metal surfaces is largely determined
by hybridization and screening by the substrate electrons. As a result, the
energy levels are significantly broadened and molecular properties, such as
vibrations are hidden within the spectral lineshapes. Insertion of thin
decoupling layers reduces the linewidths and may give access to the resolution
of electronic and vibronic states of an almost isolated molecule. Here, we use
scanning tunneling microscopy and spectroscopy to show that a single layer of
MoS$_{2}$ on Ag(111) provides a semiconducting band gap that may prevent
molecular states from strong interactions with the metal substrate. We show
that the lowest unoccupied molecular orbital (LUMO) of
tetra-cyano-quino-dimethane (TCNQ) molecules is significantly narrower than on
the bare substrate and that it is accompanied by a characteristic satellite
structure. Employing simple calculations within the Franck-Condon model, we
reveal their vibronic origin and identify the modes with strong electron-phonon
coupling.
