  The detection of vibrational excitations of individual molecules on surfaces
by scanning tunneling spectroscopy does not obey strict selection rules but
rather propensity rules. The experimental verification of these is challenging
because it requires the independent variation of specific parameters, such as
the electronic structure, while keeping the vibrational modes the same. Here,
we make use of the versatile self-assembled structures of
Fe-tetra-pyridyl-porphyrin molecules on a Au(111) surface. These exhibit
different energy-level alignments of the frontier molecular orbitals, thus
allowing the correlation of electronic structure and detection of vibrations.
We identify up to seven vibrational modes in the tunneling spectra of the
molecules in some of the arrangements, whereas we observe none in other
structures. We find that the presence of vibrational excitations and their
distribution along the molecule correlates with the observation of
energetically low-lying molecular states. This correlation allows to explain
the different numbers of vibrational signatures for molecules embedded within
different structures as well as the bias asymmetry of the vibrational
intensities within an individual molecule. Our observations are in agreement
with a resonant enhancement of vibrations by the virtual excitation of
electronic states.
