  Scanning tunneling spectroscopy requires the application of a potential
difference between the sample and a tip. In metal-vacuum-metal junctions, one
can safely assume that the potential is constant along the metallic substrate.
Here, we show that the inhomogeneous shape of the electric potential has to be
taken into account when probing spatially extended molecules on a decoupling
layer. To this end, oligothiophene-based molecules were deposited on a
monolayer of molybdenum disulfide (MoS$_2$) on a Au(111) surface. By probing
the delocalized molecular orbital along the thiophene-backbone, we found an
apparent intramolecular shift of the positive ion resonance, which can be
ascribed to a perturbation potential caused by the tip. Using a simple model
for the electrostatic landscape, we show that such a perturbation is caused by
the inhomogeneity of the applied bias potential in the junction and may be
further modified by an electric dipole of a functionalized tip. The two effects
can be disentangled in tunneling spectra by probing the apparent energy shift
of vibronic resonances along the molecular backbone. We suggest that extended
molecules on MoS$_2$ can be used as a sensor for the shape of the electrostatic
potential of arbitrary tips.
