  Molecular vibrational spectroscopy with the scanning tunneling microscope is
feasible but usually detects few vibrational modes. We harness sharp
Yu-Shiba-Rusinov (YSR) states observed from molecules on a superconductor to
significantly enhance the vibrational signal. From a lead phthalocyanin
molecule 46 vibrational peaks are resolved enabling a comparison with
calculated modes. The energy resolution is improved beyond the thermal
broadening limit and shifts induced by neighbor molecules or the position of
the microscope tip are determined. Vice versa, spectra of vibrational modes are
used to measure the effect of an electrical field on the energy of YSR states.
The method may help to further probe the interaction of molecules with their
environment and to better understand selection rules for vibrational
excitations.
