  In single-molecule transistors, we observe inelastic cotunneling features
that correspond energetically to vibrational excitations of the molecule, as
determined by Raman and infrared spectroscopy. This is a form of inelastic
electron tunneling spectroscopy of single molecules, with the transistor
geometry allowing in-situ tuning of the electronic states via a gate electrode.
The vibrational features shift and change shape as the electronic levels are
tuned near resonance, indicating significant modification of the vibrational
states. When the molecule contains an unpaired electron, we also observe
vibrational satellite features around the Kondo resonance.
