This work is the first report on nonzero molecular vibration–vibration coupling in an infrared cavity–vibration experiment. Vibration–vibration coupling strength is determined as a cavity mode of parallel spaced mirrors (etalon mode or fringe) is angle-tuned in the region between two vibrations of liquid acetonitrile which are Fermi coupled, namely, a CN stretch dominated vibration and a nearby combination band dominated by the symmetric CH3 bend and C–C stretch. All other infrared cavity–vibration work to date involving more than one vibration has used a value of zero for vibration–vibration coupling; however, this work starts with Fermi coupled vibrations and reveals that there are changes in the vibration–vibration coupling and cavity–vibration couplings as the cavity mode is angle-tuned between the interacting vibrations. The ability to change fundamental vibrational dynamics within a cavity is an exciting result which helps to build a foundation for understanding molecular vibrational dynamics in parallel plate etalon cavities.

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