  This work proposes a new photophysical phenomenon whereby UV/vis excitation
of a molecule involving a Franck-Condon (FC) active vibration yields infrared
(IR) emission by strong coupling to an optical cavity. The resulting
UV/vis-to-IR photonic down conversion process is mediated by vibrational
polaritons in the electronic excited state potential. It is shown that the
formation of such excited state vibrational polaritons (ESVP) via UV/vis
excitation only occurs with molecules having vibrational modes with both a
non-zero FC activity and IR activity in the excited state. Density functional
theory calculations are shown to effectively identify a candidate molecule,
1-Pyreneacetic acid (PAA), with this property and the dynamics of ESVP are
modeled using the truncated Wigner approximation. Overall, this work presents a
new avenue of polariton chemistry where excited state dynamics, driven by
photoexcitation, are influenced by the formation of vibrational polaritons.
Along with this, the photonic down conversion is potentially useful in both the
sensing of excited state vibrations and in quantum transduction schemes.
