The VSC condition was achieved by placing a solution of Fe(CO)5 in n-dodecane into a Fabry-Pérot microcavity. Unless specifically mentioned, we set the Fe(CO)5 concentration to ~180 mM and the cavity longitudinal thickness to ~12.5 μm. The e ' and a2 " vibrational modes of Fe(CO)5 strongly couple to a 5 th -order cavity mode. The IR spectrum (Fig. ) shows the transitions of upper, middle, and lower polaritons (UP, MP and LP) at ωUP = 2045, ωMP = 2014, and ωLP = 1976 cm -1 , respectively. By fitting to a coupled oscillator model (see SI Section 2.1), we determined that the cavity mode is 2013 cm -1 and interacts with the e' and a2" modes with amplitudes gcav-e' = 26 cm -1 and gcav-a2" = 19 cm -1 , respectively. Because the full width at half maximum (FWHM) of the e' and a2" modes are 8 cm -1 and 5 cm -1 , respectively, and that of the cavity mode is 11 cm -1 , the samples satisfy the criteria for VSC . (bottom panel) for Fe(CO)5 outside the cavity upon pumping of the a2" modes (red dots) and inside the cavity upon pumping of UP (blue dots) and the a2" dark modes (gray dots). The black dashed, dotted and solid lines are the corresponding fits. Energy is exchanged at a faster rate when pumping UP, whereas pumping the a2" dark modes leads to a similar rate to the one outside the cavity.