Similarly, for Fe(CO)5 under VSC (Fig. , bottom), we followed the dynamics of the [ωUP, ωMP] (red box in Fig. ) and [ωUP, ωLP] (blue box in Fig. ) peaks of the 2D IR spectrum (or the corresponding narrowband pump probe spectra; see SI Section 1.3 for details). Here we specifically focus on the dynamics involving pumping UP, to avoid complications of hot (i.e., highly excited) vibrational states when exciting LP modes . The interpretation of these peaks was discussed in our previous works . Basically, the polariton transitions at ωMP and ωLP overlap with the 1->2 transition of the a2" and e' modes, respectively (this assignment is further confirmed by spectral simulations (SI Section 2.6) and input-output theory (SI Section 4)). Upon exciting UP, when the waiting time is beyond the polariton lifetime, the [ωUP, ωMP] and [ωUP, ωLP] peaks correspond respectively to the excited state population of the a2" and e' modes. Therefore, the dynamics of these peaks report the energy transfer between the a2" and e' modes facilitated by pseudorotation and IVR. At a first glance, the dynamics under VSC are a bit faster than the one without (Fig. ).