To quantify the energy exchange dynamics upon pumping UP, we used the kinetic model shown in Fig. . First, the population of UP relaxes to both the a2" and e' dark modes within the polariton lifetime. Then, the a2" and e' dark modes exchange energy, through pseudorotation and IVR, and at the same time dissipate energy to their environment. The solution of this kinetic model provides a good fit to the [ωUP, ωMP] and [ωUP, ωLP] dynamics (Fig. ). Furthermore, the measured dynamics can be separated into three components: polariton relaxation to dark modes at short times (cavity leakage is implicitly accounted for; see SI Section 2.3.2), energy exchange at intermediate times, and vibrational decay at long times. From the fitted results, kex under VSC is 0.113 ± 0.009 ps -1 at room temperature, 30% faster than that outside the cavity. Using 2D IR and the same analysis, we found when exciting the a2" dark reservoir modes directly, the energy exchange dynamics have similar trends to those outside the cavity (Fig. ), and kex is 0.090 ± 0.006 ps -1 . Clearly, VSC is only modifying the dynamics when the polaritons are pumped, whereas pumping the a2" dark reservoir modes causes the system to evolve similarly to the molecules outside the cavity, agreeing with the reservoir's purely molecular character. Similar findings have been predicted by a recent theoretical work . The contrast of dynamics between pumping UP and dark a2" modes suggests that the energy exchange rates depend on the initial populated states.