For the cavity-free dynamics, the population of the S 1 state is shown for reference in Fig. . In this case, the dynamics is purely nonadiabatic without photon-emission, thus radiative decay is not permitted. However, the nonradiative decay appears starting at t = 800 a.t.u., when the photoexcited wavepacket approaches the avoided crossing at R = 4 a 0 , where nonadiabaticity is the strongest. At around t = 1050 a.t.u., the S 1 population increases again, since the wavepacket is being reflected by the right wall of the potential and goes back towards the avoided crossing. Figure reports the time trace of the S 1 population for the cavity-free dynamics calculated via QD (black line) as well as the trajectorybased results that will be discussed below. In the strong-coupling regime, which will be investigated further below, the coupling between the system and the cavity is strong since the beginning of the simulated dynamics, and leads to an immediate photon-emission event: the S 1 population decreases while the number of photons increases. Shortly after t = 400 a.t.u., the average number of photons stabilizes at approximately 0.15. At around t = 1000 a.t.u., the S 1 population decreases again since the S 1 wavepacket decays nonradiatively via the avoided crossing.