  Under vibrational strong coupling (VSC), the formation of molecular
polaritons may significantly modify the photo-induced or thermal properties of
molecules. In an effort to understand these intriguing modifications, both
experimental and theoretical studies have focused on the ultrafast dynamics of
vibrational polaritons. Here, following our recent work [J. Chem. Phys., 154,
094124, (2021)], we systematically study the mechanism of polariton relaxation
for liquid CO2 under a weak external pumping. Classical cavity molecular
dynamics (CavMD) simulations show that polariton relaxation results from the
combined effects of (i) cavity loss through the photonic component and (ii)
dephasing of the bright-mode component to vibrational dark modes as mediated by
intermolecular interactions. The latter polaritonic dephasing rate is
proportional to the product of the weight of the bright mode in the polariton
wave function and the spectral overlap between the polariton and dark modes.
Both these factors are sensitive to parameters such as the Rabi splitting and
cavity mode detuning. Compared to a Fermi's golden rule calculation based on a
tight-binding harmonic model, CavMD yields a similar parameter dependence for
the upper polariton relaxation lifetime but sometimes a modest disagreement for
the lower polariton. We suggest that this disagreement results from
polariton-enhanced molecular nonlinear absorption due to molecular
anharmonicity, which is not included in our analytical model. We also summarize
recent progress on probing nonreactive VSC dynamics with CavMD.
