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1. Collective Vibrational Strong Coupling Effects on Molecular Vibrational Relaxation and Energy Transfer: Numerical Insights via Cavity Molecular Dynamics Simulations**

   https://doi.org/10.1002/anie.202103920  

   Li, Tao E. ; Nitzan, Abraham ; Subotnik, Joseph E. ( July 2021 , Angewandte Chemie International Edition)

   For a small fraction of hot CO₂molecules immersed in a liquid‐phase CO₂thermal bath, classical cavity molecular dynamics simulations show that forming collective vibrational strong coupling (VSC) between the C=O asymmetric stretch of CO₂molecules and a cavity mode accelerates hot‐molecule relaxation. This acceleration stems from the fact that polaritons can be transiently excited during the nonequilibrium process, which facilitates intermolecular vibrational energy transfer. The VSC effects on these rates 1) resonantly depend on the cavity mode detuning, 2) cooperatively depend on Rabi splitting, and 3) collectively scale with the number of hot molecules. For larger cavity volumes, the average VSC effect per molecule can remain meaningful for up toN≈10⁴molecules forming VSC. Moreover, the transiently excited lower polariton prefers to relax by transferring its energy to the tail of the molecular energy distribution rather than distributing it equally to all thermal molecules. As far as the parameter dependence is concerned, the vibrational relaxation data presented here appear analogous to VSC catalysis in Fabry–Pérot microcavities.

    

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2. Collective Vibrational Strong Coupling Effects on Molecular Vibrational Relaxation and Energy Transfer: Numerical Insights via Cavity Molecular Dynamics Simulations**

   https://doi.org/10.1002/ange.202103920  

   Li, Tao E. ; Nitzan, Abraham ; Subotnik, Joseph E. ( July 2021 , Angewandte Chemie)

   For a small fraction of hot CO₂molecules immersed in a liquid‐phase CO₂thermal bath, classical cavity molecular dynamics simulations show that forming collective vibrational strong coupling (VSC) between the C=O asymmetric stretch of CO₂molecules and a cavity mode accelerates hot‐molecule relaxation. This acceleration stems from the fact that polaritons can be transiently excited during the nonequilibrium process, which facilitates intermolecular vibrational energy transfer. The VSC effects on these rates 1) resonantly depend on the cavity mode detuning, 2) cooperatively depend on Rabi splitting, and 3) collectively scale with the number of hot molecules. For larger cavity volumes, the average VSC effect per molecule can remain meaningful for up toN≈10⁴molecules forming VSC. Moreover, the transiently excited lower polariton prefers to relax by transferring its energy to the tail of the molecular energy distribution rather than distributing it equally to all thermal molecules. As far as the parameter dependence is concerned, the vibrational relaxation data presented here appear analogous to VSC catalysis in Fabry–Pérot microcavities.

    

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3. Polariton relaxation under vibrational strong coupling: Comparing cavity molecular dynamics simulations against Fermi’s golden rule rate

   https://doi.org/10.1063/5.0079784  

   Li, Tao E. ; Nitzan, Abraham ; Subotnik, Joseph E. ( April 2022 , The Journal of Chemical Physics)

   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 [Li et al., J. Chem. Phys. 154, 094124 (2021)], we systematically study the mechanism of polariton relaxation for liquid CO 2 under a weak external pumping. Classical cavity molecular dynamics (CavMD) simulations confirm 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. 

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4. Cavity molecular dynamics simulations of liquid water under vibrational ultrastrong coupling

   https://doi.org/10.1073/pnas.2009272117  

   Li, Tao E. ; Subotnik, Joseph E. ; Nitzan, Abraham ( July 2020 , Proceedings of the National Academy of Sciences)

   We simulate vibrational strong coupling (VSC) and vibrational ultrastrong coupling (V-USC) for liquid water with classical molecular dynamics simulations. When the cavity modes are resonantly coupled to the O−H stretch mode of liquid water, the infrared spectrum shows asymmetric Rabi splitting. The lower polariton (LP) may be suppressed or enhanced relative to the upper polariton (UP) depending on the frequency of the cavity mode. Moreover, although the static properties and the translational diffusion of water are not changed under VSC or V-USC, we do find the modification of the orientational autocorrelation function of H₂O molecules especially under V-USC, which could play a role in ground-state chemistry.

    

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5. Cavity frequency-dependent theory for vibrational polariton chemistry

   https://doi.org/10.1038/s41467-021-21610-9  

   Li, Xinyang ; Mandal, Arkajit ; Huo, Pengfei ( February 2021 , Nature Communications)

   Recent experiments demonstrate the control of chemical reactivities by coupling molecules inside an optical microcavity. In contrast, transition state theory predicts no change of the reaction barrier height during this process. Here, we present a theoretical explanation of the cavity modification of the ground state reactivity in the vibrational strong coupling (VSC) regime in polariton chemistry. Our theoretical results suggest that the VSC kinetics modification is originated from the non-Markovian dynamics of the cavity radiation mode that couples to the molecule, leading to the dynamical caging effect of the reaction coordinate and the suppression of reaction rate constant for a specific range of photon frequency close to the barrier frequency. We use a simple analytical non-Markovian rate theory to describe a single molecular system coupled to a cavity mode. We demonstrate the accuracy of the rate theory by performing direct numerical calculations of the transmission coefficients with the same model of the molecule-cavity hybrid system. Our simulations and analytical theory provide a plausible explanation of the photon frequency dependent modification of the chemical reactivities in the VSC polariton chemistry.

    

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