Phuc Nguyen Thanh, Trung Pham Quang, Ishizaki Akihito
Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, 444-8585, Japan.
Department of Structural Molecular Science, The Graduate University for Advanced Studies, Okazaki, 444-8585, Japan.
Sci Rep. 2020 Apr 30;10(1):7318. doi: 10.1038/s41598-020-62899-8.
Recent experiments showed that the chemical reaction rate is modified, either increased or decreased, by strongly coupling a nuclear vibration mode to the single mode of an optical cavity. Herein we investigate how the rate of an electron-transfer reaction depends on the molecule-cavity coupling in the ultrastrong coupling regime, where the coupling strength is comparable in magnitude with both the vibrational and the cavity frequencies. We found two main factors that determine the modification of the reaction rate: the relative shifts of the energy levels induced by the coupling and the mixing of the ground and excited states of molecular vibration in the ground state of the hybrid molecule-plus-cavity system through which the Franck-Condon factor between the initial and final states of the transition is altered. The former is the dominant factor if the molecule-cavity coupling strengths for the reactant and product states differ significantly from each other and gives rise to an increase in the reaction rate over a wide range of system's parameters. The latter dominates if the coupling strengths and energy levels of the reactant and product states are close to each other and it leads to a decrease in the reaction rate. The effect of the mixing of molecular vibrational states on the reaction rate is, however, suppressed in a system containing a large number of molecules due to the collective nature of the resulting polariton, and thus should be observed in a system containing a small number of molecules. In contrast, the effect of the relative shifts of the energy levels should be essentially independent of the number of molecules coupled to the cavity.
最近的实验表明，通过将核振动模式与光学腔的单模进行强耦合，化学反应速率会被改变，要么增加要么降低。在此，我们研究在超强耦合 regime 中，电子转移反应的速率如何依赖于分子 - 腔耦合，在该 regime 中，耦合强度在大小上与振动频率和腔频率相当。我们发现了决定反应速率改变的两个主要因素：耦合引起的能级相对位移，以及通过改变跃迁初末态之间的弗兰克 - 康登因子，混合分子加腔系统基态中分子振动的基态和激发态。如果反应物态和产物态的分子 - 腔耦合强度彼此显著不同，前者是主导因素，并在广泛的系统参数范围内导致反应速率增加。如果反应物态和产物态的耦合强度和能级彼此接近，后者占主导，导致反应速率降低。然而，由于所产生的极化激元的集体性质，分子振动态混合对反应速率的影响在包含大量分子的系统中受到抑制，因此应该在包含少量分子的系统中观察到。相比之下，能级相对位移的影响本质上应与耦合到腔的分子数量无关。
