Fischer Eric W, Saalfrank Peter
Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Potsdam-Golm, Germany.
J Chem Phys. 2021 Mar 14;154(10):104311. doi: 10.1063/5.0040853.
Recent experiments and theory suggest that ground state properties and reactivity of molecules can be modified when placed inside a nanoscale cavity, giving rise to strong coupling between vibrational modes and the quantized cavity field. This is commonly thought to be caused either by a cavity-distorted Born-Oppenheimer ground state potential or by the formation of light-matter hybrid states, vibrational polaritons. Here, we systematically study the effect of a cavity on ground state properties and infrared spectra of single molecules, considering vibration-cavity coupling strengths from zero up to the vibrational ultrastrong coupling regime. Using single-mode models for Li-H and O-H stretch modes and for the NH inversion mode, respectively, a single cavity mode in resonance with vibrational transitions is coupled to position-dependent molecular dipole functions. We address the influence of the cavity mode on polariton ground state energies, equilibrium bond lengths, dissociation energies, activation energies for isomerization, and on vibro-polaritonic infrared spectra. In agreement with earlier work, we observe all mentioned properties being strongly affected by the cavity, but only if the dipole self-energy contribution in the interaction Hamiltonian is neglected. When this term is included, these properties do not depend significantly on the coupling anymore. Vibro-polaritonic infrared spectra, in contrast, are always affected by the cavity mode due to the formation of excited vibrational polaritons. It is argued that the quantized nature of vibrational polaritons is key to not only interpreting molecular spectra in cavities but also understanding the experimentally observed modification of molecular reactivity in cavities.
近期的实验和理论表明，当分子置于纳米级腔体内时，其基态性质和反应活性会发生改变，从而导致振动模式与量子化腔场之间产生强耦合。通常认为这是由腔扭曲的玻恩-奥本海默基态势或光-物质混合态（振动极化激元）的形成所引起的。在此，我们系统地研究了腔体对单分子基态性质和红外光谱的影响，考虑了从零到振动超强耦合 regime 的振动-腔耦合强度。分别使用针对 Li-H 和 O-H 伸缩模式以及 NH 反转模式的单模模型，将与振动跃迁共振的单个腔模耦合到位置相关的分子偶极函数上。我们探讨了腔模对极化激元基态能量、平衡键长、解离能、异构化活化能以及振动极化激元红外光谱的影响。与早期工作一致，我们观察到所有上述性质都受到腔体的强烈影响，但前提是相互作用哈密顿量中的偶极自能贡献被忽略。当包含该项时，这些性质不再显著依赖于耦合。相比之下，由于激发振动极化激元的形成，振动极化激元红外光谱总是受到腔模的影响。有人认为，振动极化激元的量子化性质不仅是解释腔体内分子光谱的关键，也是理解实验中观察到的腔体内分子反应活性变化的关键。
