Department of Chemistry, Graduate School of Science, Kyoto University Kitashirakawa, Sakyoku, Kyoto 606-8502, Japan.
Acc Chem Res. 2009 Sep 15;42(9):1270-9. doi: 10.1021/ar9000444.
Spectral line shapes in a condensed phase contain information from various dynamic processes that modulate the transition energy, such as microscopic dynamics, inter- and intramolecular couplings, and solvent dynamics. Because nonlinear response functions are sensitive to the complex dynamics of chemical processes, multidimensional vibrational spectroscopies can separate these processes. In multidimensional vibrational spectroscopy, the nonlinear response functions of a molecular dipole or polarizability are measured using ultrashort pulses to monitor inter- and intramolecular vibrational motions. Because a complex profile of such signals depends on the many dynamic and structural aspects of a molecular system, researchers would like to have a theoretical understanding of these phenomena. In this Account, we explore and describe the roles of different physical phenomena that arise from the peculiarities of the system-bath coupling in multidimensional spectra. We also present simple analytical expressions for a weakly coupled multimode Brownian system, which we use to analyze the results obtained by the experiments and simulations. To calculate the nonlinear optical response, researchers commonly use a particular form of a system Hamiltonian fit to the experimental results. The optical responses of molecular vibrational motions have been studied in either an oscillator model or a vibration energy state model. In principle, both models should give the same results as long as the energy states are chosen to be the eigenstates of the oscillator model. The energy state model can provide a simple description of nonlinear optical processes because the diagrammatic Liouville space theory that developed in the electronically resonant spectroscopies can easily handle three or four energy states involved in high-frequency vibrations. However, the energy state model breaks down if we include the thermal excitation and relaxation processes in the dynamics to put the system in a thermal equilibrium state. The roles of these excitation and relaxation processes are different and complicated compared with those in the resonant spectroscopy. Observing the effects of such thermal processes is more intuitive with the oscillator model because the bath modes, which cause the fluctuation and dissipation processes, are also described in the coordinate space. This coordinate space system-bath approach complements a realistic full molecular dynamics simulation approach. By comparing the calculated 2D spectra from the coordinate space model and the energy state model, we can examine the role of thermal processes and anharmonic mode-mode couplings in the energy state model. For this purpose, we employed the Brownian oscillator model with the nonlinear system-bath interaction. Using the hierarchy formalism, we could precisely calculate multidimensional spectra for a single and multimode anharmonic system for inter- and intramolecular vibrational modes.
凝聚相中的谱线形状包含了各种调制跃迁能量的动态过程的信息，如微观动力学、分子内和分子间的耦合以及溶剂动力学。由于非线性响应函数对化学过程的复杂动力学敏感，多维振动光谱可以将这些过程分离出来。在多维振动光谱学中，使用超短脉冲测量分子偶极子或极化率的非线性响应函数，以监测分子内和分子间的振动运动。由于这种信号的复杂轮廓取决于分子系统的许多动态和结构方面，因此研究人员希望对这些现象有一个理论上的理解。在本报告中，我们探讨并描述了在多维光谱中由于体系-浴耦合的特殊性而产生的不同物理现象的作用。我们还为弱耦合多模布朗体系提出了简单的解析表达式，并用其来分析实验和模拟得到的结果。为了计算非线性光学响应，研究人员通常使用适合实验结果的特定形式的体系哈密顿量。分子振动运动的光学响应已经在振子模型或振动能态模型中进行了研究。原则上，只要选择的能态是振子模型的本征态，这两个模型都应该得到相同的结果。能态模型可以提供对非线性光学过程的简单描述，因为在电子共振光谱学中发展的图式刘维尔空间理论可以很容易地处理涉及高频振动的三个或四个能态。然而，如果我们在动力学中包括热激发和弛豫过程，将系统置于热平衡状态，那么能态模型就会失效。与共振光谱相比，这些激发和弛豫过程的作用不同且复杂。用振子模型观察这些热过程的影响更直观，因为导致波动和耗散过程的浴模也在坐标空间中进行了描述。这种坐标空间体系-浴方法补充了现实的全分子动力学模拟方法。通过比较坐标空间模型和能态模型计算得到的 2D 谱，可以检查热过程和非谐模-模耦合在能态模型中的作用。为此，我们采用了具有非线性体系-浴相互作用的布朗振子模型。利用层次形式主义，我们可以精确地计算单模和多模非谐体系中分子内和分子间振动模式的多维光谱。
