Department of Chemistry, University of Rochester, 120 Trustee Rd., Rochester, New York 14627, USA.
J Chem Phys. 2009 Dec 28;131(24):244512. doi: 10.1063/1.3276684.
We present a classical theoretical treatment of a two-dimensional Raman spectroscopy based on the initiation of vibrational coherence with an impulsive Raman pump and subsequent probing by two-pulse femtosecond stimulated Raman spectroscopy (FSRS). The classical model offers an intuitive picture of the molecular dynamics initiated by each laser pulse and the generation of the signal field traveling along the probe wave vector. Previous reports have assigned the observed FSRS signals to anharmonic coupling between the impulsively driven vibration and the higher-frequency vibration observed with FSRS. However, we show that the observed signals are not due to anharmonic coupling, which is shown to be a fifth-order coherent Raman process, but instead due to cascades of coherent Raman signals. Specifically, the observed vibrational sidebands are generated by parallel cascades in which a coherent anti-Stokes or Stokes Raman spectroscopy (i.e., CARS or CSRS) field generated by the coherent coupling of the impulsive pump and the Raman pump pulses participates in a third-order FSRS transition. Additional sequential cascades are discussed that will give rise to cascade artifacts at the fundamental FSRS frequencies. It is shown that the intended fifth-order FSRS signals, generated by an anharmonic coupling mechanism, will produce signals of approximately 10(-4) DeltaOD (change in the optical density). The cascading signals, however, will produce stimulated Raman signal of approximately 10(-2) DeltaOD, as has been observed experimentally. Experiments probing deuterochloroform find significant sidebands of the CCl(3) bend, which has an E type symmetry, shifted from the A(1) type C-D and C-Cl stretching modes, despite the fact that third-order anharmonic coupling between these modes is forbidden by symmetry. Experiments probing a 50:50 mixture of chloroform and d-chloroform find equivalent intensity signals of low-frequency CDCl(3) modes as sidebands shifted from both the C-D stretch of CDCl(3) and the C-H stretch of CHCl(3). Such intermolecular sidebands are allowed in the cascade mechanism, but are expected to be extremely small in the fifth-order frequency modulation mechanism. Each of these observations indicates that the observed signals are due to cascading third-order Raman signals.
我们提出了一种基于二维拉曼光谱的经典理论处理方法，该方法基于用脉冲拉曼泵启动振动相干性，然后用双脉冲飞秒受激拉曼光谱（FSRS）进行探测。该经典模型提供了一个直观的分子动力学图像，描述了每个激光脉冲引发的分子动力学以及沿着探测波矢传播的信号场的产生。先前的报告将观察到的 FSRS 信号归因于脉冲驱动振动与通过 FSRS 观察到的高频振动之间的非谐耦合。然而，我们表明，观察到的信号不是由于非谐耦合引起的，非谐耦合是一个五阶相干拉曼过程，而是由于相干拉曼信号的级联。具体来说，观察到的振动边带是通过平行级联产生的，其中由脉冲泵和拉曼泵脉冲的相干耦合产生的相干反斯托克斯或斯托克斯拉曼光谱（即 CARS 或 CSRS）场参与三阶 FSRS 跃迁。还讨论了额外的顺序级联，这将在基本 FSRS 频率处产生级联伪影。结果表明，由非谐耦合机制产生的预期的五阶 FSRS 信号将产生约 10(-4) DeltaOD（光密度的变化）的信号。然而，级联信号将产生约 10(-2) DeltaOD 的受激拉曼信号，这与实验观察到的情况一致。探测氘代氯仿的实验发现，尽管这些模式之间的三阶非谐耦合在对称性上是禁止的，但具有 E 型对称性的 CCl(3) 弯曲的 CCl(3) 弯曲的显著边带从 A(1) 型 C-D 和 C-Cl 伸缩模式偏移。探测氯仿和 d-氯仿的 50:50 混合物的实验发现，低频 CDCl(3) 模式的等效强度信号作为从 CDCl(3) 的 C-D 伸缩和 CHCl(3) 的 C-H 伸缩偏移的边带。这种分子间边带在级联机制中是允许的，但在五阶频率调制机制中预计非常小。这些观察结果中的每一个都表明，观察到的信号是由于级联三阶拉曼信号引起的。
