Department of Chemistry, University of Rochester, 120 Trustee Rd. Rochester, New York 14627, USA.
J Chem Phys. 2009 Dec 7;131(21):214502. doi: 10.1063/1.3263909.
A new methodology for two-dimensional Raman spectroscopy-termed two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS)-is presented and experimental results for acetonitrile are discussed. 2D-FSRS can potentially observe molecular anharmonicity by measuring the modulation of the frequency of a probed Raman mode, at frequency omega(hi), by the coherent motion of an impulsively driven mode, at frequency omega(low). In acetonitrile, the signal is generated by driving the CCN bend (379 cm(-1)) and CC stretch (920 cm(-1)) into coherence via impulsive stimulated Raman scattering and subsequently probing the stimulated Raman spectrum of the CC stretch, the CN stretch (2250 cm(-1)) and the CH stretch (2942 cm(-1)). The resultant signal can be generated by two alternative mechanisms: a fifth-order Raman process that would directly probe anharmonic coupling between the two modes, or a third-order cascade in which a third-order coherent Raman process produces a field that goes on to participate in a third-order stimulated Raman transition. The third-order cascade is shown to dominate the 2D-FSRS spectrum as determined by comparison with the predicted magnitude of the two signals, the 2D spectrum of a mixed isotope experiment, and the concentration dependence of the signal. In acetonitrile, theoretical calculations of the vibrational anharmonicity indicate that the third-order cascade signal should be 10(4) times larger than the fifth-order Raman signal. 2D-FSRS signals are observed between acetonitrile's CCN bend, of E symmetry, and several different A(1) modes but are forbidden by symmetry in the fifth-order pathway. A 2D-FSRS spectrum of a 50:50 mixture of acetonitrile and d(3)-acetonitrile shows equivalent intensity for intramolecular coupling peaks and intermolecular coupling peaks, indicating that the observed signal cannot be probing molecular anharmonicity. Finally, the magnitudes of the 2D-FSRS peaks are observed to be proportional to the square of the number density, supporting the cascade mechanism.
提出了一种用于二维拉曼光谱的新方法，称为二维飞秒受激拉曼光谱（2D-FSRS），并讨论了乙腈的实验结果。2D-FSRS 可以通过测量受激驱动模式在频率 omega(low)下的相干运动对探测拉曼模式频率 omega(hi)的调制来潜在地观察分子非谐性。在乙腈中，信号是通过通过脉冲受激拉曼散射驱动 CCN 弯曲（379 cm(-1)) 和 CC 伸展（920 cm(-1)) 进入相干，然后探测 CC 伸展的受激拉曼光谱，CN 伸展（2250 cm(-1)) 和 CH 伸展（2942 cm(-1))。所得信号可以通过两种替代机制产生：一种是直接探测两个模式之间非谐耦合的五阶拉曼过程，或者是三阶级联，其中三阶相干拉曼过程产生的场继续参与三阶受激拉曼跃迁。通过与两种信号的预测幅度、混合同位素实验的二维光谱以及信号的浓度依赖性进行比较，表明三阶级联占主导地位。在乙腈中，振动非谐性的理论计算表明，三阶级联信号应该比五阶拉曼信号大 10(4)倍。在乙腈的 CCN 弯曲（E 对称性）和几个不同的 A(1)模式之间观察到 2D-FSRS 信号，但在五阶途径中被对称性禁止。乙腈和 d(3)-乙腈的 50:50 混合物的 2D-FSRS 光谱显示分子内耦合峰和分子间耦合峰的强度相等，表明观察到的信号不能探测分子非谐性。最后，观察到 2D-FSRS 峰的幅度与数密度的平方成正比，支持级联机制。
