Ishii Kunihiko, Takeuchi Satoshi, Tahara Tahei
Molecular Spectroscopy Laboratory, Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan.
J Chem Phys. 2009 Jul 28;131(4):044512. doi: 10.1063/1.3181777.
We have studied IR-induced low-frequency coherent vibration of an intramolecularly hydrogen-bonded molecule, quinizarin, by an ultrashort IR-pump-visible-probe spectroscopy with approximately 60 fs time resolution. In this experiment, the IR excitation of the symmetric OH-stretching mode induced a low-frequency vibrational coherence, which was then detected as an oscillation of the visible absorption intensity. The observed oscillation was assigned to a "hydrogen-bond modulating" vibration by the vibrational analysis based on the density functional theory (DFT). Because the vibrational coherence formation by IR excitation requires a substantial anharmonic coupling, we carried out a DFT-based numerical analysis of the anharmonic coupling between the OH-stretching and the low-frequency mode, by evaluating the transition moment of the combination band. We took account of two types of anharmonicities, i.e., the mechanical anharmonicity and the electrical anharmonicity. Although the electrical anharmonicity is often neglected, it was found that the electrical anharmonicity had a comparable contribution to the mechanical anharmonicity, in generation of vibrational coherence of the low-frequency mode in this system. This result indicates general importance of the electrical anharmonicity in strongly hydrogen-bonded systems.
我们通过具有约60飞秒时间分辨率的超短红外泵浦-可见探测光谱，研究了分子内氢键分子醌茜因的红外诱导低频相干振动。在该实验中，对称OH伸缩模式的红外激发诱导了低频振动相干，然后将其检测为可见吸收强度的振荡。基于密度泛函理论（DFT）的振动分析将观察到的振荡归因于“氢键调制”振动。由于红外激发形成振动相干需要大量非谐耦合，我们通过评估组合带的跃迁矩，对OH伸缩与低频模式之间的非谐耦合进行了基于DFT的数值分析。我们考虑了两种类型的非谐性，即机械非谐性和电非谐性。尽管电非谐性常常被忽略，但发现在该系统中低频模式的振动相干产生过程中，电非谐性对机械非谐性有相当的贡献。这一结果表明电非谐性在强氢键系统中具有普遍重要性。
