Infrared-induced coherent vibration of a hydrogen-bonded system: effects of mechanical and electrical anharmonic couplings.

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.