Time-Domain Raman Spectroscopy and Its Application to Ultrafast Photochemical/Photobiological Reactions
Since its discovery 90 years ago, Raman spectroscopy has been developing continuously, and it is now one of the most important spectroscopies which is extensively utilized in various fields of science and technology. In traditional Raman spectroscopy, the energetically-shifted inelastic light scattering (Raman scattering) is measured, and the energy shift from the excitation light provides information about the vibrational energy of the molecules. On the other hand, using an ultrashort optical pulse that has a duration shorter than the vibrational period of molecules, we can carry out time-domain Raman spectroscopy in which we induce coherent nuclear motion of the molecule with the impulsive stimulated Raman process and observe Raman-active vibrations directly in the time domain. In principle, the information obtainable with time-domain Raman spectroscopy is equivalent to that obtained by ordinary frequency-domain Raman spectroscopy. However, because time-domain Raman spectroscopy is performed with only femtosecond pulses, we can trace the temporal change of the molecular vibrations with a femtosecond accuracy by combining it with a femtosecond pump pulse that starts chemical reactions [1-3]. In this lecture, I talk about the recent progress of our research about femtosecond time-domain Raman spectroscopy. A newly developed apparatus using 7-fs optical pulses allowed us to investigate the ultrafast dynamics of complex molecular systems such as the chromophore isomerization in photoreceptor proteins and the chemical bond formation process in molecular assemblies [4, 5, 6]. We also showed the possibility of multi-dimensional time-domain Raman spectroscopy that reveals the anharmonicity of reactive excited-state potential energy surfaces of complex molecules .
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4. T. Fujisawa, H. Kuramochi, H. Hosoi, S. Takeuchi and T. Tahara, J. Am. Chem. Soc. 138, 3942 (2016).
5. H. Kuramochi, S. Takeuchi, K. Yonezawa, H. Kamikubo, M. Kataoka and T. Tahara, Nat. Chem. 9, 660 (2017).
6. H. Kuramochi, S. Takeuchi, M. Iwamura, K. Nozaki, T. Tahara, J. Am. Chem. Soc. in press (2019).
7. H. Kuramochi, S. Takeuchi and T. Tahara, Sci. Adv. 5, eaau4490 (2019).