One of the frontiers in modern chemical science is to observe nuclear rearrangements during a chemical reaction in real time and unveil structure-function interplay underlying the sophisticated functions of complex molecular systems. In this quest, various time-resolved techniques have been developed in the last decades. Nevertheless, it has not yet been trivial to track structural changes of the molecules proceeding on the time scale of the nuclear motion, i.e., femto-to-picosecond time scale. Recently, we developed femtosecond time-resolved time-domain Raman spectroscopy using <7-fs pulses, which allows us to track structural changes of the molecules on the femtosecond time scale with exquisite sensitivity. With this technique, we realized real-time observation of the ultrafast structural dynamics in the primary photochemical/photophysical processes in condensed-phase complex molecular systems. In this article, we overview the principle and a brief history of time-domain Raman spectroscopy and then describe the apparatus and recent applications to the femtosecond dynamics of molecules as complex as photoreceptor proteins and molecular assemblies.
