Sub-5fs-nonlinear spectroscopy in molecular systems

Femtosecond pulses shorter than 10 fs have become easily available and enabled the study of real-time dynamics of various molecules. Especially, femtosecond wavepacket dynamics has been recently studied to realize the possibility of controlling molecular dynamics using tailored ultrashort pulses. Information about the dynamic coupling among vibrational modes in molecules in condensed phase is needed for the realization of such selective chemistry. Dynamic mode coupling is also important for the clarification of the mechanism of structural relaxation in the excited state of molecules. Ultrafast dynamics of conjugated polymers such as polydiacetylenes (PDA) has been attracting many scientists because they are model systems of quasi-one dimensional semiconductors with a strong electron-phonon coupling. From our extensive studies the initial kinetics after photo-excitation can be explained by a relaxation of a free exciton (FE) to a self-trapped exciton (STE) within 150 fs. The recent achievement of sub-5-fs visible pulse generation based on a novel noncollinear optical parametric amplification (NOPA) has enabled the real-time observation of the formation of a STE in a PDA for the first time.