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Elementary time scale of the nuclear motions in molecules corresponds to the periods of molecular vibrations, which occur in 10~100 fs. Femtosecond light pulses can set off coherent vibrational wave packets, and one can observe the motions directly in time domain. When a molecule is excited impulsively by a short pulse of light, coherent vibrational wave packets are created, which are manifested in the oscillations of the time trace in various time-resolved spectroscopies. The nature of the vibrational modes excited and their decay provide a wealth of information on the dynamics and molecular structures of the states involved. The coherent nuclear wave packet can be launched by any nuclear rearrangements such as chemical reaction and internal conversion that occur faster than roughly half of the vibrational period. Excited state intramolecular proton transfer (ESIPT) is one such example. Time resolved fluorescence with high enough resolution allows the observation of the motions in the electronic excited state exclusively in time domain. In this presentation, I report the observation of the coherent wave packet motions using time-resolved spontaneous fluorescence for the molecules in the excited states created by various processes such as direct electronic excitation, chemical reactions, and internal conversion from Sn (n≥2) to S1. These observations give direct information on the structures of the excited states and the reaction coordinates by examining the modes excited. Possibilities of using femtosecond X-ray pulses for the reaction dynamics of these molecules in crystalline form will also be discussed. |
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