If gas electron diffraction measurements are performed with a pulsed mode, we may be able probe the variations of geometrical structure of molecules in the gas phase in real time. However, the temporal resolution that could be achieved by the pulsed gas electron diffraction method in which a pulsed electron beam is used could not be shorter than 1 ps. This 1 ps barrier could be broken by introducing a method called laser assisted electron scattering/diffraction [1,2], in which electrons are scattered by atoms or molecules under the presence of an ultrashort intense laser field. By the laser assisted diffraction method, we have shown that instantaneous geometrical structure of polyatomic molecules can be determined with femtosecond temporal resolution . It is also interesting to probe ultrafast nuclear dynamics of molecules interacting with an intense laser field. From the series of our recent studies [3,4], it has been revealed that ultrafast hydrogen migration proceeds when hydrocarbon molecules are exposed to an intense laser field. We have recently investigated such ultrafast dynamical processes of hydrocarbon molecules by the pump-probe coincidence momentum imaging method and showed that a hydrogen atom in highly excited hydrocarbon molecules exhibits rich dynamics on a multi-dimensional potential energy surfaces .
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 T. Ando, A. Shimamoto, S. Miura, K. Nakai, H. Xu, A. Iwasaki, K. Yamanouchi,
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