Recent developments in ultrafast laser technology have made it possible to generate a laser field (10¹² ~10 ¹⁵ W/cm² ) with the electronic field strength comparable to the Coulomb field within molecules, from a lab-scale sized laser system. When molecules are exposed to such an intense laser field, they exhibit a variety of new phenomena, which have never been observed in the perturbative regime of photon-molecule interaction. Since electrons within a molecule are strongly driven in intense laser fields, the shape of the internuclear potential energy surface (PES) is significantly modified to induce a significant change in the molecular dynamics. The formation of the “light-dressed” molecule due to the strong coupling with the laser field proposes a novel scheme of chemical reaction control through the manipulation of PES by laser pulse shaping.

In order to clarify the dynamics of a molecule in intense laser fields, we have developed new experimental techniques such as coincidence momentum imaging. Based on the momentum vectors of the fragment ions produced from the Coulomb explosion, we identified new features, such as, (i) the structural deformation of CO₂ and O₃ [1], (ii) the ultrafast hydrogen migration from the methyl group to the nitrile group in acetonitrile, and (iii) the control of structural deformation by steering the electrons in the molecular frame [2]. On the other hand, these discussions were merely based on the information carried by the products formed by the interaction with the laser fields, so that there remain several points to be clarified to understand how molecules behave in the light-dressed states.

　The aim of the present research project is to develop an experimental technique utilizing the holography of inner-shell photoelectrons for the real-time probing of the nuclear dynamics of molecules in intense laser fields (Fig.1). This new experimental technique, which directly captures the ultrafast changes in the molecular structure in the course of the reaction, should promote a deeper understanding of the interaction between molecules and laser fields, and provide the key to realizing of chemical reaction control with high efficiency.

Fig.1 ： Time-resolved photoelectron holography.
(a) Molecular dynamics induced by the interaction with laser fields
(b) Innercore ionization by a ultrashort soft X-ray pulse
(c) Hologram formation by the electron scattering by the surrounding atoms .

[1] A. Matsuda, E. J. Takahashi, and A. Hishikawa, J. Chem. Phys. 127, 114318 (2007).
[2] A. Hishikawa, E. J. Takahashi, and A. Matsuda, Phys. Rev. Lett. 97, 243002 (2006).