A further pump-probe experiment: infra-red observation of N2 molecules ionized by ultra-short soft X-ray pulses

X-rays have been used as a tool for probing atomic-scale structures. Used in combination with pump by laser (usually from the infra red to visible light region), time-resolved X-ray (or soft X-ray) analysis is pushing back the frontiers in the world of materials. Recently, a further way of performing pump-probe experiments, i.e., an X-ray pump and infra-red probe, has been successfully applied to clarify the dynamics of N₂ molecules. A research team led by Professors H. Kapteyn and M. Murnane (University of Colorado, Boulder, USA) employed 43 eV soft X-rays with ca. 5 fs pulse width, which are laser-generated high-order harmonics, and an intense IR laser pulse (1.5 eV, 30 fs, 10¹³ W/cm²). The team found that substantial fragmentation occurs through an electron-shakeup process, in which a second electron is simultaneously excited during the soft X-ray photoionization process. During fragmentation, the molecular potential seen by the electron changes rapidly from nearly spherically symmetric to a two-center molecular potential. For more information, see the paper, " Soft X-ray-Driven Femtosecond Molecular Dynamics", E. Gagnon et al., Science, 317, 1374-1378 (2007).