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Even simple molecules have complex dissociation pathways when exposed to ionizing radiation. What processes are important, and on what timescales? In recent work published in Science, we performed the first experiment to directly observe the dynamic interaction between light and ionizing radiation - or radiation femtochemistry. The direct observation of molecular dynamics initiated by x-rays has been hindered to date by the lack of bright femtosecond sources of short wavelength light. In our experiment, we used soft-x-ray beams generated by high harmonic upconversion of a femtosecond laser to photoionize an N2 molecule, creating highly-excited N2+ ions. A strong infrared pulse was then used to probe the ultrafast electronic and nuclear dynamics as the molecule explodes. We found that significant 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. Our approach can capture in real time and with angstrom resolution the influence of ionizing radiation on a range of molecular systems, probing dynamics that are inaccessible using other techniques. Such dynamics cannot be understood from spectral measurements alone, and are relevant to radiation chemistry and atmospheric science.
Research Highlights: Photoionization. Probing break ups.
Nature Photonics, Vol. 1, pp 609 (Nov. 2007).
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