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. We used soft x-ray beams generated by high-harmonic upconversion of a femtosecond laser to photoionize a nitrogen molecule, creating highly excited molecular cations. A strong infrared pulse was then used to probe the ultrafast electronic and nuclear dynamics as the molecule exploded. We 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. 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 with the use of other techniques.
We study the response of graphene to high-intensity 10^11-10^12 Wcm^-2,
50-femtosecond laser pulse excitation. We establish that graphene has a fairly
high (~3\times10^12Wcm^-2) single-shot damage threshold. Above this threshold,
a single laser pulse cleanly ablates graphene, leaving microscopically defined
edges. Below this threshold, we observe laser-induced defect formation that
leads to degradation of the lattice over multiple exposures. We identify the
lattice modification processes through in-situ Raman microscopy. The effective
lifetime of CVD graphene under femtosecond near-IR irradiation and its
dependence on laser intensity is determined. These results also define the
limits of non-linear applications of graphene in femtosecond high-intensity
regime.Comment: 4 pages, 3 figure
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