Abstract
Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10−15 s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicting ten or eleven atoms, including all the hydrogens, and thus overcome a so-far impregnable barrier for complete Coulomb explosion imaging—its limitation on molecules consisting of three to five atoms. In combination with state-of-the-art multi-coincidence techniques and elaborate theoretical modelling, this allows tracing ultrafast hydrogen emission and obtaining information on the result of intramolecular electron rearrangement. Our work represents an important step towards imaging femtosecond chemistry via Coulomb explosion.
Highlights
In contrast to gas-phase X-ray or electron diffraction[1,2,3,4,5,6,7], Coulomb explosion imaging (CEI) is an experimental technique that is sensitive to light and heavy atoms, providing information in momentum space, typically in a molecular frame of reference
We demonstrate that intense, femtosecond soft X-ray pulses from a high-repetition-rate free-electron laser, the European X-ray free-electron lasers (XFELs), make it possible to image a complex molecule in its entirety, as well as to obtain detailed information on the ultrafast intramolecular electron rearrangement without measuring all the fragments in coincidence
The experiment was carried out using a cold-target recoil ion momentum spectroscopy (COLTRIMS) reaction microscope[24] that is part of the Small Quantum Systems (SQS) scientific instrument at the European XFEL25. 2-Iodopyridine (C5H4IN) or 2-iodopyrazine (C4H3IN2) molecules were prepared in a dilute gas jet through supersonic expansion, such that each X-ray pulse typically interacted with only a single molecule
Summary
With the development of X-ray free-electron lasers (XFELs) offering pulse duration of a few femtoseconds, the achievable intensity has increased by orders of magnitude. Due to the high repetition rate of the European XFEL (here up to 570 X-ray pulses per second), we could record up to eightfold ion coincidences, which produce identical-looking images in this case (Extended Data Fig. 1). This reveals that for the studied planar molecules, the reduced set of information from partial coincidences captures the essence of the asymptotic momentum density distribution of the fully fragmented molecule. We note that the Newton plots for higher molecular charges provide clearly distinct carbon-ion distributions even without normalization, as shown in Fig. 3b for the I4+, N2+ and C2+ channel (see Extended Data Fig. 6 for other charge states). The trend in the charge-state distribution shows that these effects are overcompensated by the rapid rearrangement of electrons a
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