Speaker
Description
A central tenet of attochemistry is charge-directed reactivity: ionization launches a valence hole whose motion and eventual localization across a molecule can predetermine the ensuing dynamics [1]. Yet, despite successes in capturing attosecond electron dynamics, their causal link to reaction dynamics, particularly in liquid solutions, has remained experimentally elusive. Concentrated aqueous urea is an ideal model for this link, as it assembles into hydrogen-bonded urea dimers where valence ionization selectively drives an ultrafast proton transfer within the dimer, while the surrounding water remains a spectator. Table-top carbon K-edge transient absorption spectroscopy has revealed the femtosecond proton transfer dynamics [2], but the attosecond-to-few-femtosecond electronic precursors preceding it have remained unveiled.
Here we employ all-X-ray attosecond transient absorption spectroscopy (AX-ATAS) to capture the electron dynamics within the urea dimer directly. Using the ω/2ω XLEAP setup at LCLS, an ω pulse valence-ionizes the sample while a 2ω attosecond pulse probes the oxygen K-edge valence hole [3]. We also integrated a covariance-based single-shot spectral reconstruction [4] to make an improved spectral resolution as benchmarked in pure liquid water.
In ionized urea solution, we observed a distinct transient absorption feature near 526 eV, clearly separated from the nearby water valence-hole response and from the two previously characterized urea bands. It rises within the first few femtoseconds and evolves differently from the known urea peaks near the absorption edge. This is consistent with the 1s to HOMO transition of oxygen predicted by QM/MM simulations to track the structurally driven oscillation of the valence hole between the two urea moieties, the fingerprint anticipated for charge-directed reactivity. To our knowledge, this offers the first experimental signature of the ultrafast valence-hole dynamics that direct proton transfer in a hydrogen-bonded liquid, connecting electron precursors to the proton transfer observed previously at the carbon K-edge [2] and in liquid water [3].
References
[1] F. Remacle et al., Chem. Phys. Lett. 285, 25 (1998)
[2] Z. Yin et al., Nature 619, 749 (2023)
[3] S. Li et al., Science 383, 1118 (2024)
[4] T. Driver et al., Phys. Chem. Chem. Phys. 22 (5), 2704-2712 (2020)
| Scientific Topics | Chemistry |
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