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Attosecond X-Ray Core-Level Chronoscopy of Aromatic Molecules

Jia-Bao Ji1,*, Zhaoheng Guo2,3,4,*, Taran Driver2,3, Cynthia S. Trevisan5, David Cesar2, Xinxin Cheng2, Joseph Duris2, Paris L. Franz2,3, James Glownia2 et al.

Xiaochun Gong1,6, Daniel Hammerland1, Meng Han1,7, Saijoscha Heck1, Matthias Hoffmann2, Andrei Kamalov2, Kirk A. Larsen2,3, Xiang Li2, Ming-Fu Lin2, Yuchen Liu8,9, C. William McCurdy8,9, Razib Obaid2, Jordan T. O’Neal3, Thomas N. Rescigno8, River R. Robles2,3, Nicholas Sudar2,3, Peter Walter2, Anna L. Wang2,3, Jun Wang2,3, Thomas J. A. Wolf2,3, Zhen Zhang2, Kiyoshi Ueda1,10,11, Robert R. Lucchese8,†, Agostino Marinelli2,3,‡, James P. Cryan2,3,§, and Hans Jakob Wörner1,∥

  • *These authors contributed equally to this work.
  • †Contact author: rlucchese@lbl.gov
  • ‡Contact author: marinelli@slac.stanford.edu
  • §Contact author: jcryan@stanford.edu
  • ∥Contact author: hwoerner@ethz.ch
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Phys. Rev. X 15, 041031 – Published 14 November, 2025

DOI: https://doi.org/10.1103/dp5w-qxqc

Abstract

Attosecond photoemission or photoionization delays are a unique probe of the structure and the electronic dynamics of matter. However, the spectral congestion of valence photoelectron spectra sets fundamental limits to the complexity of systems that can be studied, and the delocalization of valence electron wave functions blurs the spatial origin of the photoelectron wave packet. Using attosecond x-ray pulses from LCLS, we demonstrate the key advantages of measuring core-level delays: The photoelectron spectra remain atomlike, the measurements become element specific, and the observed scattering dynamics originate from a pointlike source when multicenter interference effects are negligible. We exploit these unique features to reveal the effects of changing functional groups (C-H vs N) and symmetry on attosecond scattering dynamics by measuring and calculating the photoionization delays between N−1s and C−1s core shells of a series of aromatic azabenzene molecules. Remarkably, the delays increase with the number of nitrogen atoms in the molecule and reveal multiple resonances. We identify two previously unknown mechanisms regulating the associated attosecond dynamics, namely the enhanced confinement of the trapped wave function with the replacement of C-H groups by N atoms and the decrease of the coupling strength among the photoemitted partial waves with increasing symmetry. This study demonstrates the unique opportunities opened by measurements of core-level photoionization delays for unraveling attosecond electron dynamics in complex matter.

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Popular Summary

Electrons in matter move on attosecond timescales—billionths of a billionth of a second—making their motion extremely difficult to capture, especially in large molecules. Tracking this motion is vital for understanding how electrons drive chemical and physical processes. Earlier experiments have used ultraviolet attosecond pulses to eject outer, or valence, electrons and measure when they escape. But in complex molecules, these signals overlap and become ambiguous, making it unclear which atom an electron came from. In this study, we overcome these challenges by directly measuring element-specific electron dynamics in complex molecules using attosecond x-ray pulses.

We achieve this by targeting core-level electrons—inner electrons bound tightly to specific atoms—that provide clean, element-specific signatures: Carbon and nitrogen atoms each produce distinct signals with no overlap. Using an “attosecond streaking” method that employs a circularly polarized infrared pulse as a stopwatch, we precisely measure the timing difference between electrons ejected from nitrogen and carbon atoms within the same molecule.

We find that electrons from nitrogen escape more slowly than those from carbon, and that this delay increases as nitrogen atoms replace carbon in a series of related molecules. The data also reveal signatures characteristic of transient trapping. Our analysis shows two main reasons for this slowdown: Nitrogen atoms deepen the potential well that holds the electrons, and symmetric arrangements of nitrogen restrict available escape paths.

Together, these results demonstrate the power of attosecond x-ray techniques for observing site-specific electron motion and open the door to studies of even more complex molecular systems.

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Phys. Rev. X 15, 041031– Published 14 November, 2025
  • Received 26 May 2025
  • Accepted 24 October 2025
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