Dr. Oksana Travnikova (CNRS, Sorbonne Université, France)

Understanding how molecules respond to radiation requires following the coupled motion of electrons and nuclei on their natural timescales – timescales where chemistry and physics merge. In the ultraviolet and optical regimes, this is typically achieved using pump-probe schemes that track changes in valence electronic structure after excitation [1]. As the photon energy increases toward the soft X-ray range, excitation becomes increasingly localized, providing atom-specific access to molecular dynamics through core-level spectroscopy.

Soft X-ray absorption creates inner-shell core holes with lifetimes of only a few femtoseconds (1 fs = 10-15 s) – already long enough for light nuclei to move [2-3]. The molecular response is then governed by a competition between electronic relaxation via Auger decay and nuclear motion triggered by the localized excitation. In extreme cases, this leads to ultrafast dissociation, where molecular fragmentation occurs during the lifetime of the inner-shell vacancy. Resonant Auger spectroscopy offers a sensitive probe of this regime: the kinetic energy of the emitted electrons reflects both the evolving electronic structure and the nuclear motion unfolding during the core-hole lifetime, which itself acts as an intrinsic ultrafast clock.

Coincidence techniques provide a more complete view of the system dynamics [3-4]. By detecting Auger  electrons in coincidence with ions, one can reconstruct fragmentation pathways, distinguish between early and late electronic decay and give direct access into nuclear dynamics through momentum correlations. At the FinEstBeAMS beamline at MAX IV, the GPES coincidence setup enables vibrationally resolved measurements [3]. Another COLTRIMS experiments at BESSY II have revealed ultrafast dissociation mechanisms, in which light molecular groups are ejected in a catapult-like manner while heavier fragments remain nearly stationary [4].

 

1. O. Travnikova et al. J. Am. Chem. Soc. 144, 21878 (2022)

2. O. Travnikova et al. J. Phys. Chem. Lett. 4 2361 (2013) and references therein

3. O. Travnikova et al. Phys. Chem. Chem. Phys., 24, 5842 (2022) & 25, 1063 (2023)

4. O. Travnikova et al. J. Phys. Chem. Lett. 15 11883 (2024)

5. O. Travnikova et al. Phys. Rev. Lett. 116, 213001 (2016); 118, 213001 (2017); 134, 063003 (2025)

6. O. Travnikova et al. Phys. Rev. A (2026) DOI: 10.1103/6bdq-fwrd

7. E. Kukk … O. Travnikova J. Synchrotron Rad. 32, 1017 (2025)