ECAMP 15

Coulomb explosion imaging (CEI) of polyatomic molecules induced by intense, femtosecond pulses at the European X-ray free-electron laser (EuXFEL) has allowed several new insights. In recent years, CEI has resolved asymmetric deformation and bond-angle opening in fragmenting water molecules [Jahnke2021], reconstructed the three-dimensional geometry of a five-atom molecule using a charge buildup model [Li2021, Li2022], and imaged a complex eleven-atom molecule—including all hydrogen atoms—while tracing intramolecular electron transfer [Boll2022]. More recently, CEI has captured the time-resolved deplanarization reaction following UV excitation of a heterocycle [Jahnke2025].

The multidimensional nature of CEI allows selective investigation of complex structural dynamics. Here, we demonstrate how X-ray induced CEI can be used to trace the correlated dynamics of a molecular elimination reaction—a minor reaction pathway involving the cleavage of two bonds and the formation of a vibrationally excited di-halogen fragment [Li2025]. In parallel, we map the light-induced bending vibration of the bound molecular wave packet and disentangle competing dissociation channels.

As CEI advances toward more quantitative structural analysis, interpretation becomes increasingly complex. Previous studies have relied on molecule-specific data processing tailored to each system and science case. To establish CEI as a more general and broadly applicable method, systematic approaches for analyzing high-dimensional ion-coincidence data are needed, as some quantitative insights can only be obtained through comparison with advanced modelling. Recent efforts in this direction include the application of principal component analysis (PCA) to simulated CEI data [Richard2021]. Building on this, we have now applied PCA to experimental data for the first time, overcoming significant technical challenges. This analysis enabled the extraction of collective ground-state structural fluctuations (zero-point motion) in a complex molecule [Richard2025]. To our knowledge, this represents the first direct experimental observation of this fundamental quantum phenomenon in a system of comparable complexity. The zero-point motion manifests as correlated variations in ion momenta, establishing CEI as a powerful tool for accessing high-dimensional quantum dynamics.

References

[Boll2022] R. Boll et al., Nat. Phys. 18, 423 (2022)
[Jahnke2021] T. Jahnke et al., Phys. Rev. X 11, 041044 (2021)
[Jahnke2025] T. Jahnke et al., Nat. Commun. 16, 2074 (2025)
[Li2021] X. Li, R. Boll, D. Rolles and A. Rudenko, Phys. Rev. A 104, 033115 (2021)
[Li2022] X. Li et al., Phys. Rev. Res. 4, 013029 (2022)
[Li2025] X. Li et al., Imaging a Light-Induced Molecular Elimination Reaction with an X-ray Free-Electron Laser, under review (2025)
[Richard2021] B. Richard et al., Journal of Physics B 54, 194001 (2021)
[Richard2025] B. Richard et al., Imaging collective quantum fluctuations of the structure of a complex molecule, under review (2025)