Coulomb explosion imaging (CEI) is a powerful experimental technique that maps a molecule's geometric structure onto the momenta of ionic molecular fragments produced by rapid multiple ionization. Here, we apply CEI induced by pulses from an X-ray free-electron laser in order to image and distinguish complex hydrocarbon isomers with the chemical formula C7H8: toluene, cycloheptatriene, and 1,6-heptadiyne. The measured fragment-ion momentum distributions show discernible differences between the three isomers and provide signatures of specific carbon and hydrogen sites in the molecule. In contrast to previous work, we demonstrate that distinct 'marker atoms' are not strictly required for constructing a meaningful molecular frame of reference for the interpretation of the momentum-space data. Our work paves the way for tracking the ultrafast motion of nuclei during isomerization reactions in pure hydrocarbons.
@article{arxiv.2602.04149,
title = {Probing the structure of cyclic hydrocarbon molecules with X-ray-induced Coulomb explosion imaging},
author = {Kurtis D. Borne and Rebecca Boll and Thomas M. Baumann and Surjendu Bhattacharyya and Martin Centurion and Keyu Chen and Benjamin Erk and Alberto De Fanis and Ruaridh Forbes and Markus Ilchen and Edwin Kukk and Huynh V. S. Lam and Xiang Li and Lingyu Ma and Tommaso Mazza and Michael Meyer and Terence Mullins and J. Pedro F. Nunes and Asami Odate and Shashank Pathak and Daniel Rivas and Philipp Schmidt and Florian Trinter and Sergey Usenko and Anbu S. Venkatachalam and Enliang Wang and Peter M. Weber and Till Jahnke and Artem Rudenko and Daniel Rolles},
journal= {arXiv preprint arXiv:2602.04149},
year = {2026}
}