Isotope effects in dynamics of water isotopologues induced by core ionization at an x-ray free-electron laser

Dynamical response of water exposed to x-rays is of utmost importance in a wealth of scienceareas. We exposed isolated water isotopologues to short x-ray pulses from a free-electron laserand detected momenta of all produced ions in coincidence. By combining experimental resultsand theoretical modelling, we identify significant structural dynamics with characteristicisotope effects in H$_2$O$^{2+}$ , D$_2$O$^{2+}$ , and HDO$^{2+}$ , such as asymmetric bond elongation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a fewfemtoseconds. A method to disentangle the sequences of events taking place upon theconsecutive absorption of two x-ray photons is described. Methodological access to thestructural properties and dynamics of dissociating water isotopologues exposed to x-rays opensthe way to a broader understanding of aqueous systems of biological interest