X-ray induced electron and ion fragmentation dynamics in iodine monobromine

Characterization of the inner-shell decay processes in molecules containing heavy elements is key to understanding x-ray damage of molecules and materials and for medical applications with Auger-electron-emitting radionuclides. The 1s hole states of heavy atoms can be produced by absorption of tunable x-rays and the resulting vacancy decays characterized by recording emitted photons, electrons, and ions. The 1s hole states in heavy elements have large x-ray fluorescence yields that transfer the hole to intermediate electron shells that then decay by sequential Auger-electron transitions that increase the ion's charge state until the final state is reached. In molecules the charge is spread across the atomic sites, resulting in dissociation to energetic atomic ions. We have used x-ray/ion coincidence spectroscopy to measure charge states and energies of I and Br atomic ions following 1s ionization at the I and Br K-edges of iodine monobromine (IBr). We present the charge states and kinetic energies of the two correlated fragment ions associated with core-excited states produced during the various steps of the cascades. We develop a computational model that combines Monte-Carlo/Molecular Dynamics simulations with a classical over-the-barrier model to track inner-shell cascades and redistribution of electrons in valence orbitals and nuclear motion of fragments. Our theory shows good agreement with the measured ion data as well as the exiting electron data. Our calculation also predicts that molecules can undergo bond length contraction in femtosecond timescale before dissociation. Future pump-probe studies can enable imaging of the x-ray induced ultrafast electron and dissociation dynamics in molecules.