Disentangling charge transfer effects and site-specific photoabsorption in multiple x-ray ionization of molecules

The ionization and fragmentation dynamics of molecules triggered by high-fluence; high-intensity x-ray free electron laser (XFEL) is key to understanding the fundamental ultrafast dynamics of molecules. We investigate the complex ionization and fragmentation dynamics of iodine chloride (ICl) exposed to a 1500-eV XFEL pulse. We track the charge and kinetic energy distribution (KED) of iodine and chlorine ionic fragments, associated with the different fragmentation channels, in comparison to the earlier study on atomic Xe^1,2. Our experimental results show that the charge distribution of ICl has the same maximum charge state of 34+, however, the shapes of charge distribution exhibit discrepancy for states above 22+ compared to Xe. The inconsistency between measured KED and simulated Coulomb-repulsion energies increases with higher total charge products as well as for the asymmetric channels, e.g., I⁶⁺Cl³⁺ and I³⁺Cl⁶⁺. To understand these discrepancies, we implemented Monte Carlo/molecular-dynamics (MCMD) simulations with a classical over-the-barrier model³. Our calculations rationalize the experimental findings and demonstrate the role of resonant excitation channel and electron transfer between I and Cl atoms.