Fragmentation Dynamics of Small Molecules upon Multiple Ionization by X-Ray Free-Electron Laser Pulses

Abstract

The ionization and fragmentation dynamics of small molecules (CH3SeH, C2H5SeH, CH3I, and ICl) triggered by intense ultrashort soft X-ray pulses delivered by the Linac Coherent Light Source are investigated employing coincident three-dimensional ion momentum spectroscopy. This work aims at investigating the role of the molecular environment in multiple inner-shell photoionization and the accompanying electronic relaxation processes by studying molecular systems containing a single constituent of high nuclear charge Z, i.e. selenium or iodine, such that photoabsorption is almost exclusively localized at the core-shells of these heavy atoms. By comparing the level of ionization for the molecules containing selenium or iodine with results on isolated krypton and xenon atoms, signatures of efficient charge redistribution within the molecular environment are observed. Measured kinetic energies and angular distributions of the ionic fragments in comparison to the outcome of a simple Coulomb explosion model allow tracking down the evolution of the molecular geometry, revealing considerable displacement of the nuclei on the time scale of sequential multiple ionization. The results obtained have considerable implications for coherent diffractive imaging, providing a direct measure of radiation damage (displacement of nuclei and electronic rearrangement) on the time scale of the X-ray pulse and the length scale of the individual atoms.