X-ray free-electron lasers (XFELs) provide highly intense, femtosecond X-ray pulses that are ideal for the study of photoionization dynamics in atoms and molecules. This thesis aims to follow and understand the X-ray induced fragmentation dynamics of molecules on the ultrafast timescale. It comprises two main bodies of work: first, the commissioning of a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) experimental setup at the Maloja endstation of SwissFEL; and second, the investigation of ultrafast dynamics in ethanol following core ionization above the oxygen K-edge, with focus on the intramolecular hydrogen transfer and roaming mechanisms.

The COLTRIMS spectrometer resolution was characterized with the recoil momentum of He+ ions upon X-ray ionization. The resolution of the COLTRIMS spectrometer was determined to be < 1 a.u. The capability of ion coincidence spectroscopy was demonstrated with the resonant photofragmentation of CO2, showing evidence that the bent Renner-Teller state was populated after ionization at the O 1s â Ï â resonance. More complex molecular dynamics was observed in the fragmentation of ethanol upon ionization in the O K-edge region. The kinetic energy release spectra of the coincident channels containing H2+ and H3+ ions suggested that the ethanol dication decays into the roaming intermediate (H2 â C2H4O)2+. Experimental evidence of the roaming mechanism formed the basis for a time-resolved study on ethanol.

For the second work, the dynamics of H+, H2+ and H3+ ions that were formed after the core ionization of ethanol were studied using time-resolved ion time-of-flight spectroscopy, employing an X-ray pump/near-infrared (NIR) laser probe scheme. The formation of H3+ ions from the roaming mechanism is consistent with previous studies, but with a faster timescale of 145 ± 40 fs. Unlike H3+ ions, H2+ ions are formed from the molecular rearrangement of highly excited doubly charged ethanol cations within 81 ± 12 fs. Lastly, H+ ions are formed from the progressive fragmentation of the destabilized ethanol dication. They exhibit interesting dynamics that suggest the coupling of electronic states of the excited dication. These results reveal dynamics that shed light on the effect of core ionization in populating excited cationic states, resulting in phenomena such as a faster roaming time and an increase in kinetic energy of the ejected fragment ions. Future work will be directed towards molecular dynamics simulations to support the current interpretations and investigation of hydrogen transfer in other molecules. In summary, by studying ultrafast X-ray induced dynamics, this thesis provides insights into fundamental processes in nature such as radiation damage and the formation of simple molecules in interstellar medium.