All-XUV pump-probe transient absorption spectroscopy on the dissociation dynamics of small molecules

In this work, the implementation of an XUV-pump–XUV-probe transient absorp- tion spectroscopy scheme at the free-electron laser facility FLASH in Hamburg is presented. It is applied to the ultrafast dissociation dynamics of the small molecules diiodomethane and oxygen. For the diiodomethane target, the XUV pump pulse enables the resonant site-specific excitation of an iodine 4d electron to an antibonding molecular orbital. The hereby initiated dissociation of the molecule includes a direct, as well as an indirect path through isomeric geometries of the molecular cation. These parallel pathways are traced with the help of simulated absorption spectra and their time constants are extracted via a global fit model. In the case of the oxygen target, dissociation is initiated by the (multiple) ionization of the oxygen molecule. The emergence of absorption resonances of the doubly- charged ionic fragment O2+ is observed in the probe spectrum as a product of the dissociation. The compatibility of the extracted rise times of the fragments with Coulomb explosion is investigated with a simple classical model