Time-resolved imaging of transient charge-transfer dynamics*

The advent of free-electron lasers capable of producing intense pulses of short-wavelength light has enabled new time-resolved studies into photodynamics in a site-selective manner. By choosing a suitable photon energy, photoabsorption can be highly localized at an individual atomic orbital within a molecule. In the presented experiment, this is utilized to study charge between dissociating fragments in gas-phase 2-iodopropane molecules. The parent molecules are photoexcited by an ultraviolet (UV) pump pulse, initiating neutral photodissociation, prior to multiple ionization selectively at the I 4d site by an extreme-ultraviolet (XUV) pulse produced by the SACLA free-electron laser. By imaging the momenta of the resultant In+ ions, signatures of charge transfer from nascent highly charged iodine ions to the neutral alkyl radical cofragment are observed. At longer pump-probe delays, and therefore extended C-I bond distances, charge transfer is no longer possible, localizing charge at the dissociating iodine atom. Detailed analysis of charge transfer signal (which occurs at higher ion momentum due to mutual Coulombic repulsion between charged fragments) as a function of pump-probe delay, iodine charge state and momentum yields new insights into the physics underpinning charge rearrangement.