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The dominant pathway of radiation damage begins with the ionization of water. Thus far, however, the underlying primary processes could not be conclusively elucidated. Here, we directly study the earliest steps of extreme ultraviolet (XUV)–induced water radiolysis through one-photon excitation of large water clusters using time-resolved photoelectron imaging. Results are presented for H2O and D2O clusters using femtosecond pump pulses centered at 133 or 80 nm. In both excitation schemes, hydrogen or proton transfer is observed to yield a prehydrated electron within 30 to 60 fs, followed by its solvation in 0.3 to 1.0 ps and its decay through geminate recombination on a ∼10-ps time scale. These results are interpreted by comparison with detailed multiconfigurational non-adiabatic ab-initio molecular dynamics calculations. Our results provide the first comprehensive picture of the primary steps of radiation chemistry and radiation damage and demonstrate new approaches for their study with unprecedented time resolution.

Chiral molecules interact and react differently with other chiral objects, depending on their handedness. Therefore, it is essential to understand and ultimately control the evolution of molecular chirality during chemical reactions. Although highly sophisticated techniques for the controlled synthesis of chiral molecules have been developed, the observation of chirality on the natural femtosecond time scale of a chemical reaction has so far remained out of reach for isolated molecules.Here, we demonstrate a general experimental technique, based on high-harmonic generation in tailored laser fields, and apply it to probe the time evolution of molecular chirality during the photodissociation of 2-iodobutane. These measurements show a change in sign and a pronounced increase in the magnitude of the chiral response over the first 100 fs, followed by its decay within less than 500 fs, revealing the photodissociation to achiral products. The observed time evolution is explained in terms of the variation of the electric and magnetic transition-dipole moments between the lowest electronic states of the cation as a function of the reaction coordinate. These results open the path to investigations of the chirality of molecular reaction pathways, light-induced chirality in chemical processes and the control of molecular chirality through tailored laser pulses. SIGNIFICANCE STATEMENTChiral molecules interact and react differently, depending on their handedness (left vs. right). This chiral recognition is the basic principle governing most biomolecular interactions, such as the activity of drugs or our perception of scents. Inspite of this fundamental importance, a real-time (femtosecond) observation of chirality during a chemical reaction has remained out of reach for isolated molecules. In the present work, we report this fundamental breakthrough with a seemingly unlikely technique: high-harmonic generation (HHG) in tailored intense near-infrared laser fields. Combining the powerful transient-grating technique with HHG in counter-rotating circularly-polarized laser fields, we follow the temporal evolution of molecular chirality during a chemical reaction from the unexcited electronic ground state through the transition-state region to the final achiral products.

Isomerization induced by laser ionization in acetonitrile (CH3CN) was investigated using pump–probe spectroscopy in combination with ion–ion coincident Coulomb explosion imaging. We deduced five primary channels indicating direct C–C breakup, single and double hydrogen migration, and H and H2 dissociation in the acetonitrile cation. Surprisingly, the hydrogen-migration channels dominate over direct fragmentation. This observation is supported by quantum chemistry calculations showing that isomerization through single and double hydrogen migration leads to very stable linear and ring isomers, with most of them more stable than the original linear structure following ionization of the parent molecule. This is unlike most molecules investigated previously using similar schemes. By varying the delay between the pump and probe pulses, we have also determined the time scales of the corresponding dynamical processes. Isomerization typically occurs in a few hundred femtoseconds, a time scale that is comparable to that found for H and H2 dissociation and direct molecular fragmentation.

Femtosecond X-ray absorption spectroscopy (XAS) is a powerful method to investigate the dynamical behavior of a system after photoabsorption in real time. So far, the application of this technique has remained limited to large-scale facilities, such as femtosliced synchrotrons and free-electron lasers (FEL). In this work, we demonstrate femtosecond time-resolved soft-X-ray absorption spectroscopy of liquid samples by combining a sub-micrometer-thin flat liquid jet with a high-harmonic tabletop source covering the entire water-window range (284–538 eV). Our work represents the first extension of tabletop XAS to the oxygen edge of a chemical sample in the liquid phase. In the time domain, our measurements resolve the gradual appearance of absorption features below the carbon K-edge of ethanol and methanol during strong-field ionization and trace the valence-shell ionization dynamics of the liquid alcohols with a temporal resolution of ∼30 fs. This technique opens unique opportunities to study molecular dynamics of chemical systems in the liquid phase with elemental, orbital, and site sensitivity.