Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H2O+, remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H2O+/OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.

We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic media, and is sensitive to the coupling and anharmonicity of thermally activated THz modes that play a central role in liquid-phase chemistry. Specifically, by varying the timing between two intense THz pulses, we control the orientational alignment of molecules in a liquid, and nonlinearly excite vibrational coherences. A comparison of experimental and simulated 2D TTR spectra of bromoform (CHBr 3 ), carbon tetrachloride (CCl 4 ), and dibromodichloromethane (CBr 2 Cl 2 ) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes.ultrafast dynamics | terahertz | coherent multidimensional spectroscopy D etailed molecular pictures of the structure and dynamics of liquids drive our understanding of chemistry and biology. Nonlinear 2D infrared and NMR spectroscopies have revealed many specifics of liquid behavior, monitoring the coupling, spectral diffusion, and homogeneous linewidths of intramolecular vibrations and nuclear spins (1, 2). However, the motions that directly participate in solvation and chemical reactivity are manifest in the terahertz (THz) region of the spectrum, making 2D THz studies especially valuable. To date, no 2D technique has been demonstrated that incorporates multiple THz interactions with a liquid.Recent advances in pulsed, high-power THz sources with electric fields exceeding 100 kV/cm have enabled a new generation of nonlinear THz spectroscopy, in which THz radiation is used to both manipulate and record the response of matter (3). It is now possible, for example, to control the alignment of gasphase molecules (4) and antiferromagnetic spin waves (5), drive an insulator-to-metal transition in oxides such as VO 2 (6), and break up Cooper pairs in a superconductor with intense THz pulses (7). Nonlinear THz interactions have also enabled the first demonstrations of 2D THz spectroscopy in a double quantum well system and graphene (8, 9).With weak transition dipole moments yet high THz absorptivity, liquids present many challenges with respect to the development of 2D THz spectroscopy. Initial successes with 2D Raman spectroscopy were later shown to suffer from the interference of cascaded processes (10-12), but new schemes using optical pulse shaping have eliminated the cascaded contributions (13). An alternative approach is resonant 2D THz spectroscopy, analogous to 2D IR spectroscopy. However, this method is hindered by a lack of THz directional phase matching, leading to signals that can be easily overwhelmed by a strong linear background. In the last few years, hybrid optical-THz techniques that circumvent these challenges have emerged, including 2D Raman-THz spectroscopy and THz Kerr effect spectroscopy (14-16). Here, we present the complementary 2D TTR spectroscopy, a natural extension of these hybrid te...

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. The simulation of these interactions requires a parametrization of the permanent and induced dipole moments. However, the underlying molecular polarizability of water and its dependence on ions are partially unknown. Here, we apply intense terahertz pulses to liquid water, whose oscillations match the timescale of orientational relaxation. Using a combination of terahertz pump / optical probe experiments, molecular dynamics simulations, and a Langevin dynamics model, we demonstrate a transient orientation of their dipole moments, not possible by optical excitation. The resulting birefringence reveals that the polarizability of water is lower along its dipole moment than the average value perpendicular to it. This anisotropy, also observed in heavy water and alcohols, increases with the concentration of sodium iodide dissolved in water. Our results enable a more accurate parametrization and a benchmarking of existing and future water models.