Capturing transient species in ionized liquid water with ultrafast x-rays

Using ultrafast x-rays to follow the dynamics of chemical reactions in liquid solutions has the potential to place condensed phase reaction dynamics on par with understanding in the gas phase.  Here I address a fundamental reaction in the solution phase: proton transfer in ionized liquid water.  Ionization of liquid water is a universal manifestation of the interaction of radiation with matter.  The ensuing cascade of electrons, ions and radicals form the basis of solution and interfacial chemistry in aqueous environments in complex systems, e.g. organisms undergoing medical diagnosis, radiation therapy, nuclear reactors and environmental remediation.  I will discuss our study of pure water and the initial ultrafast steps of the elementary proton transfer reaction : H₂O⁺ + H₂O —›  OH + H₃O⁺, triggered by strong-field ionization.  X-rays are uniquely sensitive to two species in the water window, H₂O⁺ and OH.  By monitoring three photon-in/photon-out channels, x-ray transmission, total and dispersed x-ray emission as a function of x-ray photon energy and time delay,  we find evidence of the elusive H₂O⁺ radical cation as well as a clean and distinctive signature of the OH radical, complementing earlier studies in the UV.  Theoretical studies using a QM/MM approach provide insight into the valence hole localization and nuclear dynamics in the water network during the proton transfer reaction and demonstrate the sensitivity of the x-ray absorption spectrum to structural dynamics near the reaction center.   The RIXS spectrum of the OH radical reveals the complementary nature of x-rays relative to the UV absorption probes.  Interestingly, the x-ray spectra also show sensitivity to the dynamics of the injected electron.