Direct imaging of the structural change in strongly excited liquid water

Understanding the structural properties of liquid water in warm dense conditions has numerous implications for areas including astrophysics, shock physics and solution-phase chemistry. Here we report the results of using femtosecond electron diffraction to directly image the structural change in warm dense water formed by strong optical excitation of liquid water. In this study, a 266 nm, 100 fs laser pulse was focused onto a liquid water sheet of ~650 nm thickness, reaching a maximum excitation energy density of ~5.4 MJ/kg. Structural change of the excited water was probed with time-resolved electron diffraction up to 500 ps after laser arrival. The diffraction data is converted to differential pair distribution function (DPDF) to resolve the dynamics of intermolecular O···O and O···H bonds of water. The DPDF results show that the structural change is dominated by OH(H₃O⁺) radical-cation pairs within 0.5 ps, followed by a steady state up to 50 ps and a transition to phase explosion regime on a 100-ps time scale.