High-Harmonic Generation and Femtosecond-Resolved Ultrafast Dynamics in Liquid Water

The high-harmonic generation (HHG) is of great significance for attosecond science, ultrafast detection, and control of quantum processes. Compared with gases and solids currently under extensive investigations, HHG from liquids is rarely studied, but it enables unique electron scattering processes different from those in gases and solids owing to the dense configuration without long-range order in liquids. Here using state-of-the-art <i>ab initio</i> quantum dynamics simulations, we investigated HHG in liquid water across a wide range of laser intensities. We identified the transition from isolated molecule behavior to condensed-phase dynamics and revealed the suppression of HHG due to ultrafast water plasma generation. This transition results in a scaling behavior of <i>E</i>₀^1.8 for cutoff energy <i>E</i>_c with respect to field strength <i>E</i>₀ of driving pulses before severe water dissociation. Further increasing field strength <i>E</i>₀ leads to ultrafast water dissociation and plasma generation, and in turn the enhanced decoherence and decrease of <i>E</i>_c. Via frequency filtering, the individual attosecond pulses can be obtained from liquid water. More importantly, photoinduced plasma generation and insulator-to-metal transition can be directly tracked via time-resolved HHG with femtosecond resolution. Our work offers novel insights into liquid-based HHG and reveals the femtosecond-resolved nonequilibrium dynamics of photoexcited liquid water, which can be experimentally probed by time-resolved HHG.