A liquid-phase high-order harmonic generation apparatus for investigating ultrafast dynamics in liquids.

To investigate the electronic-dominated ultrafast dynamics in liquids, we built a liquid high-order harmonic generation (HHG) apparatus, which consists of an ultrafast laser system, a liquid flatjet system, and an extreme ultraviolet (EUV) spectrometer. The performance of this apparatus was systematically examined through the generation of HHG spectra from 99.7% isopropanol [(CH3)2CHOH], 99.7% ethanol (CH3CH2OH), and liquid water (H2O). A negligible contribution from the gas-phase was verified by a direct comparison between the liquid-phase and gas-phase under an optimal configuration. Under the current experimental conditions, the measured high-order harmonic spectra (HHS) exhibited a distinct maximum photon energy of 20 eV, with intensity decreasing as the harmonic order increased. Our two-band model based on the semiconductor Bloch equations suggests that HHSs from liquids reach saturation very quickly. Moreover, we measured the time-dependent HHS of liquid isopropanol [(CH3)2CHOH] using a non-collinear 800 nm pump-probe Mach-Zehnder interferometer. These preliminary results demonstrate the operational capability of liquid-based HHG spectroscopy and highlight its potential for probing ultrafast electron dynamics and molecular properties in liquids.