Structural Fluctuation in Liquids Revealed by High-Order Harmonic Generation

High-order harmonic generation (HHG) in liquids has attracted considerable attention, yet the fundamental processes responsible for the liquid-related differences are still not well elucidated. Here we systematically investigate the liquid-HHG spectra of water, acetonitrile, methanol, ethanol, and 1- and 2-propanols. Within the framework of the three-step recollision model, we extract the values of maximum excursion length (Lmax) of the electron prior to its recapture. We establish a phenomenological scaling law linking the experimental Lmax values to the liquid properties (viscosity, mass density, and molecular thermal speed), exhibiting an intrinsic connection between the HHG spectra and structural fluctuations within liquids. This relationship is further validated through a quantitative comparison of the macroscopic self-diffusion coefficients of the liquids. Our findings also indicate that HHG spectroscopy emerges as a promising tool for probing liquid properties.