High-order harmonic generation (HHG) in liquids exhibits striking characteristics that differ fundamentally from those in gases and crystalline solids, yet its microscopic origin remains elusive due to the internal complexity and disorder in liquid structures. The hydrogen bond (HB), responsible for many unique physical and chemical properties of liquids, is expected to play important roles in liquid HHG. However, how HB interactions are encoded in liquid HHG spectra remains largely unexplored. To solve these longstanding problems, here we propose an energy-space quantum multichannel transition model to account for the mechanism of liquid HHG, which is validated by the quantitative agreement with recent experimental measurements. We identify an electronic channel associated with HB interactions in liquid water, where HB-related modifications of the electronic states give rise to distinctive harmonic features that differ from those in other phases. Both the ab initio simulation and the simplified HB model capture this behavior and confirm the essential role of the HB network. Our results establish a universal microscopic framework for understanding liquid HHG and can be naturally generalized to interpret harmonic features in other liquids. It provides an HHG spectroscopic tool for probing HB-related chemical and biological processes in liquids.

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• High-order harmonic generation