Probing dynamics of high harmonic generation in liquids

High-harmonic spectroscopy (HHS) is a successful in-situ attosecond probe of electron dynamics in gases and solids [1,2,3]. However, HHS in liquids, has so far been limited by the lack of an intuitive understanding of the high harmonic generation (HHG) process. Studying HHG over a wide range of wavelengths (λ), intensities and pulse durations, we show that the cut-off (E_c) is a characteristic liquid dependent property, independent of λ beyond a threshold intensity [4,5]. We explain these observations within an intuitive semi-classical model based on electron trajectories that are limited by scattering to a characteristic length, which is connected to the electron mean-free path (MFP). This is validated with ab initio time-dependent density-functional-theory calculations. The model is further verified experimentally through the systematic control E_c by the variation of liquid density resulting in a change in the effective MFP [4]. Our work establishes the possibility of resolving sub-femtosecond electron dynamics in a bio-chemically relevant liquid environment.

[1] S. Ghimire, et al., Observation of high-order harmonic generation in a bulk crystal, Nature Physics, 7(2):138–141, 2011.

[2] J. Itatani, et al., Tomographic imaging of molecular orbitals, Nature, 432:867–871, 2004.

[3] P. M. Kraus, et al., Measurement and laser control of attosecond charge migration in ionized iodoacetylene, Science (New York, N.Y.), 350(6262):790–5, nov 2015.

[4] A. Mondal, et al., Probing the low-energy electron-scattering dynamics in liquids with high-harmonic spectroscopy, arXiv:2203.03617, 2022.

[5] A. Mondal, et al., High-harmonic generation in liquids with few-cycle pulses: effect of laser-pulse duration on the cut-off energy, arXiv:2210.07736, 2022.