Femtosecond laser ionization measurement in liquid water with comprehensive modeling

We report on our experimental and simulation studies of ultrafast laser ionization of conduction band (CB) electrons in liquid water. A variant of spectral interferometry (SI) technique was employed to characterize CB electrons via probe laser phase shift accumulated through a thin water jet. A comprehensive chemistry model for liquid water that includes all dominant, temperature-dependent collisional processes, along with the Keldysh photoionization rate [1], were incorporated into the axisymmetric, nonlinear laser propagation code SNOPROP [2]. The production of CB electrons is highly sensitive to collisional ionization and momentum transfer, whereas published values of the associated interaction cross sections vary widely. Our SNOPROP simulations enabled determination of the cross section values from the literature in agreement with the experimental results. These results impact applications of intense underwater photochemistry, including medical laser therapeutic techniques and laser micromachining.

[1] L. V. Keldysh, “Ionization in field of a strong electromagnetic wave”, Sov. Phys. JETP-USSR 20, 1307 (1965).

[2] J. R. Peterson, “SNOPROP: a solver for nonlinear optical propagation,” https://doi.org/10.5281/zenodo.3908609 (2020).