Observing Early Ultrafast Dynamics in the Strong-Field Ionization of Liquid Water

Ionization from strong-field light is a well-established tool for studying the ultrafast dynamics of molecules, but so far has been largely dominated by gas-phase studies. Field-ionization in solution-phase systems follows the same physical principle of a binding potential being distorted to form a free electron, which can either recombine with the ionized molecule or cluster as in high-harmonic generation, or be sufficiently separated from the ion to remain unbounded. In a dipolar medium such as liquid water, these unbounded electrons form solvated electrons that on the picosecond timescale exhibit polaron oscillations, but the mechanism for the femtosecond (fs) behavior of these systems is not well understood. In particular, the underlying molecular physics of the initial charge formation below 100-fs have been experimentally elusive due to a lack of sufficient time resolution. Here we present a table-top study of the radiolysis of water that emphasizes these ultrafast dynamics.

We perform a strong-field pump, weak-field probe transient absorption (TA) experiment that uses a microfluidic chip nozzle to produce a 2-micron thick water flatjet at room temperature. Both pulses are 6-fs and 800-nm, requiring a thinner liquid jet than previous experiments. This approach provides significantly better time resolution than any liquid water strong field TA experiments previously reported. The strong-field pump field-ionizes the water, then the weak-field probe arrives at a variable time delay to measure changes in the optical absorption of the water due to the ionization. We observe the time-dependent formation of the solvated electron, changes in the water refractive index, and plasma effects when the electron concentration is sufficiently high.