Dynamics of Water Transition to the Supercritical State under Ultrafast Heating with Ultrashort Laser Pulses

The dynamics of femtosecond laser impact on water was experimentally studied and reconstructed using numerical modeling based on the classical molecular dynamics method in combination with the two-temperature model and dynamical rate equations. This process occurs in several stages. Initially, a femtosecond laser pulse interacts with the electron subsystem, generating plasma due to multiphoton, tunnel, and impact ionization. The energy transfer from plasma electrons to atoms, as shown using the two-temperature model, leads to ultrafast heating of the substance to a temperature of ~10 000 K, and the pressures achieved in the irradiated area are ~15 GPa, which leads to the generation of a shock wave. The temperatures and pressures exceeding the critical values, combined with high density fluctuations and clustering, indicate the transition of the substance to a supercritical state. The pressures and temperatures exceeding the critical values are achieved in a region slightly exceeding the cavitation zone, and this region experiences oscillations with a period close to the period of oscillations of the cavitation bubble. In the case of the femtosecond laser impact, the experimentally measured deposited energy density can be used as an initial condition under assumtion of unstantaneous heating of the medium, which significantly simplifies numerical modeling. Both the pressures achieved at the shock wave front and the dynamics of cavitation bubbles are successfully reconstructed within the framework of this approach.