Dynamic response of molecular liquids under multi-shock compression: Shock-induced cooling and thermodynamic pathways

Understanding phase transitions and physical behavior of diatomic molecules under extreme pressure and temperature conditions is crucial for high-energy-density science. This experimental study examines shock-induced phenomena in molecular liquids (N2, O2, and CO) using time-resolved pyrometry and Doppler velocimetry to assess dissociation dynamics, radiative signatures, and temperature evolution. By applying sequential shocks (second and fourth) with pressure release, we observe critical radiance variations at the sample/LiF interface. These measurements reveal the interface's transparency behavior and pressure-driven phase transitions, providing key insights into the dynamic response of molecular liquids under extreme conditions. Key findings show significant differences in radiative behavior across molecular liquids under shock compression, with radiance reduction at the sample/LiF interface strongly influenced by molecular composition. Among the studied systems, liquid nitrogen (LN2) showed a distinct shock cooling effect, where the observed temperature reduction was linked to the formation of a transient, complex molecular state under high pressure. Furthermore, LN2 demonstrated reversible energy recovery upon pressure release, suggesting the presence of pressure-dependent phase transitions that facilitate reversible thermodynamic pathways. In contrast, re-shocked liquid oxygen and carbon monoxide displayed irreversible behavior at the LiF interface during pressure release, indicating fundamentally different relaxation dynamics compared to LN2. Emissivity measurements further highlighted sample-dependent responses: while liquid-argon transitioned from partial to full optical opacity under compression, LN2 exhibited a more intricate emissivity evolution, initially peaking near unity during compression before decreasing upon release as the fluid regained brief transparency.