Reversible thermodynamic pathways in shock-compressed liquid nitrogen: Unveiling the role of molecular dissociation in shock cooling and phase transition

This study investigates the high-pressure behavior of liquid nitrogen (LN2), under dynamic compression from 19 to 89 GPa and corresponding temperatures from 4000 to 8200 K. Using Doppler velocimetry and multichannel pyrometry, we determine the Hugoniot states, shock temperatures, and emissivity. The application of sequential shocks and subsequent pressure release process lead to significant variations in radiance and radiative temperature, which help to predict the optical transparency at the LN2/LiF interface. Our results indicate the existence of two distinct thermodynamic pathways. In experiments with an initial shock >30 GPa, at the second shock (re-shock), the fluid undergoes shock cooling, which we attribute to the formation of a transient complex molecular state. Further, during pressure release, we observe thermal energy emission, which indicates that decompression primarily governs the phase transitions as well as the reversible thermodynamic pathways. By contrast, LN2 initially shocked to 19 GPa undergoes dissociation upon re-shock, followed by recombination during pressure release, with no evidence of cooling being observed. A comparison of these results suggests that shock cooling consistently appears above 30 GPa and 6500 K. Furthermore, these two regimes are differentiated by their response to pressure release: while both show a decrease in emissivity that reflects restoration of the fluid’s transparency, the magnitudes are dramatically different. A 21% decrease occurs in the shock-cooling regime, compared with a 39% reduction in the dissociation-dominated regime.