Molecular Dissociation and Shock-Induced Cooling in Fluid Nitrogen at High Densities and Temperatures

Radiative temperatures and electrical conductivities were measured for fluid nitrogen compressed dynamically to pressures of 18-90 GPa (180-900 kbar), temperatures of 4000-14 000 K, and densities of 2-3 g/${\mathrm{cm}}^{3}$. The data show a continuous phase transition above 30 GPa shock pressure and confirm that ${(\frac{\ensuremath{\partial}P}{\ensuremath{\partial}T})}_{v}<0$, as indicated previously by Hugoniot equation-of-state experiments. The first observation of shock-induced cooling is also reported. The data are interpreted in terms of molecular dissociation and the concentration of dissociated molecules is calculated as a function of density and temperature.