Phase Transition in Fluid Nitrogen at High Densities and Temperatures

Dynamic equation-of-state data are presented for liquid N₂ single- and double-shocked to pressures of 100 GPa (1 Mbar), compressions to fourfold over initial liquid density, and internal energies to 1 MJ/mole. Three double-shock points lie above the principal Hugoniot in pressure-volume space, the first such observation in condensed matter. The data are interpreted in terms of a continuous phase transition, identified as molecular dissociation by comparison of the shock compression curves of liquid N₂ and isoelectronic CO.