Shock-induced cooling in high-density fluid nitrogen

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Abstract
Radiative temperature measurements have shown that fluid nitrogen that has been shock-compressed to high density and temperature exhibits a cooling when it is reshocked to a still higher density. This shock-induced cooling occurs over a limited range of pressures and densities, and is intimately connected with molecular dissociation, which occurs in nitrogen above 30 GPa and 7000 K. In this paper the experimental evidence for shock-induced cooling against both LiF and Al2O3 anvils is discussed and compared with theoretical results. Several unusual aspects of the thermodynamics, notably negative values of the Gruniesen γ and crossing isotherms, are shown to be associated with shock-induced cooling. A brief comparison is given between the double-shock cooling effect in dense nitrogen and the cooling observed for first-sound shocks in superfluid helium II.