Fluid hydrogen at high density: Pressure ionization.

Abstract

In an earlier paper [Phys. Rev. A 44, 5122 (1991)], we presented a Helmholtz-free-energy model for nonideal mixtures of hydrogen atoms and molecules. In the present paper, we extend this model to describe an interacting mixture of ${\mathrm{H}}_{2}$, H, ${\mathrm{H}}^{+}$, and ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ in chemical equilibrium. This general model describes the phenomena of dissociation and ionization caused by pressure and temperature effects, as encountered in astrophysical situations and high-pressure experiments. The present model is thermodynamically unstable in the pressure-ionization regime and predicts the existence of a plasma phase transition with a critical point at ${\mathit{T}}_{\mathit{c}}$=15 300 K, ${\mathit{P}}_{\mathit{c}}$=0.614 Mbar, and ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$=0.35 g/${\mathrm{cm}}^{3}$. The transition occurs between a weakly ionized phase and a partially (\ensuremath{\approxeq}50%) ionized phase. Molecular dissociation and pressure ionization occur in the same narrow density range; atoms play a minor role in pressure ionization. In the high-density phase, complete pressure ionization is reached gradually. The sensitivity of the coexistence curve and of the critical point to model parameters and assumptions is discussed in detail.