Pressure ionization and transitions in dense hydrogen

Shock experiments with fluid hydrogen have shown that a transition from insulating behavior to metal-like conductivity occurs at pressures beyond 100 GPa. This requires the development of new methods to describe the transition region of dense plasmas. The traditional approach due to Saha is based on the assumption of chemical equilibrium between charged and neutral components. This is equivalent to minimizing the free energy with respect to the composition. Here we improve an expression for the free energy developed recently to determine Hugoniot curves and isentropes in dense hydrogen and deuterium plasma in the regions of partial dissociation and partial ionization. We show that at high pressures the influence of the excluded volume occupied by neutral species is crucial for the transition to full ionization. We present curves for several thermodynamic functions for the region 5000 K < T < 20 000 K and 0.6 g/cm(3) < rho < 1 g/cm(3). The influence of the effective radii of the neutral species is crucial in the transition region. (c) 2005 WILEY-VCH Vertag GmbH & Co. KGaA, Weinheim