Dissociation and thermodynamics in dense hydrogen fluid

The behavior of matter at high pressures is studied for a better understanding of planetary interiors and the structure and evolution of stars, or for inertial confinement fusion. Especially, the phase diagram of hydrogen and helium as the most abundant elements in the universe has been studied both theoretically and experimentally up to ultra-high pressures in the Mbar region where metallization is expected to occur. While static high-pressure experiments in diamond anvil cells give no evidence for metallic conduction in solid hydrogen up to 250 GPa, recent dynamic shock-wave experiments in fluid hydrogen have demonstrated metallisation for the first time at 140 GPa and 3000 K. The equation of state, possible phase transitions, and the structural properties of fluid hydrogen at ultra-high pressures as well as of hydrogen plasma at elevated temperatures are of special interest. We use here analytical calculations and Monte Carlo simulations to determine the pair distribution functions in partly dissociated hydrogen. The equation of state and the degree of dissociation are calculated. The gradual transition from molecular to atomic hydrogen occurs due to pressure dissociation. Furthermore, we show the influence of dissociation on the proton-proton pair distribution function.