Molecular Dissociation in Hot, Dense Hydrogen

We present a path-integral Monte Carlo study of dissociation in dense hydrogen ( $1.75\le r_s\le 2.2$, with $r_s$ the Wigner sphere radius). As the temperature is lowered from ${10}^5$ to 5000 K, a molecular hydrogen gas forms spontaneously from a neutral system of protons and electrons. At high density, $r_s<2.0\mathrm{}$, thermally activated dissociation is accompanied by decreasing pressure, signaling the presence of a first order transition and critical point. The decrease in electron kinetic energy during dissociation is responsible for the pressure decrease and transition. At lower density the phase transition disappears.