Molecular Dissociation in Hot, Dense Hydrogen
We present a path-integral Monte Carlo study of dissociation in dense hydrogen ( $1.75\ensuremath{\le}{r}_{s}\ensuremath{\le}2.2$, 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, ${r}_{s}<2.0$, 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.