Molecular dissociation in hot, dense hydrogen.

We present a path-integral Monte Carlo study of dissociation in dense hydrogen (1.75{le}{ital r}{sub {ital s}}{le}2.2, with {ital r}{sub {ital s}} the Wigner sphere radius). As the temperature is lowered from 10{sup 5} to 5000 K, a molecular hydrogen gas forms spontaneously from a neutral system of protons and electrons. At high density, {ital r}{sub {ital s}}{lt}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. {copyright} {ital 1996 The American Physical Society.}