Molecular Dissociation in Hot, Dense Hydrogen

TL;DR

This study uses path-integral Monte Carlo simulations to investigate how dense hydrogen molecules dissociate at various densities and temperatures, revealing a first-order transition at high density.

Contribution

It provides the first detailed quantum Monte Carlo analysis of molecular dissociation and phase transitions in dense hydrogen under extreme conditions.

Findings

Dissociation occurs spontaneously as temperature decreases from 10^5 to 5000 K.

A first-order transition with a critical point is observed at high density.

Pressure decreases during dissociation due to reduced electron kinetic energy.

Abstract

We present a path-integral Monte Carlo study of dissociation in dense hydrogen ($1.75 \leq r_s \leq 2.2$, with $r_s$ the Wigner sphere radius). As 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.