Optical properties of high pressure liquid hydrogen across molecular dissociation

TL;DR

This study uses ab-initio calculations to analyze the optical properties of liquid hydrogen under high pressure, revealing the nature of dissociation and metallization transitions and supporting recent experimental interpretations.

Contribution

It provides a detailed computational analysis of hydrogen's optical properties across phase transitions, clarifying the nature of metallization and dissociation at high pressures and temperatures.

Findings

Above 3000 K, dissociation and metallization are continuous processes.

Below 1500 K, a first-order phase transition causes discontinuities in conductivity.

Reflectivity and absorption change rapidly but continuously across the transition.

Abstract

Optical properties of compressed fluid hydrogen in the region where dissociation and metallization is observed are computed by ab-initio methods and compared to recent experimental results. We confirm that above 3000 K both processes are continuous while below 1500K the first order phase transition is accompanied by a discontinuity of the DC conductivity and the thermal conductivity, while both the reflectivity and absorption coefficient vary rapidly but continuously. Our results support the recent analysis of NIF experiments (P. Celliers et al, Science 361, 677-682 (2018)) which assigned the inception of metallization to pressures where the reflectivity is about 0.3. Our results also support the conclusion that the temperature plateau seen in laser-heated DAC experiments at temperatures higher than 1500 K corresponds to the onset of of optical absorption, not to the phase transition.