Exciton Nature of Plasma Phase Transition in Warm Dense Fluid Hydrogen: ROKS Simulation

• Published in: Chemphyschem 2023-03, Vol.24 (6), p.e202200730-n/a
• Main Authors: Fedorov, Ilya D., Stegailov, Vladimir V.
• Format: Article
• Language: English
• Subjects: exciton dissociation; Excitons; First principles; Hydrogen; Localization method; Molecular dynamics; Phase transitions; plasma chemistry; restricted open-shell Kohn–Sham method
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.202200730

The transition of warm dense fluid hydrogen from an insulator to a conducting state at pressures of about 20–400 GPa and temperatures of 500–5000 K has been the subject of active scientific research over the past few decades. However, various experimental and theoretical methods do not provide consistent results. In this work, we have applied the restricted open‐shell Kohn–Sham (ROKS) method for first principles molecular dynamics of dense hydrogen after thermal excitation to the first singlet excited state. The Wannier localization method has allowed us to analyze the exciton dynamics in this system. The model shows that a key mechanism of the transition is associated with the dissociation of electron‐hole pairs, which allows explaining several stages of the transition of fluid H2 from molecular state to plasma. This mechanism is able to give a quantitative description of several experimental results as well as to resolve the discrepancies between experimental studies. A new mechanism of plasma phase transition in warm dense fluid H2 has been proposed based on analysis of the formation and dissociation of localized excitons during heating. Comparison with experimental data is based on the DFT calculations within the ROKS method that provide the temperatures of exciton dissociation at different densities. This kinetic mechanism explains both the key experimental observations and the existing discrepancies in experimental data.