Vibron hopping and bond anharmonicity in hot dense hydrogen

Area of Science:

• Condensed matter physics
• Quantum mechanics
• Materials science

Background:

• Dense hydrogen exhibits unexpected changes in its Raman-active vibron under high pressure and temperature.
• Understanding these changes is crucial for solid-state physics and materials science.

Purpose of the Study:

• To investigate the pressure and temperature dependence of the Raman-active vibron in dense hydrogen.
• To elucidate the underlying theoretical mechanisms driving the observed spectral changes.

Main Methods:

• Supercell-based calculations employing Van Kranendonk theory, incorporating lattice vibration renormalization of the hopping parameter.
• Hybrid path integral molecular dynamics (PIMD) simulations for the fluid state.

Main Results:

• Van Kranendonk theory accurately describes experimental results up to 70 GPa, with temperature dependence primarily from rotational state populations.
• Hybrid PIMD calculations and an amorphous-solid fluid model predict minimal Raman vibron frequency shifts upon melting.
• The Van Kranendonk model, assuming fixed molecular rotational identities, overpredicts Raman spectral peaks compared to experimental observations.

Conclusions:

• The study provides a theoretical framework for understanding the behavior of dense hydrogen's vibron under extreme conditions.
• Lattice vibrations and molecular rotational states significantly influence the Raman spectra of dense hydrogen.
• Further refinement of theoretical models is needed to fully reconcile predictions with experimental data, particularly regarding spectral peak multiplicity.