Optical studies of dense hydrogen at multi-megabar pressures

Hydrogen, being the simplest and most abundant element in the Universe, is of fundamental importance to condensed matter sciences. Through advances in high pressure experimental technique, hydrogen (and its isotope deuterium) has been contained and studied using in situ optical spectroscopy to 315 (275 GPa) at 300 K, pressure and temperature conditions previously thought to be inaccessible. At 200 GPa, hydrogen undergoes a phase transformation, attributed to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental mode, new low-frequency phonon excitations, and a dramatic softening and broadening of the first vibrational fundamental mode. To impose constraints on the P-T phase diagram, the temperature stability of phase IV is investigated through a series of low temperature experiments, where the phase IV-III transformation is observed. Analysis of the Raman spectra suggests that phase IV is a mixture of graphene-like layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules. Isotopic comparisons reveal spectral differences between the phase IV-III transition of hydrogen and deuterium, which strongly indicates the presence of proton tunnelling in phase IV. Optical transmission spectra of phase IV reveals an overall increase of absorption and a closing band gap reaching 1.8 eV at 315 GPa. No differences between the isotopes were observed in absorption studies, resulting in identical values for the band gap. Extrapolation of the band gap yields 375 GPa as the minimum transition pressure to a metallic state of hydrogen (deuterium)