Bonding changes in hot fluid hydrogen at megabar pressures

Raman spectroscopy in laser-heated diamond anvil cells has been employed to probe the bonding state and phase diagram of dense hydrogen up to 140 GPa and 1,500 K. The measurements were made possible as a result of the development of new techniques for containing and probing the hot, dense fluid, which is of fundamental importance in physics, planetary science, and astrophysics. A pronounced discontinuous softening of the molecular vibron was found at elevated temperatures along with a large broadening and decrease in intensity of the roton bands. These phenomena indicate the existence of a state of the fluid having significantly modified intramolecular bonding. The results are consistent with the existence of a pressure-induced transformation in the fluid related to the presence of a temperature maximum in the melting line as a function of pressure.