Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen.

Hydrogen has been an essential element in the development of atomic, molecular and condensed matter physics. It is predicted that hydrogen should have a metal state; however, understanding the properties of dense hydrogen has been more complex than originally thought, because under extreme conditions the electrons and protons are strongly coupled to each other and ultimately must both be treated as quantum particles. Therefore, how and when molecular solid hydrogen may transform into a metal is an open question. Although the quest for metal hydrogen has pushed major developments in modern experimental high-pressure physics, the various claims of its observation remain unconfirmed. Here a discontinuous change of the direct bandgap of hydrogen, from 0.6 electronvolts to below 0.1 electronvolts, is observed near 425 gigapascals. This result is most probably associated with the formation of the metallic state because the nucleus zero-point energy is larger than this lowest bandgap value. Pressures above 400 gigapascals are achieved with the recently developed toroidal diamond anvil cell, and the structural changes and electronic properties of dense solid hydrogen at 80 kelvin are probed using synchrotron infrared absorption spectroscopy. The continuous downward shifts of the vibron wavenumber and the direct bandgap with increased pressure point to the stability of phase-III hydrogen up to 425 gigapascals. The present data suggest that metallization of hydrogen proceeds within the molecular solid, in good agreement with previous calculations that capture many-body electronic correlations.