  Hydrogen has been the essential element in the development of atomic and
molecular physics1). Moving to the properties of dense hydrogen has appeared a
good deal more complex than originally thought by Wigner and Hungtinton in
their seminal paper predicting metal hydrogen2): the electrons and the protons
are strongly coupled to each other and ultimately must be treated equally3)4).
The determination of how and when molecular solid hydrogen will transform into
a metal is the stepping stone towards a full understanding of the quantum-many
body properties of dense hydrogen. The quest for metal hydrogen has pushed
major developments of modern experimental high pressure physics, yet the
various claims of its observation over the past 30 years have remained
controversial5)6)7). Here we show a first order phase transition near 425 GPa
from insulator molecular solid hydrogen to metal hydrogen. Pressure in excess
of 400 GPa could be achieved by using the recently developed Toroidal Diamond
Anvil Cell (T-DAC)8). The structural and electronic properties of dense solid
hydrogen at 80 K have been characterized by synchrotron infrared spectroscopy.
The continuous vibron frequency shift and the electronic band gap closure down
to 0.5 eV, both linearly evolving with pressure, point to the stability of the
insulator C2/c-24 phase up to the metallic transition. Upon pressure release,
the metallic state transforms back to the C2/c-24 phase with almost no
hysteresis, hence suggesting that the metallization proceeds through a
structural transformation within the molecular solid, presumably to the Cmca-12
structure. Our results are in good agreement with the scenario recently
disclosed by an advanced calculation able to capture many-body electronic
correlations9).
