  The insulator-to-metal transition in dense fluid hydrogen is an essential
phenomenon to understand gas giant planetary interiors and the physical and
chemical behavior of highly compressed condensed matter. Using fast laser
spectroscopy techniques to probe hydrogen and deuterium precompressed in a
diamond anvil cell and laser heated on microsecond timescales, we observe an
onset of metal-like reflectivity in the visible spectral range at P>150 GPa and
T>3000 K. The reflectance increases rapidly with decreasing photon energy
indicating free-electron metallic behavior with a plasma edge in the visible
spectral range at high temperatures. The reflectivity spectra also suggest much
longer electronic collision time (>1 fs) than previously inferred, implying
that metallic hydrogens at the conditions studied are not in the regime of
saturated conductivity (Mott-Ioffe-Regel limit). Combined with previously
reported data, our results suggest the existence of a semiconducting
intermediate fluid hydrogen state en route to metallization.
