  Diatomic nitrogen is an archetypal molecular system known for its exceptional
stability and complex behavior at high pressures and temperatures, including
rich solid polymorphism, formation of energetic states, and an
insulator-to-metal transformation coupled to a change in chemical bonding.
However, the thermobaric conditions of the fluid molecular-polymer phase
boundary and associated metallization have not been experimentally established.
Here, by applying dynamic laser heating of compressed nitrogen and using fast
optical spectroscopy to study electronic properties, we observe a
transformation from insulating (molecular) to conducting dense fluid nitrogen
at temperatures that decrease with pressure, and establish that metallization,
and presumably fluid polymerization, occurs above 125 GPa at 2500 K. Our
observations create a better understanding of the interplay between molecular
dissociation, melting, and metallization revealing features that are common in
simple molecular systems.
