Evidence of a first-order phase transition to metallic hydrogen

The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed-matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from the low-temperature limit with the system in the ground state to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen in the region of 1.1--1.7 Mbars and up to 2200 K. We present evidence supportive of a first-order phase transition accompanied by changes in transmittance and reflectance, characteristic of a metal. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition.