Optical Studies of Hydrogen Above 200 Gigapascals: Evidence for Metallization by Band Overlap

Abstract

Direct optical observations of solid hydrogen to pressures in the 250-gigapascal (2.5-megabar) range at 77 K are reported. Hydrogen samples appear nearly opaque at the maximum pressures. Measurements of absorption and Raman spectra provide evidence that electronic excitations in the visible region begin at ∼200 gigapascals. The optical data are consistent with a band-overlap mechanism of metallization.