Reflectance measurements on solid hydrogen to 177 GPa (1.77 Mbar) have been performed from near-infrared to ultraviolet wavelengths (0.5 to 3 eV). Above 150 GPa characteristic free-electron behavior in the infrared region is observed to increase sharply with increasing pressure. Analysis of volume dependence of the plasma frequency obtained from Drude-model fits to the spectra indicates that the pressure of the insulator-metal transition is 149 (±10) GPa at 295 K. The measurements are consistent with metallization by closure of an indirect gap in the molecular solid.

• Received 9 May 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.484