Synchrotron Infrared Spectroscopy to 0.15 eV ofH2andD2at Megabar Pressures

New synchrotron infrared absorption and reflectivity techniques have been developed to test predicted band overlap metallization in ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ above 150 GPa at various temperatures. Measurements on both isotopes to maximum pressures of 216 GPa show no evidence for Drude excitations in the midinfrared to near-infrared spectrum. The most definitive are measurements on ${\mathrm{D}}_2$, which remains transparent down to 0.15 eV at 170 GPa and 83 K. The results put new bounds on the optical conductivity and possible band-gap closure at the 150-GPa transition. In addition to strong vibrational transitions, new infrared absorption features are observed.