Hydrogen metallization under stable conditions is a major quest for realizing the first room temperature su-
perconductor. Recent low-temperature experiments report different metallization pressures, varying from 360GPa to 490GPa. In this work, we simulate structural properties, vibrational Raman, IR and optical spectra of hydrogen phase III accounting for proton quantum effects. We demonstrate that nuclear quantum fluctuations downshift the vibron frequencies by 25%, introduce a broad line-shape in the Raman spectra, and reduce the optical gap by 3eV. We show that hydrogen metallization occurs at 380GPa in phase III due to band overlap, in good agreement with transport data. Our simulations predict this state is a black metal - transparent in the IR - so that the shiny metal observed at 490GPa is not phase III. We predict the conductivity onset and the optical gap will substantially increase if hydrogen is replaced by deuterium, underlining that metallization is driven by quantum fluctuations and is thus isotope dependent. We
show how hydrogen acquires conductivity and brightness at different pressures, explaining the apparent contradictions in existing experimental scenarios.European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (grant agreement No. 802533
