HIGH-PRESSURE DIELECTRIC MEASUREMENTS OF SOLID HYDROGEN TO 170-GPA

AT high pressure (> 100 GPa), the valence-conduction band gap in solid hydrogen is predicted to decrease and eventually to close, transforming it from an insulator to a metal 1. Recent experiments at pressures up to approximately 300 GPa suggest a gradual closing of the direct gap at very high pressures whereas closure of the indirect gap may occur at lower pressures, perhaps below 200 GPa (refs 2-6). Measurements of the refractive index, n, and its frequency dependence (the dispersion dn/d-omega) as a function of pressure can provide information on the changes in electronic structure that occur under these conditions 7,8. Here we report refractive-index measurements on solid hydrogen at visible frequencies at pressures up to 170 GPa. Unlike earlier studies 9, we find no evidence for a divergence (dielectric instability) at 150 GPa, close to the low-temperature phase transition observed previously 6 and suggested as being associated with metallization 4,9. Our results are consistent with closure of the indirect gap. A fit of our data to a dielectric model indicates that the onset of visible absorption owing to direct interband transitions should occur above 200 GPa, consistent with previous direct observations 3. Pressure-induced molecular dissociation may occur before closure of the direct gap.