Optical evidence for a nonmolecular phase of nitrogen above 150 GPa

Optical spectroscopy techniques, including visible and near infrared (IR) Raman and synchrotron IR methods have been applied to study solid nitrogen at megabar pressures. We find that nitrogen becomes totally opaque above 150 GPa, accompanied by the disappearance of Raman and IR vibrational excitations, while new broad IR and Raman bands become visible. Optical absorption measurements reveal that the semiconducting absorption edge responsible for the change of color is characterized by the presence of a wide Urbach-like tail and a high-energy (Tauc) region. These data are consistent with the dissociation of molecular nitrogen into a nonmolecular (possibly amorphous) phase.