Optical spectroscopy techniques, including visible and near infrared (IR) R
aman and synchrotron IR methods have been applied to study solid nitrogen a
t megabar pressures. We find that nitrogen becomes totally opaque above 150
GPa, accompanied by the disappearance of Raman and IR vibrational excitati
ons, 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 d
issociation of molecular nitrogen into a nonmolecular (possibly amorphous)
phase.