Optical Studies of Nitrogen to 130 GPa

Nitrogen is observed to remain a molecular solid up to 130 GPa, contrary to recent theoretical predictions of metallization below 100 GPa. Raman scattering reveals three new phases at 20, 66, and 100 GPa, which are distinguished by branching of existing vibronic modes. One mode increases in frequency to a broad maximum at 66 GPa and then decreases, similar to the case of ${\mathrm{H}}_2$. In ${\mathrm{H}}_2$, such behavior was attributed to a weakening of the H-H bond. In ${\mathrm{N}}_2$, three other vibronic frequencies continue to increase, showing the situation to be more complicated than just a weakening of the N-N bond.