N 2 and CO2 vibrational modes in solid nitrogen under pressure

Nitrogen has a complex phase diagram that has presented a formidable challenge to theoretical models. In this study, infrared (IR) spectroscopy has been performed on solid nitrogen at low temperatures and pressures up to 6 GPa. In the ε phase, two IR absorption peaks were observed that correspond to N–N stretch modes. The presence of two IR-active N2 vibrons is consistent with a tetragonal structure (P43212) and is inconsistent with an earlier proposed rhombohedral (R3̄C) structure. The matrix isolation of CO2 molecules in solid N2 led to the observation of transverse (ν2) vibrational modes, which showed a splitting consistent with the structures of the different N2 phases. The ν3 mode of CO212 and CO213 molecules was measured as a function of pressure, as well as combination modes. All the vibrational-mode frequencies shifted discontinuously at the critical pressures for N2 phase transitions. For CO2 concentrations of 0.1%, phase separation is present at low pressures, but is suppressed for pressures greater than 1 GPa. In summary, these studies have contributed to the knowledge of the N2 phase diagram and the nature of guest-host interactions in molecular systems.