Raman spectroscopy of hot compressed hydrogen and nitrogen: Implications for the intramolecular potential

Raman measurements of molecular hydrogen (H-2 and D-2) and nitrogen (N-2) have been made under simultaneous conditions of high temperature and high static pressure. Measurements have been made on H-2 and D-2 to 50 GPa and 1600 K, and on N-2 to 50 GPa and 2000 K. In all three materials the familiar molecular stretching mode (vibron) is accompanied in the high-temperature Raman spectra by one or more lower-frequency peaks due to transitions from excited vibrational states. We find that the frequency differences between these bands decreases with pressure, implying that the anharmonicity of the corresponding part of the intramolecular potential also decreases. This is accompanied by an increase in the measured linewidths of the bands that is consistent with a decrease of the depth of the potential and an approaching molecular dissociation.