Pure and Applied Chemistry

Abstract: Isotropic Raman line shapes of simple molecular fluids exhibit critical line broadening near their respective liquid-gas critical points. In order to observe this phenomenon, it is essential that the band position of a given vibrational mode is density-dependent, and that vibrational depopulation processes negligibly contribute to line broadening. Special attention was given to the fact that the isotropic (i.e., nonrotationally broadened) line shape of liquid N₂ is affected by resonant intermolecular vibrational interactions between identical oscillators. By means of the well-chosen isotopic mixture (¹⁴N₂)_.975 - (¹⁴N¹⁵N)_.025, the temperature and density dependences of shift, width, and asymmetry of the resonantly coupled ¹⁴N₂ and, depending on the S/N ratio available, of the resonantly uncoupled ¹⁴N¹⁵N were determined, with up to milli-Kelvin resolution, in the coexisting liquid and gas phases and along the critical isochore, using a highest-resolution double monochromator and modern charge-coupled device detection techniques. Clear evidence was found that vibrational resonance couplings are present in all dense phases studied.