Critical Raman line shape behavior of fluid nitrogen

Abstract

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 N 2 is affected by resonant intermolecular vibrational interactions between identical oscillators. By means of the well-chosen isotopic mixture ( 14 N 2 ) .975 - ( 14 N 15 N) .025 , the temperature and density dependences of shift, width, and asymmetry of the resonantly coupled 14 N 2 and, depending on the S/N ratio available, of the resonantly uncoupled 14 N 15 N 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.