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In order to improve the experimental database about the additional nonrotational broadening of vibrational line shapes observable when a simple fluid approaches its gas–liquid critical point, we improved the pioneering measurements of Clouter and Kiefte [for their own review see Phys. Rev. A 33, 2749 (1986)] on the critical behavior of the polarized Raman line of fluid nitrogen by using the isotopic mixture (14N2).975−(14N15N).025, giving special attention to the fact that the isotropic line shape of liquid N2 (ν̃≈2327 cm−1) is affected by intermolecular vibrational resonance couplings. Using a highest-resolution double monochromator and modern CCD detection techniques, we were able to follow the temperature dependencies of the line shape parameters (i.e., shift, width, and asymmetry) of the coupled N142 and, depending on the S/N ratio available, of the uncoupled N1415N in the range 45 K≲T≲300 K with up to mK resolution (1) in the β-solid phase, (2) in the coexisting liquid and gas phases, and (3) along the critical isochore. Comparing the line shifts of both isotopic species, clear evidence was found that vibrational resonance couplings are present in all dense phases studied, the line position ν̃0 being more density than temperature dependent. Additionally, the existence of (negative) cross correlations between resonant and nonresonant dephasing mechanisms has been confirmed by the change in sign observed for the small but non-negligible difference in the linewidths between coupled N142 and uncoupled N1415N around 90 K. The λ-shaped dependencies of the width parameters, observed when moving along the coexistence line through the critical point, Tcrit=126.192 K, and along the critical isochore, is much more evident in the line asymmetry than in the usually considered linewidth. Clear proof was found that, in accordance with theoretical predictions, the linewidth converges to a constant maximum value regardless if the critical point is reached along the coexistence line or along the critical isochore, i.e., it does not diverge approaching the critical temperature up to our closest value |T/Tcrit−1|≈10−5.

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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.

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