Pure vibrational Raman spectra of simple liquids: O2, N2, CO, CH4, CF4

Abstract

Interferometric techniques have been applied in a high resolution study of the polarized Raman components (Q branches) corresponding to the totally symmetric vibrational modes of CO, CH₄, and CF₄ in the liquid phase. Additional unpublished data for N₂ and O₂, as well as limited results of dilution studies in liquid argon, are reported. Emphasis is placed upon the measurement of variations in frequency shift and spectral width along the liquid–vapor coexistence line from the triple point to within 0.2 K of the critical point for each pure liquid. The character of the observed linewidth variations exhibits distinct differences from liquid to liquid, especially in the neighborhood of the triple and critical points. A consistent interpretation of the results suggests that a substantial contribution to the N₂ and O₂ linewidths is associated with the effect of intramolecular vibration–rotation coupling. The CH₄ data support the predictions of Hills and Madden (i) that the long wavelength, hydrodynamic, density fluctuations can play an important role in determining the linewidth in the critical region, and (ii) that a transition from fast to slow modulation conditions should be observable as the critical point is approached. CF₄ may represent a case where slow modulation conditions hold throughout a large part of the liquid range.