Molecular orientation correlations and reorientational motions in liquids carbon monoxide,nitrogen and oxygen at 77 K; A Raman and Rayleigh light-scattering study

Reorientational autocorrelation functions have been determined from measurements of depolarized vibrational Raman scattering for liquid carbon monoxide, nitrogen and oxygen at 77 K and atmospheric pressure. The autocorrelation functions, which for these liquids are not significantly affected by vibration-rotation interaction, reveal that free rotation is an important feature of the molecular motion in liquid nitrogen but is less important for carbon monoxide and oxygen. The differences in behaviour are discussed in terms of intermolecular forces.

New values for the depolarized Rayleigh scattering cross section have been determined from intensity measurements made relative to the 992 cm^-1 Raman line of benzene. These values are compared to those reported previously by the authors using a different intensity standard (Chem. Phys. Lett., 31, 355 (1975)). The scattering cross sections yield the following values , where ?_ij is the angle between the major axes of molecules i and j (i≠j) and P ₂ indicates the second Legendre polynomial: -0·15 ± 0·2 for CO, +0·30 ± 0·2 for N₂ and +0·40 ± 0·2 for O₂. The large errors result from uncertainties in the local field correction factor. The negative value for CO can be explained as a result of strong quadrupole interactions which tend to align neighbouring molecules perpendicular to one another. The forms of the reorientational cross-correlation functions determined from the current Raman data and previous Rayleigh data are briefly discussed.