ROTATIONAL WINGS OF RAMAN BANDS AND FREE ROTATION IN LIQUID OXYGEN, NITROGEN, AND METHANE

Abstract

Rotation-vibrational Raman bands have been observed in liquid oxygen, nitrogen, and methane, and in solid methane. In every case the Q branches associated with isotropic Raman scattering are sharp lines; but the Q branches and rotational wings associated with anisotropic scattering form broad continuous bands, with extent and intensity distribution consistent with free molecular rotation. The lack of discrete structure in the broad bands is attributed to a removal of the m degeneracy of the rotational energy levels in the intermolecular force fields. Removal of the m degeneracy broadens those transitions for which the transition probability depends on the anisotropic part of the rate of change of the polarizability, but not those for which J and m are unchanged and for which the transition probability depends only on the isotropic part. The Raman spectrum of liquid oxygen yields no evidence for O₄ aggregates. The Raman spectrum of liquid methane displays all fundamental vibrations of the molecule and two overtones, with Raman shifts (in cm.⁻¹): 1300, band (ν₄); 1535, band (ν₂); 2570, line, and 2600, band (2ν₄); 2906, line (ν₁); 3020, band (ν₃); 3053, line (2ν₂).