On the Pressure Broadening of Molecular Multiplet Spectra

Abstract

Gas-phase line shapes and band contours are calculated for microwave spectra involving fine structure levels due to electron spin and/or orbital angular momentum, or vibrational angular momentum. We use the impact approximation, and a semiclassical treatment of the molecular rotation. The line shapes are expressed in terms of a set of cross sections which depend on the transfer of angular momentum by collisions. Different results are found, depending on whether the spin and rotation are coupled together strongly [Case (a)] or weakly [Case (b)]. Good agreement is obtained with microwave measurements on NO [Case (a)] and O2 [Case (b)]. The results may be applied to study the angle-dependent shape of intermolecular forces.