Polarization and M-dependent effects in rotationally inelastic collisions of open-shell diatomic molecules: Ar–NO(X 2Π1/2)

Abstract

Our earlier quantum study of collisions of NO(X 2Π) with Ar [T. Orlikowski and M. H. Alexander, J. Chem. Phys. 79, 6006 (1983)] is here extended to the calculation of M-dependent cross sections in the case of laboratory-frame quantization, as would be appropriate to collision cell experiments. The calculations confirm the earlier prediction of Alexander and Davis [J. Chem. Phys. 79, 227 (1983)] that the JM→J, -M cross sections will be vanishingly small for transitions which preserve the e or f Λ-doublet symmetry. This propensity rule is here shown to be rigorous within the coupled-states (CS) limit. The effect of collisions on the initial rotational density matrix can be expressed in terms of multipole transfer efficiencies. The calculated values for Ar–NO collisions indicate that e/f changing collisions are much more strongly depolarizing, in the sense of destroying the multipole moments of the initial density matrix, than those collisions which preserve the e/f Λ-doublet label. Since the circular or linear polarization of the fluorescence from a laser excited molecule can be written in terms of these multipole transfer efficiencies, we are able to use the Ar–NO(X 2Π) cross sections reported here to model qualitatively the depolarization effects one might expect to see in experiments involving excited molecules in 2Π electronic states.