Dipolar model for collisional energy transfer between dark and radiating excited electronic states: CaO(A′ 1Π, a 3Π) +N2O ⇄ CaO(A 1Σ+)+N2O

Abstract

Experimental evidence indicates that collisional transfer between low-lying excited electronic states occurs readily in a wide variety of diatomic molecules, in particular the alkaline earth oxides. We present here a model for this process, based on the long-range coupling of a permanent dipole of a polar collision partner and a transition dipole between the electronic states of the molecule of interest. We specifically investigate how spin–orbit or orbit–rotation mixing of two Born–Oppenheimer states can lead to substantial transition dipoles between the eigenfunctions of the full Hamiltonian for a diatomic molecule. The collision dynamics are treated within the time-dependent Born approximation, modified to ensure statistical microreversibility. A formulation in terms of spherical tensors facilitates the application of known techniques to the collision of molecules whose eigenfunctions are linear combinations of states of nonzero electronic angular momentum. We describe the calculation of cross sections and rate constants for transfer within and between the rotational manifolds of selected near resonant vibrational levels of the (nominally) A 1Σ+, A′ 1Π, and a 3Π states of CaO, as well as for transfer between the Λ-doubling components of the A′ 1Π state. The rate constants for electronic state transfer are substantial (≳1×10−10 cm3/molecule s) over a wide range of rotational levels in the neighborhood of the various isoenergetic points, where, for a particular pair of vibrational quantum numbers, the rotational ladders cross. No evidence is seen for a continuous transfer rate, independent of rotational quantum number, even at high values of J.