A perturbation theory guide to open-shell complexes: OH--Ar(X 2Π)

Perturbation theory is used to understand the experimentally observed stimulated emission spectra of OH--Ar(X 2Π). A useful zero-order Hamiltonian for an open-shell van der Waals complex is presented, and the most important perturbation terms are identified: rotational decoupling ( j·s), Renner--Teller coupling (V2), and a Coriolis interaction (J·j). This treatment reveals those parts of the Hamiltonian which are responsible for various unusual features in the spectra of open-shell complexes, such as the large parity splittings in certain vibrational bands and spin--orbit--induced predissociation of the OH--Ar(X 2Π) complex. The magnitude of the parity splitting is shown to be directly proportional to the change in the intermolecular potential when the odd electron in the free radical lies in or out of the O--H--Ar plane, the A' and A double-prime surfaces. The measured splitting is used to infer the magnitude of the difference between the A' and A double-prime potential-energy surfaces (∼12 cm-1) in the region sampled by the first excited bend