Complete partial-wave treatment of compound-state (rotational excitation) resonances in subthreshold scattering of an atom by a diatomic molecule

The present study deals with quantitative prediction of compound-state resonance energies and widths in the elastic scattering of a diatomic molecule at energies below the threshold for rotational excitation. Previous work had been restricted to the weak interchannel coupling limit or to s-wave scattering only (but with no restriction on coupling strength). The two-state adiabatic decoupling approximation, adequate to describe the s-wave resonance phenomenon, requires modification to be practical for higher partial waves. This extension is carried out and calculations yield good agreement with exact (close-coupled) results. It is then used in the numerical solution of the complete subthreshold resonance problem for an example involving moderately strong coupling, for which other approximate methods are inadequate. This decoupling approximation, which yields accurate energies and semiquantitative lifetimes of the compound states, leads also to a spectroscopic correlation scheme, i.e., an indexing of the spectrum of resonances.