Coupled-channel calculations of excited-atom collisions

Recent calculations of excited-atom collisions based on the coupled-channel formalism are reviewed. Such calculations of electronic excitation processes require additional information (such as radial coupling matrix elements) beyond the adiabatic potential surfaces normally provided by nb-initio electronic structure calculations. Several examples are discussed that illustrate various strategies for obtaining this additional information. The first example involves collisions of heavy atoms, particularly xenon. In this case, the inner-shell electrons exhibit relativistic effects, and the effects of the spin-orbit operator can be quite large. In addition, the open-shell core has non-zero spin and orbital angular momentum. For these collisions the inner-shell electrons are treated using effective core potentials, and a model Hamiltonian is developed to handle the large spin-orbit effects and the open-shell core. The second example involves collisions in which the target atoms are spatially aligned. These calculations required radial coupling terms as well as the determination of explicit state-to-state cross-sections between specific angular momentum states. These examples demonstrate that collisions of a high level of complexity can now be quantitatively treated.