MULTICHANNEL MULTIPLE-SCATTERING THEORY WITH GENERAL POTENTIALS

We outline a many-body description of the photoemission and photoabsorption processes that incorporates the multichannel treatment of the atomic dynamical excitations into the framework of multiple-scattering (MS) theory. This generalization is a most natural one, in that the internal structure of the atomic constituents of the physical system under study is taken into account by the introduction of an interchannel atomic t matrix that fixes the probability amplitude of a particular excitation (channel) of the internal degrees of freedom of the atom by the photoelectron impinging on it. For the rest the MS structure of the theory is left unchanged, provided the propagation vector of the photoelectron between successive scattering events is changed according to the energy loss suffered. In this way the interplay between excitation dynamics and electronic and geometrical structure of the ground state is elucidated. At the same time this approach provides a theoretical model for the study of the evolution from the adiabatic to the sudden regime. In this context we describe a new MS expansion that reproduces the results of the sudden approximation for photoemission and photoabsorption cross sections in the limit of high photoelectron energies. As expected, the expansion parameter that controls the crossover between the two regimes is substantially the maximum eigenvalue of the interchannel atomic t matrix (Ta)LL, where is a channel index and L is an angular-momentum index: If this quantity is much less than one, then the deviations from the sudden approximation are negligible. Physical applications of the theory are briefly described. © 1990 The American Physical Society.