Resonances in molecular photoionization. I. Model calculations and analysis of general phenomena

Near-threshold autoionization structure in molecular photoionization is described using the projection-operator formalism of scattering theory. We address ourselves to the common situation of a number of discrete valence states interacting with the ionization continuum and the Rydberg series converging to the ionization threshold. The infinite Rydberg series are included as a whole in the treatment of the nuclear dynamics. We consider, in particular, a simplified model (single valence state and harmonic potential energy curves) which allows a very simple, fast, and numerically stable evaluation of the photoionization cross section. Model calculations are performed for a wide range of parameters (shift of potential energy curves, coupling strengths, optical transitions moments). We discuss and analyze a variety of general features of autoionization structures in total and partial photoionization cross sections.