Multiple scattering description of multicenter coherent emission with applications to photoionization and electron scattering in diatomic molecules

In the light of a multiple scattering (MS) description of molecular photodiffraction and photoabsorption processes, we present an investigation of the interplay between initial state coherence and intramolecular scattering, leading to a reinterpretation of the molecular interference in diatomic molecules and in particular of the Cohen-Fano (CF) interference term in photoabsorption. Indeed, the delocalization of the initial state electron over different atoms at positions Rn introduces in the language of MS theory as many virtual emitters as there are atoms in the molecule, giving rise to new MS paths as compared to the case of a single emitter. Their emission amplitudes interfere via the usual phase factor eik·Rn and in the case of two emitter photoemission describe how the usual picture of the microscopic Young's experiment is modified by the presence of intramolecular scattering. Photoabsorption follows from photoemission by integrating over the emission directions of the photoelectron and characterizes the CF oscillations as the remnants on the energy scale of the photoemission interference patterns introduced by the new paths joining the two centers, exactly like the extended x-ray absorption fine structure signals are the remnants of the closed paths that begin and end at the same atom. In the same context of initial state coherence we also show that the orientationally averaged scattering of electrons off small molecules can give access to CF type of oscillations, although in a more complicated way, due to the lack of site selectivity in comparison with the photoemission process and the absence of a dipole selection rule. It is shown that this type of modulation has the same physical origin as that found in photoabsorption.