Theory of x-ray emission of conjugated molecules

A theory of nonresonant x-ray emission spectra of conjugated molecules is presented. The role of relaxation effects on the spectral shape of x-ray emission is advanced and examined in detail for the transpolyenes. In some contrast to unconjugated systems, the relaxation of valence charge around the core hole is shown to influence the distribution of x-ray intensities significantly. Site and size dependences of the spectral shape of x-ray emission are predicted; the site dependence as being alternant among the π levels. Comparative calculations of x-ray emission spectra at the ab initio Hartree-Fock and the semiempirical levels show that the relaxation qualitatively changes the spectral distribution and shifts intensity from the σ to the π parts of the spectra, creating excitonlike features in the latter. The exciton character is site and size dependent; for longer polyenes it is stronger for mid-atoms than for end-atoms, and is found stronger at the top than at the bottom of the π band. Complication/simplification trends of the spectra along the oligomer sequence are predicted for both the relaxed and frozen orbital models.