THEORY OF X-RAY EXCITONS IN CONJUGATED MOLECULES AND POLYMERS

A theory for excitons in x-ray spectra of conjugated molecules is pres ented. Due to the interaction with the core hole the lowest unoccupied pi level splits from other empty levels. The energy position of this state is defined by the renormalized Coulomb interaction with the core hole and by the width of the unoccupied pi band. The ratio of these t wo quantities determine whether the x-ray state is excitonic or not. I t is shown that the conditions for excitonic character are qualitative ly different between occupied levels (x-ray emission) and unoccupied l evels (x-ray absorption). From the specified normalization we find tha t only half an electron density of the unoccupied molecular orbital ca n be localized to the atom with the core hole, while in the x-ray emis sion case the electron density of the lowest occupied pi orbital can b e totally localized. The degree of localization of the lowest unoccupi ed pi level decreases when the length of the molecule increases. Contr ary to lowest occupied and unoccupied molecular orbitals the electron density of higher occupied (emission) and higher unoccupied (absorptio n) pi orbitals is pushed out from the core excited atom for sufficient ly large Coulomb interactions between core hole and valence shell. Str ong site and size dependences of the intensity and frequency of excito nic peaks in x-ray absorption spectra are predicted, the site dependen cy being alternant for conjugated molecules. The model predicts strong depression of the absorption intensity for the pi levels along the p olyene series in good accord with ab initio data.