Comparison of the size of excitonic effects in molecular π systems as measured by core and valence spectroscopies

In aromatic molecules characterised by extended π-systems valence and core excitons can have very similar binding energies. This is shown for a number of compounds, and in each case the core exciton binding energies are analysed in terms of the Coulomb screening and rehybridisation contributions. The latter semiquantitatively explain the binding energy differences between the valence and core excitons. This suggests that core and valence excitons have a similar spatial extent in such systems. In favourable cases core level techniques can thus be used to elucidate valence excited state properties.