NEAR-EDGE CORE PHOTOABSORPTION IN POLYACENES - MODEL MOLECULES FOR GRAPHITE

We present calculations on near-edge X-ray absorption fine structures (NEXAFS) for pyrene and for molecules in the polyacene series; benzene, naphthalene, anthracene, and tetracene. Results from these model molecules are used to characterize the NEXAFS spectrum of graphite. Calculations are carried out with an ab initio static exchange method recently made applicable to large species. The trends of different NEXAFS features with respect to the site of ionization and with respect to the number of hydrocarbon rings are studied. In contrast to the linear polyene series, which shows a decay in intensity and delocalization of the first pi* level with the size of the system, the polyacene series shows a rapid build-up of pi* excitons, with constant energy and intensity, conforming with a recent experimental observation of a pi* excitonic state for graphite. The excitonic character is though different for the symmetry non-related carbon atoms in the polyacene series. The salient double peak feature in these spectra is firmly established as due to chemical shifts. Except for the end atoms, there is only one pi* exciton per site. Possible excitonic character of the sigma* resonances in graphite is discussed in terms of trends found for the polyacene series of the NEXAFS spectra.