High-Resolution Near-Edge X-ray Absorption Fine Structure Study of Condensed Polyacenes

International audienceWe present a systematic study of high-resolution NEXAFS spectra of well-ordered films of condensed benzene and polyacenes, namely naphthalene, anthracene, tetracene, and pentacene. An increased spectral complexity with increasing molecular size is observed: NEXAFS features decrease in intensity and moves to lower photon energy (redshift), as the size of the aromatic system grows. Moreover, a second group of transitions arises. The dichroism in the C K-edge spectra increases with the molecular size. While benzene molecules are randomly oriented, the polyacenes are preferentially lying flat with increasing molecular size. Vibrational fine structures coupled to the C 1s π* transitions are apparent for all investigated molecules. The energy position of the onset of the first resonance decreases from 284.86 eV for benzene to 283.26 eV for pentacene. Calculations of absolute band envelopes with TDDFT followed by the analysis of the transition densities were performed for the whole series of molecules, revealing the nature of the spectroscopic features