Carbon K Edge Structures of Molecular Crystals From First-Principles: A Comparison Between Phenanthrene and Anthracene

By means of abinitio calculations on the basis of the fplapw method, we compared the energy loss near edge structure (elnes) of carbon k edges in crystalline phenanthrene and its isomer, anthracene. in these two organic compounds, different nonequivalent carbon atoms can result in distinct k edge spectra due to the different carboncarbon bond lengths, as a characteristic behavior of the molecular crystals. the smaller bond lengths push the elnes features to the higher energies. in anthracene, the energy position of the edgeonset appears at lower energies due to its smaller electronic band gap. at the onset of the c k edge of anthracene, the strong splitting of the pi;* peak into two peaks is observable. compared to the c k edge in anthracene, due to the slightly larger c ndash;c bond length in phenanthrene, the peak position of the main sigma; structure has a red shift. the elnes spectrum of crystalline phenanthrene includes electron transition of 1s carbon orbital to pi;* and sigma;* states. in anthracene, the first two intense features have contributions of pi;* orbitals. consideration of the corehole approximation by means of supercells and the collection of semiangles at magic value are essential to obtain reasonable elnes spectra.