Carbon K edge structures of molecular crystals from first-principles: A comparison between phenanthrene and anthracene

• Main Authors: H Nejatipour, M Dadsetani
• Format: Article
• Language: English
• Published: Isfahan University of Technology 2018-12-01
• Series: Iranian Journal of Physics Research
• Subjects: organic molecular crystals; phenanthrene; anthracene; ELNES; density functional theory
• Online Access: http://ijpr.iut.ac.ir/browse.php?a_code=A-10-293-1&slc_lang=en&sid=1

By means of ab-initio 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 non-equivalent carbon atoms can result in distinct K edge spectra due to the different carbon-carbon 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 edge-onset 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 π* peak into two peaks is observable. Compared to the  C K edge in anthracene, due to the slightly larger C–C bond length in phenanthrene, the peak position of the main σ structure has a red shift. The ELNES spectrum of crystalline phenanthrene includes electron transition of 1s carbon orbital to π* and σ* states. In anthracene, the first two intense features have contributions of π* orbitals. Consideration of the core-hole approximation by means of super-cells and the collection of semi-angles at magic value are essential to obtain reasonable ELNES spectra.