fused ring aromatics

J. Phys. Chem A 107 (2003) 8512 - 8520

Inner-Shell Excitation Spectroscopy of Fused-Ring Aromatic Molecules by Electron Energy Loss and X-ray Raman Techniques

M.L. Gordon (1), D.Tulumello (1), G. Cooper (1), A.P. Hitchcock (1), P. Glatze (2), O.C. Mullins (3), S.P. Cramer (4) and U.. Bergmann (4)

1.Department of Chemistry, McMaster University, Hamilton, ON, L8S 4M1, Canada,
2.Department of Chemistry, Utrecht University, 3584 CA Utrecht, The Netherlands,
3. Schlumberger-Doll Research, Ridgefield, Connecticut 06877,
4. Department of Applied Science, University of California, Davis, California 95616, and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720

Received: June 6, 2003; In Final Form: August 6, 2003

Abstract—Oscillator strengths for C 1s excitation spectra of gaseous benzene, naphthalene, anthracene, phenanthracene, triphenylene, pyrene, and 1,2-benzanthracene have been derived from inner-shell electron energy loss spectroscopy recorded under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, ~2° corresponding to a product of momentum transfer and C 1s orbital size (qr) of 0.08). These spectra are interpreted with the aid of ab initio calculations on selected species. They are compared to the C 1s spectra of solid samples of benzene, naphththalene, anthracene, triphenylene, and 1,2-benzanthracene, recorded with inelastic X-ray Raman scattering in the dipole limit (qr < 0.5). When differences in resolution are taken into account, good agreement is found between the inelastic electron scattering spectra of the gases and the inelastic photon scattering spectra of the corresponding solid. Small differences are attributed to quenching of transitions to Rydberg states in the solids. Characteristic differences related to the degree of symmetry or spatial arrangement of the fused ring aromatic hydrocarbons (e.g., linear versus bent structures) indicate that C 1s X-ray Raman spectroscopy should be useful for characterizing aromatics in bulk samples that are opaque to soft X-rays, such as coals and heavy hydrocarbon deposits.

Keywords: fused ring aromatics, NEXAFS, X-ray Raman, electric dipole excitation, electron and photon inelastic scattering