VUV photoprocessing of oxygen-containing polycyclic aromatic hydrocarbons: Threshold photoelectron spectra

Abstract

Oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) have been considered as potential carriers of the 11.3 μm band in interstellar observations. To begin to probe their potential contribution to chemistry in the interstellar medium, we report here threshold photoelectron spectra (TPES) for seven oxygen-containing polycyclic aromatic hydrocarbons 2,3-benzofuran (BF), 2,3-dihydrobenzofuran (DHF), phthalan (PH), dibenzofuran (DBF), dibenzo(B,F)oxepine (DBO), benzo[b]naphtho[2,3-d]furan (BNF) and benzo[b]naphtho[1,2-d]furan (cBNF). Vertical ionization energies to the ground and excited ion states were calculated with the outer-valence Green’s function method, and Franck–Condon simulations, based on harmonic (time-dependent) density functional theory results, were used to explore vibrational excitation upon ionization. CBS-QB3 calculations are also reported. Adiabatic ionization energies (IE) could be measured accurately for five of the molecules: BF (8.35 ± 0.01 eV), PH (8.76 ± 0.01 eV), DBF (8.12 ± 0.02 eV), BNF (7.64 ± 0.02 eV), and cBNF (7.62 ± 0.02 eV). For BF and cBNF, excited state optimizations also allowed Franck–Condon simulations of excited-state bands. The TPES of DHF and DBO feature broad ionization onsets due to large geometry changes upon ionization, as both become planar when an electron is removed. The band maxima yield the vertical IE for these two molecules: 8.2 ± 0.2 eV and 8.0 ± 0.2 eV, respectively, while the best estimate of the adiabatic IE for DHF is 7.98 ± 0.05 eV.