A study of the valence shell electronic structure of hexafluorobenzene using photoabsorption and photoelectron spectroscopy, and TDDFT calculations

The valence shell electronic structure of hexafluorobenzene (C(6)F(6)) has been studied both experimentally and theoretically. The absolute photoabsorption cross section has been measured between 6 and 42 eV, using synchrotron radiation, and is dominated by prominent broad bands associated with intravalence transitions. In contrast, the structure due to Rydberg excitations is weak, but series have been observed converging onto the (X) over tilde (2)E(1g), (A) over tilde (2)A(2u), (B) over tilde (2)B(2u) or (C) over tilde (2)E(2g) limits. At photon energies above 13.7 eV the absorption bands become strongly perturbed, possibly as a result of Rydberg/valence state mixing, and Rydberg series can no longer be identified. The TDDFT approach has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet-singlet valence transitions, and also to obtain the excitation energies for electric-dipole forbidden and/or spin forbidden transitions. These theoretical results have allowed many of the experimentally observed bands to be assigned and have proved particularly useful in clarifying the identifications of several of the weak absorption peaks appearing below 7 eV. The valence shell threshold photoelectron spectrum has been recorded to help characterize the vibrational structure and assign autoionizing Rydberg states. Evidence has been found for the participation of the Jahn-Teller active nu(+)(17) and nu(+)(18) modes in the (X) over tilde (2)E(1g) and the (C) over tilde (2)E(2g) state photoelectron bands.