The Vacuum UV Photoabsorption Spectrum of 1,1-Dibromo-2,2-Difluoroethylene (1,1-Br2C2F2) in the 5-15 eV Photon Energy Range. Combining Experimental and Theoretical Investigations.

Using synchrotron radiation, we have measured the vacuum UV photoabsorption spectrum (PAS) of 1,1-Br2C2F2 for the first time between 5 eV and 15 eV photon energy. Quantum chemical calculations have been performed for the determination of excitation and ionization energies. At low energy a weak broad band spreads between 5.2 eV and 6.0 eV and is assigned to nBr→Rpz (31B2) valence transition. A broad continuum starting at 6.18 eV corresponds to the (3b1)→5s (41B1) Rydberg transition. Starting at Eexcad=6.595 eV a group of four narrower peaks likely corresponds to the vibrational excitation of the characteristic (3b1)→*(71A1) valence transition. The features evenly spaced by 1315 cm-1 are assigned to the C=C stretching vibration. Between 7.0 eV and 11.5 eV, several weak or very strong sharp features are observed. The 3b1→5p Rydberg transition is characterized by a short progression starting at 7.208 eV. A longer progression, assigned to 3b1→4d Rydberg transition, is observed up from 7.932 eV. Both Rydberg states converge to the 2B1 ionic ground state calculated at IEad=9.40 eV with quantum defects δ=2.51 and δ=0.96 respectively. The vibrational analysis provides vibrational wavenumbers ω1=1525±20 cm-1, ω2=975±40 cm-1, ω3=620±40 cm-1, ω4=355±20 cm-1 and ω5=185±30 cm-1 averaged over the 5p and 4d Rydberg states. An interaction has likely to be considered between the 6s and 4d Rydberg states. Above 11.5 eV, several strong sharp and broad bands are tentatively assigned to transitions to Rydberg states converging to excited ionic states of 1,1-Br2C2F2+. For one of these a vibrational structure is observed and a tentative assignment is proposed.