THE VACUUM ULTRAVIOLET PHOTOABSORPTION SPECTROSCOPY OF 1-BROMO-2,2-DIFLUOROETHYLENE (BrHC=CF2). A JOINT EXPERIMENTAL AND QUANTUM CHEMICAL INVESTIGATION.

The vacuum UV photoabsorption spectrum of 1-bromo-2,2-difluoro-ethylene has been observed using synchrotron radiation. It is measured and analyzed between 5 eV (40300 cm-1) and 15 eV (121000 cm-1) for the first time. Several very weak features are observed at low photon energy, between 5.0 eV and 6.5 eV, and assigned to valence transitions. To support the assignment of these features, quantum chemical calculations have been performed on the excitation energies of the neutral states of BrHC=CF2. The characteristic CC(4a”)→CC*(a”) valence transition for the ethylene derivatives is observed at 6.955 eV. The detected vibrational structure is assigned to ν2 (C=C stretching) and ν6 (C-Br stretching) vibrations with vibrational wavenumbers ω2=1550±60 cm-1 (192±7 meV) and ω6=790±60 cm-1 (98±7 meV). Between 7.4 eV and 9.8 eV numerous weak but sharp transitions are observed, classified and assigned to CC(4a”)→np (n=5-10, λ=0,1) and nd (n=4-7) Rydberg transitions converging to IEad( 2A")=9.695 eV [5]. Their vibrational structure has been analyzed and assigned to ν2 (C=C stretching), ν6 (C-Br stretching) and ν8 (CF2 rocking) vibrational normal modes. Averaging over all the npλ and nd Rydberg states, the vibrational wavenumbers 2= 1520±20 cm-1 (188±3 meV), 6=770±20 cm-1 (95±3 meV) and 8=363±20 cm-1 (45±3 meV) are obtained. The 9.2 eV to 11.0 eV region is dominated by strong and sharp Rydberg transitions converging to IEad( 2A’)=11.362 eV [5]. Transitions from nBr(11a’)→ns (n=5-9), np (n=5-9) and nd (n=4-9, λ=0,1) are observed. Their analysis shows the excitation of ν4 (C-F symmetric stretching) and ν8 (CF2 rocking) vibrational normal modes. These are characterized by averaged vibrational wavenumbers 4=1125±40 cm-1 (139±5 meV) and 8=366±40 cm-1 (45±5 meV). Above 11.0 eV and up to 15.0 eV sharp strong absorption is followed by long series of weak modulations superimposed on strong broad bands. An analysis and interpretations of these features have been attempted in terms of Rydberg transitions converging to the successive vertical ionization energies at 12.59 eV, 14.48 eV, 15.62 eV and 16.14 eV [5]. In the former case a long vibrational progression is assigned to 7=508±20 cm-1 (63±3 meV) combined with 9=130±16 cm-1 (16±2 meV). In the three latter cases, the weak modulations have mainly been assigned to ν8 (CF2 rocking) vibrations with 8=325±30 cm-1 (45±4 meV).