The Vacuum UV photoabsorption spectroscopy of the geminal ethylene difluoride (1,1-C2H2F2). The vibrational structure and its analysis.

The vacuum UV photoabsorption spectrum of 1,1-C2H2F2 has been examined in detail between 6 eV and 25 eV photon energy by using synchrotron radiation. The broad band observed at 7.52 eV includes the →∗ and the 2b1→3s Rydberg transitions. An analysis is proposed and applied to the fine structure belonging to these transitions. For the (2b1)→∗ transition, one long vibrational progression is observed with ω2=1 475±80 cm-1 combined with one quantum of vibration of ω4= 976±24 cm-1 and its adiabatic excitation energy is determined at 6.742 eV. The 2b1→3s Rydberg transition is characterized by a single progression with ω2=1532±80 cm-1 starting at 6.957 eV. These vibrations are ascribed to the C=C and C-F stretching motions respectively. The abundant structure observed between 8.2 eV and 11.2 eV has been analyzed in terms of vibronic transitions to ns (δ= 0.98) and two different np (δ= 0.50 and 0.31) and nd (δ= 0.13 and 0.044) Rydberg series all converging to the 1,1-C2H2F2+(B1) ionic ground state. An analysis of the associated vibrational structure of the individual Rydberg states has been attempted providing average values of the wavenumbers ω2= 1 549±16 cm-1, ω4= 839±40 cm-1 and ω5= 589±16 cm-1. Eight other Rydberg states were also analyzed. The vacuum UV spectrum of 1,1-C2H2F2 has been recorded for the first time above 11.2 eV and up to 25 eV. Several broad and strong bands are tentatively assigned to transitions to Rydberg states which are members of Rydberg series converging to excited ionic states of 1,1-C2H2F2. For several of these Rydberg states vibrational, structures are observed and assignments are proposed.