The vacuum UV photoabsorption spectroscopy of vinyl fluoride (C 2H 3F): The vibrational fine structure and its analysis

Abstract

The vacuum UV photoabsorption spectrum of C 2H 3F has been examined in detail between 6 eV and 25 eV photon energy by using synchrotron radiation. The analysis of the data is supported by ab initio quantum mechanical calculations applied to valence and Rydberg excited states of C 2H 3F. At 7.6 eV the π → π ∗ and the 2a″ → 3s transitions are observed. An analysis is proposed and applied to the mixed fine structure belonging to these transitions. For the π → π ∗ transition one single long vibrational progression is observed with hcω e = 95 ± 7 meV (766 ± 56 cm −1) and its adiabatic excitation energy is 6.892 eV (55 588 cm −1). The 2a″ → 3s transition is characterized by a single short progression with hcω e = 167 ± 10 meV (1350 ± 80 cm −1) starting at 6.974 eV (56 249 cm −1). From the present ab initio calculations these two wavenumbers best correspond to the vibrational modes v 9 (CH 2 rock in-plane, FCC-bend) and v 6 (CH 2 rock in-plane, CF stretch) calculated at 615 cm −1 in the π ∗ state and 1315 cm −1 in the ( 2A″)3s Rydberg state respectively. The C C stretching could not be excluded. The dense structured spectrum observed between 8.0 eV and 10.5 eV has been analyzed in terms of vibronic transitions to Rydberg states all converging to the C 2 H 3 F + ( X ∼ A ″ ) ionic ground state. An analysis of the associated complex fine structure of the individual Rydberg states has been attempted providing average values of the wavenumbers, e.g., for the ( 2A″)3p Rydberg state hcω 9 = 60 ± 1 meV (or 484 ± 8 cm −1), hcω 7 = 151 ± 7 meV (or 1218 ± 60 cm −1), hcω 4 = 191 ± 3 meV (or 1540 ± 24 cm −1). The assignment of hcω = 105 ± 5 meV (or 823 ± 40 cm −1) is discussed. These experimental values are in good agreement with the theoretical predictions for C 2H 3F + [R. Locht, B. Leyh, D. Dehareng, K. Hottmann, H. Baumgärtel, Chem. Phys. (in press)]. Above 10.5 eV and up to 25 eV several broad and strong bands are tentatively assigned to transitions to valence (V–V) and/or Rydberg (V–R) states converging to excited ionic states of C 2H 3F.