The vacuum UV photoabsorption spectroscopy of vinyl fluoride (C2H3F): The vibrational fine structure and its analysis

Abstract

The vacuum UV photoabsorption spectrum of C 2 H 3 F 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 2 H 3 F. 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 υ 9 (CH 2 rock in-plane, FCC-bend) and υ 6 (CH 2 rock in-plane, CF stretch) calculated at 615 cm -1 in the π * state and 1315 cm -1 in the ( 2 A")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 ( 2 A")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 2 H 3 F + [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 2 H 3 F.

A photoelectron spectroscopic investigation of vinyl fluoride (C2H3F): the HeI, threshold and CIS photoelectron spectroscopy

“…[2] is fairly large and outside the uncertainty limits. The different works agree for v 9 for which ω 9 = 500 cm -1 is measured. …”

“…This wavenumber has also been detected unambiguously in the Rydberg series converging to the C 2 H 3 F + ( X 2 A") cationic state [9]. In their Hel-PES Sze et al [6] also mentioned a wavenumber of 850 cm -1 and assigned it to CH 2 deformation (see table 1 in [6]).…”

“…In all these curves only broad structures are observed. For the easiness in the interpretation and discussion, the photoabsorption spectrum of C 2 H 3 F measured in the same energy range [9] has been added to figure (figure 3(b)). …”

“…The deconvoluted Hel-PES of C 2 H 3 F above 13 eV is displayed in figure 6(a) In a recent analysis of the vacuum UV photoabsorption spectrum of C 2 H 3 F [9], several structured broad bands were observed near and above 11 eV, i.e. at 10.94 eV, 11.3 eV, 12.5 eV and 12.9 eV.…”

“…The nature of the neutral states involved in the transitions observed in the 10-22 eV photon energy has been discussed in detail (see table 1 in [9]). Several excited valence and Rydberg transitions have been identified and were assigned to the successive features observed in the PAS.…”