Vibronic valence and Rydberg transitions in geminal chloro-fluoro-ethene (1,1-C2H2FCl): a spectroscopic and quantum chemical investigation

Abstract

The vacuum ultraviolet photoabsorption spectrum of 1,1-C2H2FCl has been examined in detail between 5 and 15 eV photon energy by using synchrotron radiation dispersed by three different monochromators. Quantum chemical calculations are performed to help in the analysis of the valence/Rydberg transition region centred at 7.05 eV including the 3a″(π)→π* and the 3a″(π)→3s Rydberg transitions. Interactions between states involving transitions to the 3s, 4d and σ* orbitals are identified. A vibrational analysis is proposed for the structures belonging to these transitions. For the π(3a″)→π* transition, one vibrational progression is observed with ω3 = 1410 ± 50 cm−1 and its lowest excitation energy is determined at about 6.398 ± 0.003 eV. The π(3a″)→3s Rydberg transition is characterised by a single progression with ω3 = 1410 ± 80 cm−1 likely starting at about 6.45 eV. These vibrations are ascribed to the C=C stretching motion. The abundant structure observed in the spectrum between 7.8 and 10.5 eV has been analysed in terms of vibronic transitions to ns (δ = 0.97), np (δ = 0.63 and 0.40) and nd (δ = 0.13 and −0.11) Rydberg states, which belong to series converging to the 1,1-C2H2FCl+() ionic ground state. The analysis of the vibrational structure of the individual Rydberg states has been attempted leading to average values of the wave numbers ω3 = 1420 ± 20 cm−1, ω7 = 720 ± 50 cm−1 and ω9 = 390 ± 50 cm−1. Between 10.5 and 12.5 eV, nine other Rydberg states converging to the 1,1-C2H2FCl+() first excited state were analysed by the same way. The vibrational structure of these Rydberg states results from the excitation of one vibrational normal mode ν7 with an average value of ω7 = 520 ± 20 cm−1, which is assigned to the C–Cl stretching vibration as inferred from quantum chemical calculations.