The vacuum UV photoabsorption spectrum of the geminal dichloroethylene (1,1-C2H2Cl2) in the 5–20 eV range. A vibrational analysis of the valence and Rydberg states

Abstract

The vacuum UV photoabsorption spectrum of 1,1-C2H2Cl2 is presented and discussed in detail in the 6–20 eV photon energy range. The broad band observed at 6.488 eV includes the 2b1(π) → σ*, 2b1(π) → π* valence transitions and the 2b1(π) → 3s Rydberg transitions. A quantum chemical calculation analysis is proposed and applied to the intertwined vibrational structure belonging to these transitions. For the π → σ* and π → π* transitions short vibrational progressions are observed, analyzed and tentatively assigned. The π → 3s Rydberg transition is characterized by a single progression starting at 6.746 eV. The fine structure observed between 7.5 and 10.1 eV has been analyzed in terms of vibronic transitions to ns- (δ = 0.89), np- (δ = 0.59 and 0.37) and nd-type (δ = 0.16) Rydberg states all converging to the 1,1-C2H2Cl2+(2B1) ionic ground state. The vibrational structure analysis excitation leads to the following wavenumbers: ω2 ≈ 1310 cm−1 (162 meV), ω4 ≈ 650 cm−1 (81 meV) and ω5 ≈ 290 cm−1 (37 meV). These modes can be assigned to the C=C stretching, to the symmetric C–Cl stretching and to the symmetric Cl–C–Cl bending vibrations respectively. By the same way, eight other Rydberg states were analyzed. For the first time the vacuum UV spectrum of 1,1-C2H2Cl2 has been recorded in the 10–20 eV range, revealing intense broad bands which are assigned to transitions to Rydberg states converging to excited states of 1,1-C2H2Cl2+. Assignments are also proposed for the vibrational excitations observed in this range.