The vacuum UV photoabsorption spectroscopy of the cis-1,2-dichloroethylene ((1,2-ClHC=CHCl) in the 5-20 eV range. An experimental and theoretical investigation

The photoabsorption spectrum of cis-1,2-C2H2Cl2 has been examined in detail in the vacuum UV range between 5 eV and 20 eV photon energy by using synchrotron radiation. Quantum chemical calculations are proposed and applied to the electronic transitions and to the vibrational structures belonging to these transitions. The broad band observed at 6.568 eV includes the 1A1→11B1, 31B2 and 21B1 transitions. The two latter excitations correspond to the valence 2b1()→* and to the Rydberg 2b1()→3s transitions. The former excitation is described by a more complex 2b1()→nCl +σ*CH/Rpσ transition where valence and Rydberg characters are strongly mixed. For these transitions short vibrational progressions are observed, analyzed and tentatively assigned. The abundant structure observed between 7.0 eV and 10.0 eV has been analyzed in terms of vibronic transitions to one ns- ( = 0.960), two np- ( = 0.525 and 0.337), and two nd-type ( = 0.080 and 0.002) Rydberg series, all converging to the 2B1 ionic ground state. The vibrational structure analysis of the Rydberg states leads to the following average wave numbers: ω2 ≈ 1420 cm-1 (C=C stretching), ω3 ≈ 1190 cm-1 (symmetric C-H bending), ω4 ≈ 800 cm-1 (symmetric C-Cl stretching) and ω5 ≈ 190 cm-1 (symmetric C-Cl bending). These numbers are compared to previously reported values. Many other transitions are observed between 10 eV and 20 eV and are assigned to transitions to Rydberg states converging to the successive excited states of cis-1,2-C2H2Cl2+. For several of these Rydberg states, a vibrational structure is also observed and interpreted.