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 (X) over tilde (1)A(1)-> 1(1)B(1), 3(1)B(2) and 2(1)B(1) transitions. The two latter excitations correspond to the valence 2b(1)(pi)->pi* and to the Rydberg 2b(1)(pi)-> 3s transitions. The former excitation is described by a more complex 2b(1)(pi)-> n(Cl) + sigma* CH/R sigma 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- ((delta) over bar = 0.960), two np- ((delta) over bar = 0.525 and 0.337), and two nd-type ((delta) over bar = 0.080 and 0.002) Rydberg series, all converging to the (X) over tilde B-2(1) ionic ground state. The vibrational structure analysis of the Rydberg states leads to the following average wave numbers: omega(2) approximate to 1420 cm(-1) (C= C stretching), omega(3) approximate to 1190 cm(-1) (symmetric C-H bending), omega(4) approximate to 800 cm(-1) (symmetric C-Cl stretching) and omega(5) approximate to 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. (C) 2019 Author(s).