THE PHOTOELECTRON SPECTROSCOPY OF THE DICHLOROETHYLENES: THE VICINAL ISOMER trans-1,2-HClC=CHCl. QUANTUM CHEMICAL CALCULATIONS AND EXPERIMENT

Highlights: • HeI-, NeI-, ArII-photoelectron spectra of Trans-1,2-C₂H₂Cl₂. • Threshold, Constant Ion State photoelectron spectra of Trans-1,2-C₂H₂Cl₂. • Quantum chemical calculations of electronic and vibrational states of Trans-1,2- C₂H₂Cl₂⁺. • Major importance of autoionization. The photoelectron spectrum (PES) of trans-1,2-C₂H₂Cl₂ has been measured with the resonance lines in He, Ne and Ar. These spectra are compared to the threshold photoelectron spectrum (TPES) recorded using synchrotron radiation. Nine mostly well separated photoelectron bands are observed at adiabatic ionization energies of 9.633 eV, 11.840 eV, 12.044 eV, 12.582 eV, 13.581 eV, 14.081 eV, 15.724 eV and at about 16.325 eV and 19.00 eV. Most of them exhibit a well resolved and extended vibrational structure. The He-I photoelectron band associated with the ground electronic state of the ion and the corresponding TPES band are characterized by a vibrational structure extending over 0.7 eV and 2.0 eV respectively. In the latter case the strong correlation between the TPES and the photoabsorption spectrum in the same energy region has been established. Several excited states also show an extended vibrational structure. The assignments of the electronic bands and of the vibrational wavenumbers were supported by quantum chemical calculations. The good correlation between predicted vibrational wavenumbers and the experimental values allowed us to assign most of the vibrational structures. The constant ion state (CIS) spectra have been recorded and discussed for selected vibrational levels of the ground state and for the adiabatic transition to the first excited state of the ion.