Momentum-space ionization spectroscopy of iso-dichloroethylene: A comparative (e,2e) study of electron momentum distributions of the frontier-orbital and innervalence many-body states in iso-dichloroethylene and its cis and trans isomers

Abstract

The ionization energy spectra of iso-dichloroethylene (iso-C2H2Cl2) at electron momentum of ∼0 and 0.7 a.u. have been obtained by molecular (e,2e) spectroscopy in the symmetric noncoplanar geometry at 1200 eV impact energy. Momentum distributions (MDs) of selected ionic states, including the outervalence states X(3b1)−1 (frontier-orbital state) and G(6b2)−1, and the innervalence states H(9a1)−1, S1(8a1 )−1, S2(5b2)−1, and S3(7a1)−1 as well as an unassigned many-body feature, have also been determined and compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. Combined with our earlier (e,2e) results on the cis- and trans-C2H2Cl2, these MD measurements can be used to investigate the effects of isomeric changes on the valence-shell electronic structure of these disubstituted ethylene derivatives. The sensitivity of the experimental MDs to chemical bonding details can be demonstrated. In addition, ‘‘new’’ many-body features at 19–22 eV and above ∼26 eV have been observed in our ionization energy spectra, which support the breakdown of Koopmans’ theorem as predicted by an earlier Green’s function calculation. Furthermore, our MD measurements can be used to attribute the new many-body features observed at 19–22 eV to (9a1)−1 satellite states and to characterize the many-body states above ∼26 eV predominantly as (7a1)−1 satellite states.