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The influence of vibronic coupling on the outer valence ionic states of cis -dichloroethene has been investigated by recording photoelectron spectra over the excitation range 19-90 eV using plane polarized synchrotron radiation, for two polarization orientations. The photoelectron anisotropy parameters and electronic state branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that the photoionization dynamics of the à 2 B 2 , B ̃ 2 A 1 , C ̃ 2 A 2 , and D ̃ 2 B 1 states, all of which are formed through the ejection of an electron from a nominally chlorine lone-pair orbital, exhibit distinct evidence of the Cooper minimum associated with the halogen atom. While retaining a high degree of atomic character, these orbital ionizations nevertheless display clear distinctions. Simulations, assuming the validity of the Born-Oppenheimer and the Franck-Condon approximations, of the X ̃ 2 B 1 , à 2 B 2 , and D ̃ 2 B 1 state photoelectron bands have allowed some of the vibrational structure observed in the experimental spectra to be assigned. The simulations provide a very satisfactory interpretation for the X ̃ 2 B 1 state band but appear less successful for the à 2 B 2 and D ̃ 2 B 1 states, with irregularities appearing in both. The B ̃ 2 A 1 and C ̃ 2 A 2 state photoelectron bands exhibit very diffuse and erratic profiles that cannot be reproduced at this level. Photoelectron anisotropy parameters, β, have been evaluated as a function of binding energy across the studied photon energy range. There is a clear step change in the β values of the à 2 B 2 band at the onset of the perturbed peak intensities, with β evidently adopting the value of the B ̃ 2 A 1 band β. The D ̃ 2 B 1 band β values also display an unexpected vibrational level dependence, contradicting Franck-Condon expectations. These various behaviors are inferred to be a consequence of vibronic coupling in this system.

The valence shell photoelectron spectrum of cis -dichloroethene has been studied both experimentally and theoretically. Photoelectron spectra have been recorded with horizontally and vertically plane polarized synchrotron radiation, thereby allowing the anisotropy parameters, characterizing the angular distributions, to be determined. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function has been employed to compute the complete valence shell ionization spectrum. In addition, the vertical ionization energies have been calculated using the outer valence Green's function (OVGF) method and the equation-of-motion coupled-cluster, with single and double substitutions for calculating ionization potentials (EOM-IP-CCSD) model. The theoretical results have enabled assignments to be proposed for most of the structure observed in the experimental spectra, including the inner-valence regions dominated by satellite states. The linear vibronic coupling model has been employed to study the vibrational structure of the lowest photoelectron bands, using parameters obtained from ab initio calculations. The ground state optimized geometries and vibrational frequencies have been computed at the level of the second-order Møller-Plesset perturbation theory, and the dependence of the ionization energies on the nuclear configuration has been evaluated using the OVGF method. While the adiabatic approximation holds for the X ̃ 2 B 1 state photoelectron band, the A ̃ 2 B 2 , B ̃ 2 A 1 , and C ̃ 2 A 2 states interact vibronically and form a complex photoelectron band system with four distinct maxima. The D ̃ 2 B 1 and E ̃ 2 B 2 states also interact vibronically with each other. The potential energy surface of the D ̃ 2 B 1 state is predicted to have a double-minimum shape with respect to the out-of-plane a 2 deformations of the molecular structure. The single photoelectron band resulting from this interaction is characterized by a highly irregular structure, reflecting the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces forming a conical intersection close to the minimum of the E ̃ 2 B 2 state.