An experimental and theoretical study of the photoelectron spectra of cis-dichloroethene: Valence shell vertical ionization and vibronic coupling in the low-lying cationic states

Abstract

The valence shell photoelectron spectrum of -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 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 theB state photoelectron band, the B, A, and A states interact vibronically and form a complex photoelectron band system with four distinct maxima. The B and B states also interact vibronically with each other. The potential energy surface of the B state is predicted to have a double-minimum shape with respect to the out-of-plane 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 theB state.

Photoionization dynamics of cis-dichloroethene from investigation of vibrationally resolved photoelectron spectra and angular distributions

“…The influence of vibronic coupling, which we therefore infer from our experimental results, has been examined theoretically in a companion paper. 12 These calculations predict conical intersections between the potential energy surfaces associated with theà 2 B 2 andB 2 A 1 states and those associated with theB 2 A 1 andC 2 A 2 states, respectively, at 11.87 and 12.39 eV. These results go some way to corroborate that the low binding energy region of theà 2 B 2 state photoelectron band might be interpreted within the adiabatic approximation.…”

“…The vibrational structure is analyzed at the Franck-Condon level of approximation with the help of simulations of the vibrational envelope, and the photoionization dynamics are likewise modeled using electron multiple scattering calculations. 11 In a companion paper, 12 we give a more complete theoretical treatment of the vibronic interactions coupling theà 2 B 2 ,B 2 A 1 andC 2 A 2 states and theD 2 B 1 andẼ 2 B 2 states that can be inferred from the present results and analysis. Consideration of the inner valence region is also deferred to that paper.…”

“…Figure 2 shows an overview of the outer valence shell photoelectron spectrum of cis-dichloroethene obtained at a photon energy of 20.5 eV. The calculations reported by Trofimov et al 12 find that at binding energies below 14.58 eV the single particle model of ionization 42 is adequate; theF 2 A 1 band may, in contrast, contain contributions from satellite states. Consequently, the lowest six bands,X-Ẽ, will receive most attention in the following discussion.…”

“…However, this is effectively a long extrapolation back down the ladder of harmonic oscillator levels, and so its reliability may be questioned. In our companion paper, 12 we obtain by calculation an alternative estimate of 11.43 eV for theà 2 B 2 state adiabatic ionization energy.…”

“…Such procedures often provide a satisfactory overall interpretation of the vibrational structure associated with isolated electronic states, but, as will be shown, more sophisticated methods are required for perturbed states. 12…”