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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 ̃ 2B1 state photoelectron band, the A ̃ 2B2, B ̃ 2A1, and C ̃ 2A2 states interact vibronically and form a complex photoelectron band system with four distinct maxima. The D ̃ 2B1 and E ̃ 2B2 states also interact vibronically with each other. The potential energy surface of the D ̃ 2B1 state is predicted to have a double-minimum shape with respect to the out-of-plane a2 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 ̃ 2B2 state. [ABSTRACT FROM AUTHOR]

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 2B2, B ̃ 2A1, C ̃ 2A2, and D ̃ 2B1 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 ̃ 2B1, à 2B2, and D ̃ 2B1 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 ̃ 2B1 state band but appear less successful for the à 2B2 and D ̃ 2B1 states, with irregularities appearing in both. The B ̃ 2A1 and C ̃ 2A2 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 2B2 band at the onset of the perturbed peak intensities, with β evidently adopting the value of the B ̃ 2A1 band β. The D ̃ 2B1 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. [ABSTRACT FROM AUTHOR]

The valence shell photoelectron spectra of 2-chloropyridine and 3-chloropyridine have been studied both experimentally and theoretically. Synchrotron radiation has been employed to record angle resolved photoelectron spectra in the photon energy range 20-100 eV, and these have enabled anisotropy parameters and branching ratios to be derived. The experimental results have been compared with theoretical predictions obtained using the continuum multiple scattering Xα approach. This comparison shows that the anisotropy parameter associated with the nominally chlorine lone-pair orbital lying in the molecular plane is strongly affected by the atomic Cooper minimum. In contrast, the photoionization dynamics of the second lone-pair orbital, orientated perpendicular to the molecular plane, seem relatively unaffected by this atomic phenomenon. The outer valence ionization has been studied theoretically using the third-order algebraic-diagrammatic construction (ADC(3)) approximation scheme for the one-particle Green's function, the outer valence Green's function method, and the equation-of-motion (EOM) coupled cluster (CC) theory at the level of the EOM-IP-CCSD and EOM-EE-CC3 models. The convergence of the results to the complete basis set limit has been investigated. The ADC(3) method has been employed to compute the complete valence shell ionization spectra of 2-chloropyridine and 3-chloropyridine. The relaxation mechanism for ionization of the nitrogen σ-type lone-pair orbital (σN LP) has been found to be different to that for the corresponding chlorine lone-pair (σCl LP). For the σN LP orbital, π-π* excitations play the main role in the screening of the lone-pair hole. In contrast, excitations localized at the chlorine site involving the chlorine πCl LP lone-pair and the Cl 4p Rydberg orbital are the most important for the σCl LP orbital. The calculated photoelectron spectra have allowed assignments to be proposed for most of the structure observed in the experimental spectra. The theoretical work also highlights the formation of satellite states, due to the breakdown of the single particle model of ionization, in the inner valence region. [ABSTRACT FROM AUTHOR]

The valence shell ionization spectrum of pyridine was studied using the third-order algebraicdiagrammatic construction approximation scheme for the one-particle Green's function and the outer-valence Green's function method. The results were used to interpret angle resolved photoelectron spectra recorded with synchrotron radiation in the photon energy range of 17-120 eV. The lowest four states of the pyridine radical cation, namely, 2A2(1a2-1), 2A1(7a1-1), 2B1(2b1-1) and 2B2(5b2-1), were studied in detail using various high-level electronic structure calculation methods. The vertical ionization energies were established using the equation-of-motion coupled-cluster approach with single, double and triple excitations (EOM-IP-CCSDT) and the complete basis set extrapolation technique. Further interpretation of the electronic structure results was accomplished using Dyson orbitals, electron density difference plots and a second-order perturbation theory treatment for the relaxation energy. Strong orbital relaxation and electron correlation effects were shown to accompany ionization of the 7a1 orbital, which formally represents the nonbonding ó-type nitrogen lone-pair (nó) orbital. The theoretical work establishes the important roles of the л-system (л-л* excitations) in the screening of the nó-hole and of the relaxation of the molecular orbitals in the formation of the 7a1(nó) 1 state. Equilibrium geometric parameters were computed using the MP2 (second-order Møller-Plesset perturbation theory) and CCSD methods and the harmonic vibrational frequencies were obtained at the MP2 level of theory for the lowest three cation states. The results were used to estimate the adiabatic 0-0 ionization energies, which were then compared to the available experimental and theoretical data. Photoelectron anisotropy parameters and photoionization partial cross sections, derived from the experimental spectra, were compared to predictions obtained with the continuum multiple scattering approach. [ABSTRACT FROM AUTHOR]