Abstract
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 2B2, 2A1, 2A2, and 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 2B1, 2B2, and 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 2B1 state band but appear less successful for the 2B2 and 2B1 states, with irregularities appearing in both. The 2A1 and 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 2B2 band at the onset of the perturbed peak intensities, with β evidently adopting the value of the 2A1 band β. The 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.
Highlights
The three isomers of dichloroethene (C2H2Cl2) provide an ideal means of studying substitution effects on the valence shell electronic structure and photoionization dynamics
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
Previous experimental investigations on cis-dichloroethene include photoelectron spectra recorded with HeI 22-26 HeII,[26] Al Kα,[27] and synchrotron[28] radiation, mass analysed threshold ionization (MATI)[29] and pulsed field ionization photoelectron (PFI-PE)[30] spectra of the ground ionic state, and electron momentum spectroscopy.[31]
Summary
The three isomers (cis, trans and iso) of dichloroethene (C2H2Cl2) provide an ideal means of studying substitution effects on the valence shell electronic structure and photoionization dynamics. The Cooper minimum[15,16] associated with a Cl 3p orbital[17] will, in the atomic limit, result in a fairly isotropic photoelectron angular distribution and a reduction in the partial cross section in the photon energy range around 40 eV Previous experimental investigations on cis-dichloroethene include photoelectron spectra recorded with HeI 22-26 HeII,[26] Al Kα,[27] and synchrotron[28] radiation, mass analysed threshold ionization (MATI)[29] and pulsed field ionization photoelectron (PFI-PE)[30] spectra of the ground ionic state, and electron momentum spectroscopy.[31] Fragmentation studies have been performed.[32,33] Theoretical predictions for the orbital binding energies[25,26,34] and the valence shell photoelectron spectra[26,34,35] have been obtained. Such procedures often provide a satisfactory overall interpretation of vibrational structure associated with isolated electronic states, but, as will be shown, more sophisticated methods are required for perturbed states.[12]
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