The vacuum UV photoabsorption spectrum of the geminal dichloroethylene (1,1-C₂H₂Cl₂) in the 5–20 eV range. A vibrational analysis of the valence and Rydberg states

Abstract

The vacuum UV photoabsorption spectrum of 1,1-C 2 H 2 Cl 2 is presented and discussed in detail in the 6-20 eV photon energy range. The broad band observed at 6.488 eV includes the 2b 1 (π)→σ * , 2b 1 (π)→π * valence transitions and the 2b 1 (π)→3s Rydberg transitions. A quantum chemical calculation analysis is proposed and applied to the intertwined vibrational structure belonging to these transitions. For the π→σ * and π→π * transitions short vibrational progressions are observed, analyzed and tentatively assigned. The π→3s Rydberg transition is characterized by a single progression starting at 6.746 eV. The fine structure observed between 7.5 and 10.1 eV has been analyzed in terms of vibronic transitions to ns-(δ=0.89), np-(δ=0.59 and 0.37) and nd-type (δ=0.16) Rydberg states all converging to the 1,1-C 2 H 2 Cl 2 + (X 2 B 1 ) ionic ground state. The vibrational structure analysis excitation leads to the following wavenumbers: ω 2 ≈1310 cm −1 (162 meV), ω 4 ≈650 cm −1 (81 meV) and ω 5 ≈290 cm −1 (37 meV). These modes can be assigned to the C=C stretching, to the symmetric C-Cl stretching and to the symmetric Cl-C-Cl bending vibrations respectively. By the same way, eight other Rydberg states were analyzed. For the first time the vacuum UV spectrum of 1,1-C 2 H 2 Cl 2 has been recorded in the 10-20 eV range, revealing intense broad bands which are assigned to transitions to Rydberg states converging to excited states of 1,1-C 2 H 2 Cl 2 + . Assignments are also proposed for the vibrational excitations observed in this range.One of the latest vacuum UV photoabsorption spectra of the same molecule, in the same spectral region, has been reported by Walsh et al [6]. Besides a revision of the Rydberg series classification, a vibrational analysis and a corrected value of the lowest ionization energy have been presented. In a work on the photochemistry of chloroethylenes, Berry [7] measured the vacuum UV absorption spectrum of 1,1-C 2 H 2 Cl 2 between 260 and 140 nm (4.77-8.86 eV) but limited his analysis to the broad band between 240 and 180 nm (5.17-6.89 eV).The present paper focuses on the vacuum UV photoabsorption spectrum of 1,1-C 2 H 2 Cl 2 (i) in the still unexplored range between 11.3 and 20 eV with medium resolution and (ii) in the 5.0-13.0 eV spectral region at higher resolution. The vibrational structure observed in the latter region will be analyzed.

The photoelectron spectroscopy of the dichloroethylenes: the geminal isomer 1,1-Cl2C2H2. An experimental and quantum chemical study

“…It may also be related to the resonance observed at 16.1 eV in the PAS. This resonance shows an asymmetric profile in the absorption spectrum [8], which might be interpreted as a Fano profile. The decrease of the state-resolved ionization cross sections (CIS spectra) at this energy is compatible with a partial population transfer from the continuum to the discrete state, a situation linked to the interference between the continuum and Rydberg wave functions and which is expected to give rise to Fano profiles.…”

“…Above this energy it increases as do the CIS spectra recorded at 9.905, 9.986, 10.076 and 10.400 eV which all show the same autoionizing contributions. These are identified in the vacuum UV PAS [8] and shown in table 7. In the HeI-PES these vibrations are all populated by direct Franck-Condon transition from the neutral ground state of the molecule.…”

“…Above 14 eV, we may infer from the shape of the CIS spectra that the population of theD -Ẽ B 2 2 2 B 1 andF -G A 1 2 2 B 2 states has to occur resonantly or nearly resonantly through the autoionization of highly excited (Rydberg) states of the neutral molecule. For clarity of this discussion, the Δ-plot of the vacuum UV PAS of 1,1-C 2 H 2 Cl 2 is reproduced in figure 5(b) [8]. Both converge to theC 2 A 1 ionic state.…”

“…It seems that, in this energy region, autoionization strongly perturbs the regular progression which was observed by HeI-PES. The energy range of 13.5-17.3 eV corresponds to several broad absorption bands of 1,1-C 2 H 2 Cl 2 , with strong to weak intensities [8] (see figure 5(b)).…”

“…In an ongoing work on 1,1-C 2 H 2 Cl 2 , its vacuum UV PAS [8] and photoelectron spectrum have been remeasured. These data are complemented by the measurements of the NeI-and ArII-PES.…”