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Analysis of the vibronic structure of trans-Stilbene fluorescence and excitation spectra: the S0 and S1 potential energy surfaces along the Ce=Ce and Ce-Cph torsions

Abstract

We analyze the highly resolved vibronic structure of the low energy (≤200 cm-1) region of fluorescence and fluorescence excitation spectra of trans-Stilbene in supersonic beams. In this spectral region the vibronic structure is associated mainly with vibrational levels of the Ce-Ce torsion (τ) and the au combination of the two Ce-Cph bonds twisting (φ). We base the analysis on the well-established S0(τ, φ) two-dimensional potential energy surface (PES) and on a newly refined S1(τ, φ) PES. We obtain the vibrational eigenvalues and the eigenvectors of the anharmonic S0(τ, φ) and S1(τ, φ) PES by a numerical procedure based on the Meyer’s flexible model [R. Meyer, J. Mol. Spectrosc. 76, 266 (1979)]. Then we derive Franck-Condon factors and therefore intensities of the relevant vibronic bands for the S0 → S1 excitation and S1 → S0 fluorescence spectra. Furthermore, we assess the role of the bg combination of the two Ce-Cph bonds twisting (ν) in the structure of the S1 → S0 fluorescence spectra. By the use of these results we are able to assign most of the low energy vibrational levels of S0 → S1 excitation spectra and of the fluorescence spectra emitted from several low energy S1 vibronic levels. The good agreement between the observed and the computed vibrational structure of the S0→S1 and S1→S0 spectra suggests that the proposed picture of the E1(τ, φ) and E0(τ, φ) PES, in particular along the coordinate τ governing the trans-cis photo-isomerization in S1, is accurate. In S0, the barriers for the Ce=Ce torsion and for the au type Ce-Cph bond twisting are 16080 cm-1 and 3125 cm-1, respectively, while in S1, where the bond orders of the Ce=Ce and Ce-Cph bonds are reversed, the two barriers become 1350 cm-1 and 8780 cm-1, respectively.

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Publication details

The article was received on 13 Mar 2017, accepted on 08 Aug 2017 and first published on 09 Aug 2017


Article type: Paper
DOI: 10.1039/C7CP01594A
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    Analysis of the vibronic structure of trans-Stilbene fluorescence and excitation spectra: the S0 and S1 potential energy surfaces along the Ce=Ce and Ce-Cph torsions

    G. Orlandi, M. Garavelli and F. Zerbetto, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP01594A

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