Analysis of the vibronic structure of the trans-stilbene fluorescence and excitation spectra: the S and S PES along the C[double bond, length as m-dash]C and C-C torsions.

We analyze the highly resolved vibronic structure of the low energy (≤200 cm) region of the 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 C-C torsion (τ) and the a combination of the two C-C bond twisting (ϕ). We base this analysis on the well-established S(τ, ϕ) two-dimensional potential energy surface (PES) and on a newly refined S(τ, ϕ) PES. We obtain vibrational eigenvalues and eigenvectors of the anharmonic S(τ, ϕ) and S(τ, ϕ) PES using a numerical procedure based on the Meyer's flexible model [R. Meyer, J. Mol. Spectrosc., 1979, 76, 266]. Then we derive Franck-Condon factors and therefore intensities of the relevant vibronic bands for the S → S excitation and S → S fluorescence spectra. Furthermore, we assess the role of the b combination of the two C-C bond twisting (ν) in the structure of the S → S fluorescence spectra. By the use of these results we are able to assign most of the low energy vibrational levels of the S → S excitation spectra and of the fluorescence spectra of the emission from several low energy S vibronic levels. The good agreement between the observed and the computed vibrational structure of the S → S and S → S spectra suggests that the proposed picture of the E(τ, ϕ) and E(τ, ϕ) PES, in particular along the coordinate τ governing trans-cis photo-isomerization in S, is accurate. In S, the barriers for the C[double bond, length as m-dash]C torsion and for the a type C-C bond twisting are 16 080 cm and 3125 cm, respectively, while in S, where the bond orders of the C[double bond, length as m-dash]C and C-C bonds are reversed, the two barriers become 1350 cm and 8780 cm, respectively.