Vibrational assignments and intensity considerations of the fluorescence excitation spectra of jet-cooled S1 t r a n s-stilbene and its three isotopic analogues

Abstract

Vibrational mode assignments of S1 trans-stilbene (tSB) in a jet, particularly those for fundamental vibrational levels above the isomerization barrier, are reported with three partially isotope-substituted analogues (C6 H5 –13CH=13CH–C6 H5 , C6 H5 –CD=CD–C6 H5 , and C6 D5 –CH=CH–C6 D5 ). Vibrational levels are assigned on the basis of the frequency shifts due to isotopic substitution. The established mode assignments then enable us to discuss the individual band intensities in the fluorescence excitation spectra which bear mode specific information on the isomerization dynamics in isolated tSB molecules. Upon deuteration, the intensities of several CH deformation modes increase drastically as their frequencies become lower than the isomerization barrier. Intensities of several fundamental bands are found much smaller than those of nearby combination bands, contrary to what is expected from ordinary absorption spectra in condensed phases. The intensity ratio of the 71 (C=C stretch) to the 00 band is semiquantitatively studied with reference to a molecular-orbital calculation, the resonance Raman intensity, and the absorption intensity in a low-temperature crystal. The unusually low fluorescence excitation intensities are ascribable to large isomerization rates specific to certain kinds of fundamental vibrational levels.