Excited state structure and femtosecond ring‐opening dynamics of 1,3‐cyclohexadiene from absolute resonance Raman intensities

Resonance Raman spectra of 1,3‐cyclohexadiene have been obtained with excitation wavelengths ranging from 300 to 253 nm. The absolute excitation profiles were analyzed to determine the initial dynamics that occur following photoexcitation in the lowest energy 260 nm 1B2 absorption band. The initial atomic displacements parallel the Woodward–Hoffmann, symmetry‐allowed, ring‐opening pathway towards all‐cis‐hexatriene. The potential surface displacements in the optically prepared 1B2 excited state are, however, a small fraction of the total distance along the reaction coordinate. The low absolute scattering cross sections indicate an excited state dephasing time of 10−14 s, and the fluorescence quantum yield of 2×10−6 shows that the total dephasing rate is dominated by ultrafast internal conversion. The Gaussian homogeneous line shape indicates a nonexponential excited electronic state population decay. We attribute these observations to an allowed crossing of the 1B2 state with a second excited 2A1 state th...