Vibrational spectra of cis and trans m-fluorostyrene in the S1 state studied by one color resonant two photon ionization and theoretical calculations

The optimized molecular structures of the cis and trans rotamers of m-fluorostyrene in the ground S0 and excited S1 states are searched by ab initio and density functional theory (DFT) calculations. Their vibrational spectra in the S1 state are studied by one color resonant two photon ionization (1C-R2PI) method. In consideration of the optimized structural geometries, the interaction of the vinyl group with the ring is stronger in the S1 state than that in the S0 state, and the C1-Cα bond exhibits a partial double bond character in the S1 state. Most of the active vibrations assigned from the R2PI spectrum are found to be the in-plane ring modes. Analysis on the vibational frequencies shows that there is an increase for the C1-Cα stretching, and a decrease for the CC stretching of the vinyl group in the S1 state with respect to those in the S0 state.