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We report the experimental resonance enhanced multiphoton ionization spectrum of isoquinoline between 315 and 310 nm, along with correlated electronic structure calculations on the ground and excited states of this species. This spectral region spans the origin transitions to a π–π* excited state, which previous work has suggested to be vibronically coupled with a lower lying singlet n–π* state. Our computational results corroborate previous density functional theory calculations that predict the vertical excitation energy for the n–π* state to be higher than the π–π* state; however, we find an increase in the C–N–C angle brings the n–π* state below the energy of the π–π* state. The calculations find two out-of-plane vibrational modes of the n–π* state, which may be brought into near resonance with the π–π* state as the C–N–C bond angle increases. Therefore, the C–N–C bond angle may be important in activating vibronic coupling between the states. We fit the experimental rotational contour with a genetic algorithm to determine the excited state rotational constants and orientation of the transition dipole moment. The fits show a mostly in-plane polarized transition, and the projection of the transition dipole moment in the a-b plane is about 84° away from the a axis. These results are consistent with the prediction of our electronic structure calculations for the transition dipole moment of the π–π* excited state. 

We report results from experiments with the quinoline-O₂ complex, which was photodissociated using light near 312 nm. Photodissociation resulted in formation of the lowest excited state of oxygen, O₂ a ¹Δ_g, which we detected using resonance enhanced multiphoton ionization and velocity map ion imaging. The O₂⁺ ion image allowed for a determination of the center-of-mass translational energy distribution, P(E_T), following complex dissociation. We also report results of electronic structure calculations for the quinoline singlet ground state and lowest energy triplet state. From the CCSD/aug-cc-pVDZ//(U)MP2/cc-pVDZ calculations, we determined the lowest energy triplet state to have ππ* electronic character and to be 2.69 eV above the ground state. We also used electronic structure calculations to determine the geometry and binding energy for several quinoline-O2 complexes. The calculations indicated that the most strongly bound complex has a well depth of about 0.11 eV and places the O₂ moiety above and approximately parallel to the quinoline ring system. By comparing the experimental P(E_T) with the energy for the singlet ground state and the lowest energy triplet state, we concluded that the quinoline product was formed in the lowest energy triplet state. Finally, we found the experimental P(E_T) to be in agreement with a Prior translational energy distribution, which suggests a statistical dissociation for the complex.