The ground and ionized states of azulene: a combined study of the vibrational energy levels by photoionization, configuration interaction, and density functional calculations

A synchrotron-based photoionization spectrum of azulene shows significant additional vibrational fine structure when compared to previous studies. This spectrum was successfully analysed by Franck-Condon (FC) methods. Previously reported zero-kinetic-energy electron spectra (ZEKE) for azulene, have been reinterpreted in FC terms, leading to some alternative assignments to the earlier work. The sequence of ionic states has been determined by ab initio configuration interaction (CI) methods, leading to reliable theoretical values for both the calculated adiabatic and vertical ionization energies (AIE and VIE respectively). VIE were calculated by both symmetry-adapted cluster (SAC-CI), together with Green’s function (GF) and Tamm Dancoff approximation (TDA), single excitation CI methods; AIE for highest states of each symmetry, were determined by open-shell self-consistent field (SCF) methods at the restricted Hartree-Fock level. Complete active space SCF(CASSCF) was used for the pairs of states 1²A₂ + 2²A₂ and 1²B₁ + 2²B₁ each of which occurs as antisymmetric and symmetric (higher energy) combinations. The combined ionic state sequences (AIE and VIE) from these methods are 1²A₂ < 1²B₁ < 2²A₂ < 2²B₁. The PES shows a series of broad bands above 11 eV, each of which are attributed to more than one ionisation. The calculated PES sequence of states up to 19 eV shows the SAC-CI and GF results are in almost exact agreement. The internal spacing of the bands is best reproduced by the simpler GF and TDA methods. States involving simultaneous ionization and electronic excitation are considered by both SAC-CI and TDA methods.