Vibronic structure of the valence π-photoelectron bands in furan, pyrrole, and thiophene

The (2)A(2) and B-2(1) states formed in the ionization of the outermost pi orbitals in furan, pyrrole and thiophene are shown to interact vibronically via nontotally symmetric b(2) vibrational modes. The interaction is strongest in pyrrole and thiophene, where the conical intersection between the two adiabatic surfaces occurs near the minimum of the upper (B-2(1)) state. The resulting nonadiabatic effects manifest themselves in the B-2(1) bands by a lack of resolved structure in case of pyrrole and thiophene, and by a line broadening in case of furan. The spectra are investigated using a linear vibronic coupling model. All totally symmetric a(1) (tuning) modes and nontotally symmetric b(2) (coupling) modes describing the ring motion are taken into account. The parameters of the model are obtained with the aid of ab initio calculations. The ground state optimized-geometries and. vibrational frequencies are computed at the level of the second-order Moller-Plesset perturbation theory, while the dependence of the ionization energies on. the nuclear configuration is evaluated using the outer valence Green's function method. Where appropriate; assignments of the observed structure are given. (C) 1998 American Institute of Physics.