title: A combined theoretical and experimental study of the ionic states of iodopentafluorobenzene.
abstract: A new synchrotron radiation photoelectron spectral (PES) study of iodopentafluorobenzene, together with a theoretical analysis of the spectrum, where Franck-Condon factors are discussed, gives detailed insight into the ionization processes, and this exposes the need for a reinvestigation of the vacuum ultraviolet spectral (VUV) assignments. We have calculated adiabatic ionization energies (AIEs) for several ionic states, using the equation-of-motion coupled cluster method for ionic states combined with multi-configuration self-consistent field calculation study. The AIE sequence is: X2B1< A2A2< B2B2< C22B1< D2A1< E32B1. This symmetry sequence has a major impact on previous VUV spectral assignments, which now appear to be to optically forbidden states. Changes in the equilibrium structures for these ionic states are relatively small, but a significant decrease and increase in the C-I bond length relative to the X1A1structure occurs for the X2B1and C2B1states, respectively. The PES shows major vibrational overlaps between pairs of ionic states, X with A, and A with B. The result of these overlaps is the loss of vibrational structure and considerable broadening of the higher energy PES state. Although the baseline is nearly re-established between the A and B states, where the two bands are nearly separate, the B state is also broadened by the A state. Only the C ionic state, which shows the most highly developed vibrational structure, can be regarded as free from vibrational coupling to a neighbor state. The Franck-Condon analysis of the PES bands X, A, B, and C is described in detail; the apparent simplicity of some of these bands is illusory, since almost all the observed peaks arise from super-position of several calculated vibrational states. The experimental AIE of the A state, which is submerged under the X state envelope, has been determined by the subtraction of the calculated X state envelope from the observed PES spectrum. The overlap of these PES bands and the apparent closeness of the potential energy curves describing them have been investigated, using the state-averaged, complete active space self-consistent field method. We have identified two structures, one where the potential energy curves for the X and A states cross and another for the A and B states. At these two conical intersections (ConInts), there is zero-energy difference within each pair of states. Although similar in energy, the ConInt for the crossing of the X with A states, and that for the A with B states, shows that the open-shell occupancies correspond to the 4 lowest AIE states, and all four states that are quite different from each other.
authors.affiliation: School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, Scotland, United Kingdom.
authors.forename: Michael H
authors.initials: MH
authors.lastname: Palmer
authors.affiliation: CNR-IMIP, Montelibretti, c/o Laboratorio Elettra, Trieste, Italy.
authors.forename: Marcello
authors.identifier: 000000034376808X
authors.initials: M
authors.lastname: Coreno
authors.affiliation: CNR-IOM Laboratorio TASC, Trieste, Italy.
authors.forename: Monica
authors.identifier: 0000000294910173
authors.initials: M
authors.lastname: de Simone
authors.affiliation: ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark.
authors.forename: Søren Vrønning
authors.initials: SV
authors.lastname: Hoffmann
authors.affiliation: ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark.
authors.forename: Nykola C
authors.initials: NC
authors.lastname: Jones
authors.affiliation: CNR-IMIP, Montelibretti, c/o Laboratorio Elettra, Trieste, Italy.
authors.forename: Cesare
authors.identifier: 0000000262552041
authors.initials: C
authors.lastname: Grazioli
authors.affiliation: Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, USA.
authors.forename: Kirk A
authors.identifier: 0000000349013235
authors.initials: KA
authors.lastname: Peterson
authors.affiliation: Scuola Normale Superiore, Piazza Cavalieri 7, 56126 Pisa, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Baiardi
authors.affiliation: Department of Physics and Astronomy, University of Uppsala, Uppsala, Sweden.
authors.forename: Teng
authors.identifier: 0000000187397773
authors.initials: T
authors.lastname: Zhang
authors.affiliation: International Centre for Quantum and Molecular Structures, College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, China.
authors.forename: Malgorzata
authors.initials: M
authors.lastname: Biczysko