title: A Near-Threshold Shape Resonance in the Valence-Shell Photoabsorption of Linear Alkynes.
abstract: The room-temperature photoabsorption spectra of a number of linear alkynes with internal triple bonds (e.g., 2-butyne, 2-pentyne, and 2- and 3-hexyne) show similar resonances just above the lowest ionization threshold of the neutral molecules. These features result in a substantial enhancement of the photoabsorption cross sections relative to the cross sections of alkynes with terminal triple bonds (e.g., propyne, 1-butyne, 1-pentyne, ...). Based on earlier work on 2-butyne [ Xu et al., J. Chem. Phys. 2012, 136, 154303 ], these features are assigned to excitation from the neutral highest occupied molecular orbital (HOMO) to a shape resonance with g (l = 4) character and approximate π symmetry. This generic behavior results from the similarity of the HOMOs in all internal alkynes, as well as the similarity of the corresponding gπ virtual orbital in the continuum. Theoretical calculations of the absorption spectrum above the ionization threshold for the 2- and 3-alkynes show the presence of a shape resonance when the coupling between the two degenerate or nearly degenerate π channels is included, with a dominant contribution from l = 4. These calculations thus confirm the qualitative arguments for the importance of the l = 4 continuum near threshold for internal alkynes, which should also apply to other linear internal alkynes and alkynyl radicals. The 1-alkynes do not have such high partial waves present in the shape resonance. The lower l partial waves in these systems are consistent with the broader features observed in the corresponding spectra.
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: U
authors.initials: U
authors.lastname: Jacovella
authors.affiliation: STFC, Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, U.K.
authors.forename: D M P
authors.initials: DM
authors.lastname: Holland
authors.affiliation: Institut des Sciences Moléculaires d'Orsay, UMR 8214, CNRS & Univ. Paris-Sud, F-91405 Orsay, France.
authors.forename: S
authors.initials: S
authors.lastname: Boyé-Péronne
authors.affiliation: Institut des Sciences Moléculaires d'Orsay, UMR 8214, CNRS & Univ. Paris-Sud, F-91405 Orsay, France.
authors.forename: Bérenger
authors.initials: B
authors.lastname: Gans
authors.affiliation: Synchrotron Soleil, L'Orme des Merisiers, F-91192 Gif-sur-Yvette, France.
authors.forename: N
authors.initials: N
authors.lastname: de Oliveira
authors.affiliation: Synchrotron Soleil, L'Orme des Merisiers, F-91192 Gif-sur-Yvette, France.
authors.forename: K
authors.initials: K
authors.lastname: Ito
authors.affiliation: Synchrotron Soleil, L'Orme des Merisiers, F-91192 Gif-sur-Yvette, France.
authors.forename: D
authors.initials: D
authors.lastname: Joyeux
authors.affiliation: Synchrotron Soleil, L'Orme des Merisiers, F-91192 Gif-sur-Yvette, France.
authors.forename: L E
authors.initials: LE
authors.lastname: Archer
authors.affiliation: Department of Chemistry, Texas A&M University , College Station, Texas 77843, United States.
authors.forename: R R
authors.initials: RR
authors.lastname: Lucchese
authors.affiliation: Argonne National Laboratory, Argonne, Illinois 60439, United States.
authors.forename: Hong
authors.initials: H
authors.lastname: Xu
authors.affiliation: Argonne National Laboratory, Argonne, Illinois 60439, United States.
authors.forename: S T
authors.initials: ST
authors.lastname: Pratt