title: Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: experiment and theory.
abstract: The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to pi* virtual orbitals, and broader structures at higher energy, involving sigma* virtual orbitals. The sharp absorption structures to the pi* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of pi* symmetry, from the six chemically shifted C 1s core orbitals.
authors.affiliation: ISMN-CNR, Sez. Roma 1, Piazzale Aldo Moro 5, I-00185 Roma, Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Alagia
authors.forename: Chiara
authors.initials: C
authors.lastname: Baldacchini
authors.forename: Maria Grazia
authors.initials: MG
authors.lastname: Betti
authors.forename: Fabio
authors.initials: F
authors.lastname: Bussolotti
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Carravetta
authors.forename: Ulf
authors.initials: U
authors.lastname: Ekström
authors.forename: Carlo
authors.initials: C
authors.lastname: Mariani
authors.forename: Stefano
authors.initials: S
authors.lastname: Stranges