title: An optical spectrum of a large isolated gas-phase PAH cation: C78H26.
abstract: A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C78H26+- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2symmetry instead of the more symmetric planar D2hsymmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26+fragments and ultimately the formation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France.
authors.forename: Junfeng
authors.initials: J
authors.lastname: Zhen
authors.affiliation: CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France ; Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Mulas
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France.
authors.forename: Anthony
authors.initials: A
authors.lastname: Bonnamy
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Joblin
keywords: ISM — ISM; astrochemistry — methods; molecules — molecular processes