ELECTRONIC SPECTROSCOPY OF COLD POLYCYCLIC AROMATIC HYDROCARBON CATIONS BY PHOTODISSOCIATION OF THEIR VAN DER WALLS COMPLEXES
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Date
1999
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Ohio State University
Abstract
The Diffuse Interstellar Bands (DIBs) are absorption features systematically observed throughout the visible and near infra-red region of the Interstellar Extinction curve. These bands represent the longest-standing mystery of astrophysical spectroscopy since no carriers have been identified after more than fifty years. The purpose of the present work is to obtain near-infrared and visible spectra of PAH cations that can be directly compared to the observational data, so that the knowledge of the electronic spectra of $PAHs+$ can provide a direct test for the proposal of these molecules as DIBs carriers. We have developed a new spectroscopic method especially adapted to obtain absorption spectra of such large aromatic cations by measuring the photodissociation spectra of aromatic-rare gas van der Walls clusters in a molecular beam. The cluster cations produced are rotationally $(<10K)$ and vibrationally cold. Because the binding energy of Ar in these van Der Walls clusters is small (of the order of 500 cm-1), they can easily be fragmented by exciting the species above the dissociation threshold (this process occurs on a subnanosecond time-scale). We have applied this method to a number of cations including PAH+ species and others, which have been measured in the near-IR and the visible region. This set of results represents the first ``high resolution'' spectroscopic database available today on such large polyatomic cations.
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Author Institution: Laboratoire de Photophysique Mol\'{e}culaire, CNRS, Universit\'{e} Paris-Sud