Polycyclic Aromatic Hydrocarbon (PAHs) molecules are attracting much attention in the astrophysical and astrochemical communities because of their ubiquitous presence in space due to their ability to survive in the harsh environmental conditions of the interstellar medium (ISM). PAHs have been proposed as carriers of the aromatic infrared bands (AIBs) and as potential carriers of the still unidentified diffuse interstellar bands (DIBs). The so-called PAH-DIB proposal has been based on the abundance of PAHs in the ISM and their stability against photo and thermo dissociation. Nitrogen is one of the most abundant elements after hydrogen, helium, and carbon [1]. Nitrogenated PAHs (PANHs) exhibit spectral features similar to PAHs and may also contribute to the unidentified spectral bands.<br/>Laboratory absorption spectra of aromatic measured under astrophysical relevant conditions are of crucial importance to test the PAHs-DIBs hypothesis and for comparison with the observed DIB spectra. The most challenging task is to reproduce as closely as technically possible, the physical and chemical conditions that are present in space. Interstellar PAHs are expected to be present as free, cold, neutral molecules and/or <i>charged </i>species [2]. In our laboratory, comparable conditions are achieved using a dedicated platform developed at NASA Ames. Our cosmic simulation chamber (COSmIC) allows the measurement of gas phase spectra of neutral and ionized interstellar PAHs analogs by associating a free supersonic jet expansion with an ionizing discharge to generate a cold plasma expansion (≈100 K). Our approach, to assign PAH as carriers of some DIBs, consists in recording the electronic spectra of the cold PAHs in the gas phase and performing a systematic search for possible correspondences with features in astronomical DIB spectra [3]. In this work, we report the UV-visible absorption spectra of neutral PAHs and PAHNs recently measured using the cavity ring down spectroscopy (CRDS) technique. We discuss the effects of the substitution of C–H bond(s) with nitrogen atom(s) in the spectral signatures of PAHs and the astrochemical implications.

UV-visible spectroscopy of PAHs and PANHs in supersonic jet: Astrochemical implications

254th National Meeting (2017)

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