Atomic hydrogen interactions with small polycyclic aromatic hydrocarbons cations
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THE EUROPEAN PHYSICAL JOURNAL D
Regular Article
Atomic hydrogen interactions with small polycyclic aromatic hydrocarbons cations? Thomas Schlath¨olter1,a , Yahia Mostafa1 , Amber Kamman1 , Arnold Dongelmans1 , Yann Arribard1 , Stephanie Cazaux2,3 , and Ronnie Hoekstra1 1 2 3
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands University of Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands Received 24 February 2020 / Received in final form 6 April 2020 Published online 11 June 2020 c The Author(s) 2020. This article is published with open access at Springerlink.com
Abstract. When exposed to a thermal beam of hydrogen atoms, gas-phase coronene cations C24 H+ 12 can be sequentially hydrogenated. This process is accompanied by a gradual transition of the electronic structure from aromatic to aliphatic. The planar very stable coronene structure transforms into the significantly weaker corrugated structure, typical for aliphatic molecules. In this study, we have investigated the hydrogenation of 5 smaller polycyclic aromatic hydrocarbon cations using a combination of radiofrequency + ion trapping with time-of-flight mass spectrometry. Anthracene (C14 H+ 10 ), pyrene (C16 H10 ), triphenylene + + + (C18 H12 ), tetracene (C18 H12 ) and 8-9-benzofluoranthene (C20 H12 ) only cover a small mass range, but differ in carbon/hydrogen ratio, number of outer-edge sites and overall structure. We have observed qualitatively similar initial hydrogenation patterns for all 5 molecular ions, with odd hydrogenation states being dominant. Strong quantitative differences in hydrogenation and in attachment-induced fragmentation were found. For the case of pyrene cations, we have also investigated exposure to atomic D. Clear lines of evidence for HD/D2 abstraction reactions of Eley–Rideal type were found, as previously observed for coronene cations.
1 Introduction Polycylic aromatic hydrocarbons (PAHs) are most certainly the carriers of the aromatic infrared bands emitted by many galactic and extragalactic sources [1,2]. In the interstellar medium (ISM), PAHs can be present in neutral form or as ions [3,4]. PAHs are of great astrochemical relevance because they provide large surfaces for chemical reactions to occur, while such reactions are inefficient in the gas phase [5]. In a pioneering study, Snow et al. have studied the reactivity of small PAH cations such as C16 H+ 10 (pyrene) and found evidence for atomic hydrogen attachment, suggesting that hydrogenated PAH ions are abundant species in the ISM [6]. Rauls and Hornekær have later predicted that H2 molecules could be formed in an Eley– Rideal process from two hydrogen atoms co-adsorbed on a neutral PAH under conditions typical of the ISM [7]. Successive hydrogen attachment and H2 formation was first experimentally confirmed for neutral coronene (C24 H12 ) thin films on a solid substrate [8]. ?
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