Molecular hydrogen formation on interstellar PAHs through Eley-Rideal abstraction reactions

Abstract

We present experimental data on H 2 formation processes on gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This process was studied by exposing coronene radical cations, confined in a radio-frequency ion trap, to gas phase H atoms. Sequential attachment of up to 23 hydrogen atoms has been observed. Exposure to atomic D instead of H allows one to distinguish attachment from competing abstraction reactions, as the latter now leave a unique fingerprint in the measured mass spectra. Modeling of the experimental results using realistic cross sections and barriers for attachment and abstraction yield a 1:2 ratio of abstraction to attachment cross sections. The strong contribution of abstraction indicates that H 2 formation on interstellar PAH cations is an order of magnitude more relevant than previously thought.

Atomic hydrogen interactions with small polycyclic aromatic hydrocarbons cations

“…For an estimated trap content of 10 4 ions within a volume of 0.5 mm diameter and a collision cross section of 121Å 2 [19], we estimate a collision rate of a few 100 s −1 . This is in agreement with the experimental data in the sense that we observe almost full single hydrogenation after 0.1 s of exposure, with a H attachment rate that is about one order of magnitude smaller than the collision rate [14].…”

“…(2) Abstraction processes can occur for cationic and neutral coronene and have been first experimentally confirmed by Menella et al for supported coronene thin films (neutral coronene), albeit with a cross section of 0.06Å 2 which is about 20 times smaller than the corresponding cross section for H attachment 1.1Å 2 [13]. We have recently studied abstraction processes on gas-phase coronene cations using atomic deuterium and observed an order of magnitude larger abstraction cross sections of about 0.45Å 2 for this system [14]. Such large cross sections of cationic coronene compared to neutral coronene, could render this process relevant for H 2 formation in the ISM, and make cationic PAHs important interstellar catalysts.…”

Probing the size and charge of polycyclic aromatic hydrocarbons

“…PAHs have been extensively utilized in the literature E-mail: amaragko@uwo.ca to probe the local physical conditions of their irradiating sources as well as the characteristics of the surrounding environments. Due to their large cross sections, PAH species have a direct impact on the ionization balance of Photo-Dissociation Regions (PDRs) through their interactions with electrons and cations (Lepp & Dalgarno 1988;Bakes & Tielens 1994), and are also considered to be an effective route for molecular hydrogen formation (Bauschlicher 1998;Mennella et al 2012;Thrower et al 2012;Foley et al 2018). PAH emission has been calibrated from spectroscopic and photometric bands and used for the estimation of star-formation rates on both galaxy-wide and spatially resolved galactic scales (e.g.…”

Do defects in PAHs promote catalytic activity in space? Stone–Wales pyrene as a test case

“…6 It has been proposed that both neutral and cationic PAH molecules can first react with atomic hydrogen with little (few meV) to no barrier becoming superhydrogenated and, then, promote barrierless H 2 abstraction through the Eley-Rideal mechanism. 7,8 Since detailed photochemical evolution models of PAHs in the ISM have questioned the efficiency of the Eley-Rideal H 2 abstraction, 9 more recently other mechanisms have been invoked, such as photodissociation of H 2 from normally hydrogenated PAHs. 10 Stone-Wales (SW) defects are topological changes in the structure ubiquitous in sp 2 carbon materials such as fullerene, graphene, nanoribbons, and carbon nanotubes.…”