Near edge X-ray absorption mass spectrometry on coronene

Abstract

We have investigated the photoionization and photodissociation of free coronene cations C24H12 (+) upon soft X-ray photoabsorption in the carbon K-edge region by means of a time-of-flight mass spectrometry approach. Core excitation into an unoccupied molecular orbital (below threshold) and core ionization into the continuum both leave a C 1s vacancy, that is subsequently filled in an Auger-type process. The resulting coronene dications and trications are internally excited and cool down predominantly by means of hydrogen emission. Density functional theory was employed to determine the dissociation energies for subsequent neutral hydrogen loss. A statistical cascade model incorporating these dissociation energies agrees well with the experimentally observed dehydrogenation. For double ionization, i.e., formation of intermediate C24H12 (3+⋆)trications, the experimental data hint at loss of H(+) ions. This asymmetric fission channel is associated with hot intermediates, whereas colder intermediates predominantly decay via neutral H loss.

Single and Double Photoionization and Photodissociation of Toluene by Soft X-Rays in a Circumstellar Environment

“…In environments with some contribution of soft X-rays, these dications can be produced by the absorption of photons of energy around the C1s edge, followed by the Auger process of both neutral (emitting two electrons) and cationic molecules (emitting just one electron). Reitsma et al (2015) studied the production and stability of dications of coronene from X-ray photon interaction with cations. This may be an important route for producing dications, but it still lacks the rate production of doubly charged ions from neutral PAHs, since part of them are destroyed by photoionization.…”

Destruction and multiple ionization of PAHs by X-rays in circumnuclear regions of AGNs

“…Such estimates were based on PAH experiments with lower energy photons, up to the VUV range (Leach et al 1989a,b;Jochims et al 1994Jochims et al , 1996Jochims et al , 1999, and on the photoabsorption profile of the C atom in a photon energy range up to 30 keV (Henke et al 1982(Henke et al , 1993. In addition, the interaction of PAHs and their precursors with photons around the C1s→ π* resonance (285 eV) has been explored by several authors (Rennie et al 2000;Boechat-Roberty et al 2009;Reitsma et al 2014Reitsma et al , 2015Monfredini et al 2016;Quitián-Lara et al 2018). However, there is still a lack of data when it comes to investigating the effects of keV photons on these astrophysically relevant molecules.…”

“…In addition, the competition between VUV photoionization, photofragmentation and the photoproduction of dications in the context of interstellar PAH population were discussed by Zhen et al 2016. Using soft X-ray photons with energies around the C1s→ π* resonance (285 eV), Reitsma et al (2014Reitsma et al ( , 2015 have investigated the fragmentation of PAHs cations. However, from our perspective, there is still a lack of information on the ionization and dissociation of PAHs driven by photons of higher energies, which would be useful in the context of the circumnuclear regions of AGNs.…”

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

“…Superhydrogenation dramatically alters the response of neutral and ionic gas-phase PAHs in various astrochemically relevant interaction processes. Attachment of small numbers of H atoms to coronene cations can for instance quench photoionization-induced H loss from a C 24 H + 12 precursor cation (Reitsma et al 2014(Reitsma et al , 2015. For superhydrogenation of neutral pyrene molecules, an opposite effect was observed.The C-backbone is weakened and fragmentation upon ion collisions or photoionization is increased (Wolf et al 2016;Gatchell et al 2015).…”