arXiv subjects

Anthony Bonnamy

Publications and source records attributed to Anthony Bonnamy.

6 recordsLinked to original sources

arXiv2026

Acenaphthene Derivatives as Signatures of C$_{11}$H$_9^+$ Reactivity with Methylated Naphthalenes

C$_{11}$H$_9^+$ ion is the dominant fragment cation formed from methyl-naphthalene (MeNp) and dimethyl-naphthalene (diMeNp). Using the multiplex capabilities of PIRENEA, a setup dedicated to laboratory astrophysics, we studied the reactivity of the benzylium-like isomers of C$_{11}$H$_9^+$ with diMeNp under isolated conditions relevant to radiative association. Two reaction products are observed, C$_{12}$H$_{11}^+$ -- also formed in the reaction with MeNp -- and C$_{13}$H$_{13}^+$, with branching ratios that depend on the specific diMeNp isomer. The reaction products were subsequently exposed to UV-visible irradiation to gain insight into their structures. The acenaphthylene radical cation, C$_{12}$H$_{8}^{\bullet +}$, was identified as the most stable photofragment. We show that this experimental approach, supported by density functional theory calculations and molecular dynamics simulations, provides new constraints on the chemistry of benzylium-type species. We highlight the role that long-lived ion-molecule complexes can have in promoting C-C coupling and the formation of a pentagonal cycle. Moreover, the chemistry uncovered here highlights new pathways for the formation of pentagonal rings during PAH growth under low-pressure and cold conditions. In particular, it can lead to efficient formation of acenaphthylene-like species, recently detected in the TMC-1 cold cloud.

astro-ph.GA↗

arXiv2025

Long-Lived Isomers of C$_{11}$H$_9^+$: New Experimental Insights from the PIRENEA Setup

The dehydrogenated cation of methylated benzene is known to exist in two isomeric forms: benzylium and tropylium. Structurally similar forms have been proposed for the -H cations of methylated polycyclic aromatic hydrocarbons, but their spectroscopic characterization remains limited, and their photophysical properties are still poorly understood. Previous studies identified 2-naphthylmethylium and benzyltropylium as specific long-lived isomers of the -H fragment of methylnaphthalene cations. Here, we investigate the photodissociation spectroscopy and photoprocessing of gas-phase C$_{11}$H$_9^+$ ions in the visible range. Experiments are conducted using the versatile laboratory astrophysics setup PIRENEA, which enables studies over long timescales ($\sim$1000 s) and allows photoprocessing to be combined with ion-molecule reaction experiments. We confirm the presence of 2-naphthylmethylium and benzyltropylium and additionally identify 1-naphthylmethylium, previously undetected in experiments. Moreover, we present the first complete quantitative analysis of the relative abundances of these isomers and provide clear evidence of interconversion among the three long-lived species. These isomerization processes occur below the dissociation threshold, a finding supported by molecular dynamics simulations. The photophysical properties of C$_{11}$H$_9^+$ isomers -- including isomerization and fluorescence -- make them intriguing candidates for consideration in astrophysical environments exposed to mild UV irradiation (h$\nu$ < 7 eV). Moreover, their detection via rotational spectroscopy in such regions is facilitated by their closed-shell electronic structure.

astro-ph.GA↗

arXiv2021

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands (AIBs) but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects that favor specific sizes and structures. We investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations under conditions applicable for photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene (C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups, superhydrogenation) are submitted to VUV (10.5 eV) photons over long timescales (~1000 s) in the cryogenic PIRENEA setup. The fragmentation cascades are analyzed with a simple kinetics model; fragmentation pathways, rates and branching ratios are derived. Aliphatic PAH derivatives are found to have a higher fragmentation rate and carbon loss compared to regular PAHs. The fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be more stable than the bare PAH cations. This stability is quantified and the most stable species, for which there is an effective competition of the fragmentation with isomerization and radiative cooling are identified. This work supports a scenario in which the evaporation of nanograins with a mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4um AIB by methyl sidegroups and in an abundant source of small hydrocarbons at the border of PDRs. An additional side effect is the efficient formation of stable PAHs that contain some peripheral pentagonal rings. Our experiments also support the role of isomerization processes in PAH photofragmentation, including the H-migration process, which could lead to an additional contribution to the 3.4um AIB.

astro-ph.GA↗

arXiv2019

Photo-processing of astro-PAHs

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, C48H20+ , and compare its behavior under VUV processing with that of smaller species. We suggest that C2H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.

astro-ph.GA↗

arXiv2017

Identification of PAH Isomeric Structure in Cosmic Dust Analogues: the AROMA setup

We developed a new analytical experimental setup called AROMA (Astrochemistry Research of Organics with Molecular Analyzer) that combines laser desorption/ionization techniques with ion trap mass spectrometry. We report here on the ability of the apparatus to detect aromatic species in complex materials of astrophysical interests and characterize their structures. A limit of detection of 100 femto-grams has been achieved using pure polycyclic aromatic hydrocarbon (PAH) samples, which corresponds to 2x10^8 molecules in the case of coronene (C24H12). We detected the PAH distribution in the Murchison meteorite, which is made of a complex mixture of extraterrestrial organic compounds. In addition, collision induced dissociation experiments were performed on selected species detected in Murchison, which led to the first firm identification of pyrene and its methylated derivatives in this sample.

astro-ph.IM↗

arXiv2015

An optical spectrum of a large isolated gas-phase PAH cation: C78H26+

A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C78H26 +- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C78H26 stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C2 symmetry instead of the more symmetric planar D2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C78H26 + fragments and ultimately theformation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.

astro-ph.IM↗