Relaxation de l'énergie de molécules polycycliques aromatiques hydrogénées : Une étude en astrophysique de laboratoire

As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) in so-called photodissociation regions (PDRs) are processed by the interaction with ultraviolet (UV) photons that are emitted by hot stars. After absorption of a UV photon, an isolated PAH can undergo different relaxation processes: ionization, dissociation, and radiative cooling, including infrared (IR) fluorescence which results in the aromatic infrared bands (AIBs) observed in many astronomical objects. This interaction influences their charge state and photodissociation dynamics, ultimately determining their photostability in space. In return, it impacts the energy balance of the interstellar gas by photoelectric heating. PAHs are also proposed to act as catalysts for the most abundant molecule in space, H2, and play thus an ubiquitous role in the physics and chemistry of PDRs. This PhD thesis concentrates on the experimental investigation of cationic PAHs, addressing independently and for different molecular families two major aspects which concern the evolution of PAHs under UV irradiation and the possibility to form several isomers. The experimental measurements are supported by complementary theoretical calculations. Two main experimental campaigns have been carried out throughout this PhD work. At the SOLEIL synchrotron, large PAH cations (30 − 48 carbon atoms) were submitted to tunable vacuum UV (VUV) radiation in the range of 9.5 to 20.0 eV and their photofragments and dications were mass-analyzed as a function of photon energy. We could derive branching ratios, action spectra, and photoionization and photodissociation cross sections, demonstrating that ionization is the dominant channel in the photoprocessing of these large PAHs by VUV irradiation. Using theoretical photoabsorption cross sections from time-dependent density functional theory (TD-DFT), we were also able to determine photoionization yields and to give recipes which can directly be implemented in astrochemical modeling studies. At the FELIX facility, IR predissociation spectroscopy of cold species tagged with Ne atoms was employed to disentangle isomers according to their IR fingerprints. The study focused on the –H fragments of methylated PAH cations in order to investigate the formation of the tropylium-like isomer relative to the benzylium-like isomer, which carries a methylene sidegroup. Theoretical harmonic IR spectra have been calculated from DFT to support the identification of the ions. We found that methylene-PAHs are the only formed structures for PAHs containing more than 2 − 3 cycles. Methylated PAHs have been proposed as the carriers of the 3.4 μm emission feature in PDRs. We conclude that the observed variations of the AIB spectrum are consistent with the photoprocessing of these species by VUV irradiation leading to the formation of methylene sidegroups.Composantes de la poussière interstellaire, les molécules polycycliques aromatiques hydrogénées (PAH) qui peuplent les régions dites de photodissociation (PDR) sont soumises au rayonnement ultraviolet (UV) provenant des étoiles chaudes. Après l’absorption d’un photon UV, un PAH isolé peut subir différents processus de relaxation: ionisation, dissociation et refroidissement radiatif, y compris la fluorescence infrarouge (IR) qui se traduit par les bandes infrarouges aromatiques (AIB) observées dans de nombreux objets astronomiques. Cette interaction influence l’état de charge du PAH et sa photostabilité via la dynamique de dissociation. D’autre part, elle impacte le bilan énergétique du gaz interstellaire par chauffage par effet photoélectrique. Les PAH pourraient aussi contribuer à la formation de H2, la molécule la plus abondante et jouent donc un rôle important dans la physique et la chimie des PDR. Cette thèse se concentre sur l’étude expérimentale de PAH cationiques, traitant de manière indépendante et pour différentes familles de molécules, deux aspects majeurs que sont l’évolution sous irradiation UV et la possibilité de former différents isomères. Les mesures expérimentales sont complétées par des calculs théoriques. Deux campagnes expérimentales principales ont été menées tout au long de cette thèse. Au synchrotron SOLEIL, des cations PAH de grande taille (30 − 48 atomes de carbone) ont été soumis à un rayonnement UV du vide (VUV) accordable dans le domaine de 9,5 à 20,0 eV et leurs photofragments et dications ont été analysés en masse en fonction de l’énergie des photons. Nous en avons déduit des rapports de branchement, des spectres d’action et des sections efficaces de photoionisation et de photodissociation, démontrant que l’ionisation est la voie dominante d’évolution chimique sous irradiation VUV de ces PAH de grande taille. En utilisant des valeurs des sections efficaces d’absorption provenant de la théorie de la fonctionnelle de la densité dépendante du temps (TD-DFT), nous avons également pu déterminer des rendements de photoionisation et donner des recommandations pour leur utilisation dans des codes de modélisation astrochimique. L’étude des isomères a été réalisée à l’infrastructure FELIX grâce à une spectroscopie IR de prédissociation par complexation à basse température avec des atomes de Ne. Elle a porté sur les fragments –H de PAH cations méthylés afin de rechercher la formation de l’isomère de type tropylium relativement à celle de l’isomère de type benzylium qui comprend un groupement méthylène. Les fréquences harmoniques théoriques des différentes espèces ont été calculées en utilisant la théorie DFT afin de faciliter l’identification des ions formés. Nous avons montré que seules les espèces PAH-méthylène sont formées pour des PAH plus grands que 2 − 3 cycles carbonés. Les PAH méthylés ont été proposés comme porteurs de la bande d’émission à 3,4 μm dans les PDR. Notre étude montre que les variations observées dans le spectre AIB sont cohérentes avec l’évolution de ces espèces sous l’irradiation de photons VUV conduisant à la formation de groupements de type méthylène

Relaxation of Energized Polycyclic Aromatic Hydrocarbons : A Laboratory Astrophysical Study

Composantes de la poussière interstellaire, les molécules polycycliques aromatiques hydrogénées (PAH) qui peuplent les régions dites de photodissociation (PDR) sont soumises au rayonnement ultraviolet (UV) provenant des étoiles chaudes. Après l'absorption d'un photon UV, un PAH isolé peut subir différents processus de relaxation : ionisation, dissociation et refroidissement radiatif, y compris la fluorescence infrarouge (IR) qui se traduit par les bandes infrarouges aromatiques (AIB) observées dans de nombreux objets astronomiques. Cette interaction influence l'état de charge du PAH et sa photostabilité via la dynamique de dissociation. D'autre part, elle impacte le bilan énergétique du gaz interstellaire par chauffage par effet photoélectrique. Les PAH pourraient aussi contribuer à la formation de H2, la molécule la plus abondante et jouent donc un rôle important dans la physique et la chimie des PDR. Cette thèse se concentre sur l'étude expérimentale de PAH cationiques, traitant de manière indépendante et pour différentes familles de molécules, deux aspects majeurs que sont l'évolution sous irradiation UV et la possibilité de former différents isomères. Les mesures expérimentales sont complétées par des calculs théoriques. Deux campagnes expérimentales principales ont été menées tout au long de cette thèse. Au synchrotron SOLEIL, des cations PAH de grande taille (30 - 48 atomes de carbone) ont été soumis à un rayonnement UV du vide (VUV) accordable dans le domaine de 9,5 à 20,0 eV et leurs photofragments et dications ont été analysés en masse en fonction de l'énergie des photons. Nous en avons déduit des rapports de branchement, des spectres d'action et des sections efficaces de photoionisation et de photodissociation, démontrant que l'ionisation est la voie dominante d'évolution chimique sous irradiation VUV de ces PAH de grande taille. En utilisant des valeurs des sections efficaces d'absorption provenant de la théorie de la fonctionnelle de la densité dépendante du temps (TD-DFT), nous avons également pu déterminer des rendements de photoionisation et donner des recommandations pour leur utilisation dans des codes de modélisation astrochimique. L'étude des isomères a été réalisée à l'infrastructure FELIX grâce à une spectroscopie IR de prédissociation par complexation à basse température avec des atomes de Ne. Elle a porté sur les fragments -H de PAH cations méthylés afin de rechercher la formation de l'isomère de type tropylium relativement à celle de l'isomère de type benzylium qui comprend un groupement méthylène. Les fréquences harmoniques théoriques des différentes espèces ont été calculées en utilisant la théorie DFT afin de faciliter l'identification des ions formés. Nous avons montré que seules les espèces PAH-méthylène sont formées pour des PAH plus grands que 2 - 3 cycles carbonés. Les PAH méthylés ont été proposés comme porteurs de la bande d'émission à 3,4 µm dans les PDR. Notre étude montre que les variations observées dans le spectre AIB sont cohérentes avec l'évolution de ces espèces sous l'irradiation de photons VUV conduisant à la formation de groupements de type méthylène.As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) in so-called photodissociation regions (PDRs) are processed by the interaction with ultraviolet (UV) photons that are emitted by hot stars. After absorption of a UV photon, an isolated PAH can undergo different relaxation processes: ionization, dissociation, and radiative cooling, including infrared (IR) fluorescence which results in the aromatic infrared bands (AIBs) observed in many astronomical objects. This interaction influences their charge state and photodissociation dynamics, ultimately determining their photostability in space. In return, it impacts the energy balance of the interstellar gas by photoelectric heating. PAHs are also proposed to act as catalysts for the most abundant molecule in space, H2, and play thus an ubiquitous role in the physics and chemistry of PDRs. This PhD thesis concentrates on the experimental investigation of cationic PAHs, addressing independently and for different molecular families two major aspects which concern the evolution of PAHs under UV irradiation and the possibility to form several isomers. The experimental measurements are supported by complementary theoretical calculations. Two main experimental campaigns have been carried out throughout this PhD work. At the SOLEIL synchrotron, large PAH cations (30 - 48 carbon atoms) were submitted to tunable vacuum UV (VUV) radiation in the range of 9.5 to 20.0 eV and their photofragments and dications were mass-analyzed as a function of photon energy. We could derive branching ratios, action spectra, and photoionization and photodissociation cross sections, demonstrating that ionization is the dominant channel in the photoprocessing of these large PAHs by VUV irradiation. Using theoretical photoabsorption cross sections from time-dependent density functional theory (TD-DFT), we were also able to determine photoionization yields and to give recipes which can directly be implemented in astrochemical modeling studies. At the FELIX facility, IR predissociation spectroscopy of cold species tagged with Ne atoms was employed to disentangle isomers according to their IR fingerprints. The study focused on the -H fragments of methylated PAH cations in order to investigate the formation of the tropylium-like isomer relative to the benzylium-like isomer, which carries a methylene sidegroup. Theoretical harmonic IR spectra have been calculated from DFT to support the identification of the ions. We found that methylene-PAHs are the only formed structures for PAHs containing more than 2 - 3 cycles. Methylated PAHs have been proposed as the carriers of the 3.4 µm emission feature in PDRs. We conclude that the observed variations of the AIB spectrum are consistent with the photoprocessing of these species by VUV irradiation leading to the formation of methylene sidegroups

Gas-phase electronic action absorption spectra of protonated oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs)

Context. Extended red emission (ERE) denotes a broad unassigned feature extending from 540 to 800 nm observed in many regions of the interstellar medium (ISM), and is thought to originate from photoluminescence of cosmic dust. However, definitive assignment of specific carriers remains to be achieved. Aims. Our aim is to investigate the photoabsorption spectra of astrophysically relevant protonated oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs) to probe their ability to absorb photons in the near-ultraviolet (UV) and visible (vis) spectral region and to search for any low-lying electronic states that may account for the ERE. Methods. Gas-phase electronic action absorption spectra of the protonated OPAHs were recorded in the spectral range of 200–700 nm using the ELISA ion-storage ring. Additional time-dependent density functional theory (TD-DFT) calculations were performed to compute excited state transitions that complement the experimental spectra. Results. A set of five protonated (O)PAHs was considered, namely pentacene and the four oxygen-functionalized PAHs, pentacenequinone, pentacenetetrone, anthraquinone, and phenathrenequinone. All pentacene-related species show a main absorption band between 400 and 500 nm, while the smaller OPAHs, anthraquinone and phenanthrenequinone, generally absorb further to the blue compared to the pentacenes. Interestingly, pentacenequinone and phenanthrenequinone exhibit wide absorption plateaus towards the red side of their main absorption band(s), which places them among the potential candidates to contribute to ERE. Additional photodissociation mass spectra reveal the formation of smaller functionalized PAHs and small oxygen-bearing species. Conclusions. Our results demonstrate the ability of OPAHs to absorb in the UV/vis spectral region. Among the four studied OPAHs, two revealed very broad absorption characteristics at wavelengths up to 700 nm, which makes them suitable candidates to contribute to a part of the ERE spectrum. Moreover, these two OPAHs, pentacenequinone and phenanthrenequinone, could dissociate efficiently into oxygen-bearing molecules and smaller functionalized PAHs in photon-dominated regions (PDRs) of the ISM

Identification of the fragment of the 1-methylpyrene cation by mid-IR spectroscopy

International audienceThe fragment of the 1-methylpyrene cation, C17H11+, is expected to exist in two isomeric forms, 1-pyrenemethylium PyrCH2+ and the tropylium containing species PyrC7+ . We measured the infrared (IR) action spectrum of cold C17H11+ tagged with Ne using a cryogenic ion trap instrument coupled to the FELIX laser. Comparison of the experimental data with density functional theory calculations allows us to identify the PyrCH2+ isomer in our experiments. The IR Multi-Photon Dissociation spectrum was also recorded following the C2H2 loss channel. Its analysis suggests combined effects of anharmonicity and isomerisation while heating the trapped ions, as shown by molecular dynamics simulations

Infrared spectroscopy of the benzylium-like (and tropylium-like) isomers formed in the -H dissociative ionization of methylated PAHs

Dataset for article "Infrared spectroscopy of the benzylium-like (and tropylium-like) isomers formed in the --H dissociative ionization of methylated PAHs". DOI: 10.1016/j.jms.2022.111620 - folder Experimental contains - folder Fig2_IRPDspectra containing - with IRPD spectra data of all three species (Fig. 2) - folder Fig3_Depletion containing - the saturation depletion measurements on six bands (Fig. 3) - folder FigS1_MassSpectra containing - the mass spectra of all three species in Trap ON/OFF modes (Fig. S1) - folder Theoretical contains - .log files for each considered species - e.g.: folder C11H9+ contains folders for the - NapC7+ - NapC7+Ne - NapCH2+ - NapCH2+Ne These contain all .log files needed to reproduce Figs. 4, 5, 6 of the main mansucript and Figs. S3, S4, S5, S6, S7, S8, S9 of the supplementary material - folder Fig7_EnergyProfile containing - all minima and transition states for the computed energy profile (doublet spin state surface) for the H loss from NapCH3+ leading to NapCH2+ and NapC7+ (Fig. 7) Copyright: Creative Commons Attribution 4.0 International Open Acces

Infrared spectroscopy of the benzylium-like (and tropylium-like) isomers formed in the -H dissociative ionization of methylated PAHs

Cationic benzylium and tropylium are known to be two competitive isomers for the -H fragment of the methylbenzene (toluene) cation. Methylated polycyclic aromatic hydrocarbon (PAH) cations are expected to be abundant in space and their dehydrogenation could lead to the formation of both the benzyliumand tropyliumlike cations, which are expected to be the two lowest-energy isomers. Here, we considered 1-methylpyrene and two less compact acene-substituted species, namely 2-methylnaphthalene and 2-methylanthracene, as precursors. The cationic -H fragments, C17H11+, C11H9+, and C15H11+, were produced by dissociative ionization, and their neon tagged complexes were formed in the 22-pole cryogenic ion trap instrument FELion that is coupled to the FELIX free electron laser. Infrared (IR) predissociation spectroscopy was performed showing that the strongest depletion band is located at about 1620 cm(-1), which reveals the predominance of the benzyliumlike, XCH2+, isomers, where X = Pyr, Nap, or Ant. Saturation depletion measurements showed that only this isomer is present in the case of C17H11+, whereas for the acene-derived species at least two are present with a large abundance. Synthetic spectra were generated from the theoretical anharmonic IR spectra of the two lowest-energy isomers, namely XCH2+ and the tropylium-like isomers, XC7+. Spectral comparison led us to conclude that there is no evidence for PyrC(7)(+) but clear evidence for NapC(7)(+). No specific spectral features could be identified for AntC(7)(+) due to a high spectral congestion. These results support the important role of PAH compactness in preventing the formation of XC7+ species. They also reveal the potential of XCH(2)(+ )species to account for the aromatic infrared band observed in emission at 6.2 mu m in astrophysical environments

VUV photo processing of large cationic PAHs in astrophysical conditions: coupling a VUV source to the PIRENEA setup

International audienceStudying the interaction of polycyclic aromatic hydrocarbons (PAHs) with UV and VUV light is crucial for the understanding of the physical and chemical evolution of the interstellar medium. Photo-processing by VUV irradiation leads to variation of the hydrogenation and ionization states of PAHs [1] and plays an important role in the heating of the gas [2]. Recent laboratory studies have shown that the VUV photoexcitation of large PAHs rather induces further ionization than fragmentation [3,4] and, more surprisingly, it can also induce non-statistical dissociation [5]. We have recently coupled a VUV source based on a Xe-Ar tripling cell to the PIRENEA (Piège à Ions pour la Recherche et l’Etude de Nouvelles Espèces Astrochimiques) setup which consists of a cryogenic (35 K) ion cyclotron resonance cell that is especially well suited to investigate ionization and dissociation on long timescales [1,5]. The produced VUV photons (10.5 eV) are sufficiently energetic to trigger dissociation in small/medium-size PAHs [6] and ionization in large PAH cations [4]. We present first measurements of VUV processing of several PAHs in a cryogenic and collisionless environment which is appropriate for interstellar conditions

2022 Banhatti et al. PCCP article

This repository contains source data for the Figures in the main article

Formation of the acenaphthylene cation as a common C2H2-loss fragment in dissociative ionization of the PAH isomers anthracene and phenanthrene

International audiencePolycyclic aromatic hydrocarbons (PAHs) are thought to be a major constituent of astrophysical environments, being the carriers of the ubiquitous aromatic infrared bands (AIBs) observed in the spectra of galactic and extra-galactic sources that are irradiated by ultraviolet (UV) photons. Small (2-cycles) PAHs were unambiguously detected in the TMC-1 dark cloud, showing that PAH growth pathways exist even at low temperatures. The processing of PAHs by UV photons also leads to their fragmentation, which has been recognized in recent years as an alternative route to the generally accepted bottom-up chemical pathways for the formation of complex hydrocarbons in UV-rich interstellar regions. Here we consider the C12H8+ ion that is formed in our experiments from the dissociative ionization of the anthracene and phenanthrene (C14H10) molecules. By employing the sensitive action spectroscopic scheme of infrared pre-dissociation (IRPD) in a cryogenic ion trap instrument coupled to the free-electron lasers at the FELIX Laboratory, we have recorded the broadband and narrow line-width gas-phase IR spectra of the fragment ions (C12H8+) and also the reference spectra of three low energy isomers of C12H8+. By comparing the experimental spectra to those obtained from quantum chemical calculations we have identified the dominant structure of the fragment ion formed in the dissociation process to be the acenaphthylene cation for both isomeric precursors. Ab initio molecular dynamics simulations are presented to elucidate the fragmentation process. This result reinforces the dominant role of species containing a pentagonal ring in the photochemistry of small PAH