Threshold Dissociation of the 1-ethynylpyrene Cation at Internal Energies Relevant to H i Regions

Rouillé, Gaël; Steglich, Mathias; Hemberger, Patrick; Jäger, Cornelia; Henning, Thomas

doi:10.3847/1538-4357/ab4418

Cited by 5 publications

(8 citation statements)

References 47 publications

(68 reference statements)

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“…8). These results are in line with earlier studies (Rouillé et al 2015(Rouillé et al , 2019, which suggest that ethynyl-substitued PAHs are potentially good candidates for astroPAHs, if they can be formed efficiently as discussed by Rouillé et al (2019).…”

Section: Astrophysical Implicationssupporting

confidence: 92%

“…It likely leads to the formation of ethynyl-substituted PAHs, starting from C 18 H + 10 and C 26 H + 12 in the case of EtPyr + and EtCor + , respectively. Ethynyl-substitued PAHs are expected to be as stable as bare PAHs (Rouillé et al 2015(Rouillé et al , 2019. The fragmentation rate derived for C 18 H + 10 , k M 226 frag = 0.8 × 10 −3 s −1 suggests that these species could even be more stable than the bare PAH (k M 202 frag = 1.9 × 10 −3 s −1 ) upon VUV irradiation.…”

Section: Methylated and Ethylated Pahsmentioning

confidence: 98%

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Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Marciniak

Joblin

Mulas

et al. 2021

A&A

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Context. 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. Aims. Our aim is to investigate the impact of aliphatic bonds on the VUV photostability of PAH cations in a cryogenic and collisionless environment with conditions applicable for photodissociation regions (PDRs). Methods. The studied species are derived from pyrene (C16H10) and coronene (C24H12) and contain aliphatic bonds either in the form of methyl or ethyl sidegroups or of superhydrogenation. Their cations are produced by laser desorption ionization and isolated in the cryogenic ion cell of the PIRENEA setup, where they are submitted to VUV photons of 10.5 eV energy over long timescales (~1000 s). The parent and fragment ions are mass-analyzed and their relative intensities are recorded as a function of the irradiation time. The fragmentation cascades are analyzed with a simple kinetics model from which we identify fragmentation pathways and derive fragmentation rates and branching ratios for both the parents and their main fragments. Results. Aliphatic PAH derivatives are found to have a higher fragmentation rate and a higher carbon to hydrogen loss compared to regular PAHs. On the other hand, the fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be as stable as, or even more stable than, the bare PAH cations. This stability is quantified for the main ions involved in the fragmentation cascades by the comparison of the fragmentation rates with the photoabsorption rates derived from theoretical photoabsorption cross sections. The most stable species for which there is an effective competition of fragmentation with isomerization and radiative cooling are identified, providing clues on the structures favored in PDRs. Conclusions. 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 3.4 μm 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 3.4 μm AIB.

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Section: Astrophysical Implicationssupporting

confidence: 92%

Section: Methylated and Ethylated Pahsmentioning

confidence: 98%

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Marciniak

Joblin

Mulas

et al. 2021

A&A

View full text Add to dashboard Cite

show abstract

“…3.2). These results are in line with earlier studies (Rouillé et al 2015(Rouillé et al , 2019, which suggest that ethynyl-substitued PAHs are potentially good candidates for astro-PAHs, if they can be formed efficiently as discussed by Rouillé et al (2019).…”

Section: Astrophysical Implicationssupporting

confidence: 92%

“…It is interesting to note that it likely leads to the formation of ethynylsubstituted PAHs, starting from C 18 H + 10 and C 26 H + 12 , in the case of EtPyr + and EtCor + , respectively. Ethynyl-substitued PAHs are expected to be as stable as bare PAHs (Rouillé et al 2015(Rouillé et al , 2019. The fragmentation rate derived for C 18 H + 10 , k M 226 frag = 0.8×10 −3 s −1 suggests that these species could even be more stable than the bare PAH (k M 202 frag = 1.9 × 10 −3 s −1 ) upon VUV irradiation.…”

Section: Methylated and Ethylated Pahsmentioning

confidence: 98%

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Marciniak,

Joblin,

Mulas

et al. 2021

Preprint

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Context. The interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects favoring specific sizes and structures. Aims. Our aim is to investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations in a cryogenic and collisionless environment with conditions applicable for photodissociation regions (PDRs). Methods. The studied species are derived from pyrene (C 16 H 10 ) and coronene (C 24 H 12 ) and contain aliphatic bonds either in the form of methyl or ethyl sidegroups or of superhydrogenation. Their cations are produced by laser desorption-ionization and isolated in the cryogenic ion cell of the PIRENEA setup where they are submitted to VUV photons of 10.5 eV energy over long timescales (∼1000 s). The parent and fragment ions are mass-analyzed and their relative intensities are recorded as a function of the irradiation time. The fragmentation cascades are analyzed with a simple kinetics model from which we identify fragmentation pathways and derive fragmentation rates and branching ratios for both the parents and their main fragments. Results. Aliphatic PAH derivatives are found to have a higher fragmentation rate and a higher carbon to hydrogen loss compared to regular PAHs. On the other hand, the fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be as stable as, or even more stable than, the bare PAH cations. This stability is quantified for the main ions involved in the fragmentation cascades by comparison of the fragmentation rates with the photo-absorption rates derived from theoretical photoabsorption cross sections. The most stable species, for which there is an effective competition of fragmentation with isomerization and radiative cooling, are identified, providing clues on the structures favored in PDRs. Conclusions. This work supports a scenario in which the evaporation of nanograins with mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4 µm 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 containing 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 3.4 µm AIB.

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“…McLuckey and Goeringer have reviewed methods for slow heating activation of ions, with collision-induced dissociation (CID) being the one commonly implemented in ion trapping instruments. Concerning PAHs, such experiments revealed notable differences between isomers regarding their dehydrogenation pathways. − Among photon activation methods, infrared multiple photonic dissociation spectroscopy (IRMPD) has been most widely used to characterize PAH cations and identify their fragments by taking advantage of the free electron laser for infrared experiments (FELIX). − In order to study the isomers formed during the dissociative -H ionization of methylated PAHs, an alternative technique using Ne-tagged ions produced in cryogenic traps has proven to be more appropriate. , Finally, a number of studies have explored the unimolecular dissociation of PAH cations using vacuum ultraviolet (VUV) synchrotron irradiation and photoion mass spectrometry , or the more advanced imaging photoelectron photoion coincidence spectroscopy (iPEPICO). ,− These studies concern neutrals that experience dissociative ionization by absorption of a single VUV photon. Our group has studied the fragmentation (and ionization) of trapped PAH cations subjected to VUV irradiation. − This work has in particular revealed differences in the dehydrogenation characteristics (H, 2H, and H 2 losses) of two dibenzopyrene isomers …”

Section: Introductionmentioning

confidence: 99%

Isomer Differentiation of Trapped C₁₆H₁₀⁺ Using Low-Energy Collisions and Visible/VUV Photons

et al. 2022

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Polycyclic aromatic hydrocarbons are major species in astrophysical environments, and this motivates their study in samples of astrochemical interest such as meteorites and laboratory analogues of stardust. Molecular analyses of carbonaceous matter in these samples show a dominant peak at m/z = 202.078 corresponding to C 16 H 10 . Obtaining information on the associated isomeric structures is a challenge for the molecular analysis of samples available in very small quantities (mg or less). Here we show that coupling laser desorption ionization mass spectrometry with ion trapping opens up the possibility of unraveling isomers by activating ion fragmentation via collisions or photon absorption. We report the best criteria for differentiating isomers with comparable dissociation energies, namely pyrene, fluoranthene, and 9-ethynylphenanthrene, on the basis of the parent dissociation curve and the ratio of dehydrogenation channels. Photoabsorption schemes (multiple photon absorption in the visible range and single photon absorption at 10.5 eV) are more effective in differentiating these isomers than activation by low energy collisions. The impact of the activation scheme on the fragmentation kinetics and dehydrogenation pathways is discussed. By analyzing the 10.5 eV photodissociation measurements with a simple kinetic model, we were able to derive a branching ratio for the H and 2H/H 2 loss channels of the parent ions. The results suggest a role in the formation of H 2 for bay hydrogens that are present in both fluoranthene and 9-ethynylphenanthrene. In addition, we suggest for the latter the presence of a highly competitive 2H loss channel, possibly associated with the formation of a pentagonal ring.

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Threshold Dissociation of the 1-ethynylpyrene Cation at Internal Energies Relevant to H i Regions

Cited by 5 publications

References 47 publications

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Isomer Differentiation of Trapped C₁₆H₁₀⁺ Using Low-Energy Collisions and Visible/VUV Photons

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Threshold Dissociation of the 1-ethynylpyrene Cation at Internal Energies Relevant to H i Regions

Cited by 5 publications

References 47 publications

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Isomer Differentiation of Trapped C16H10+ Using Low-Energy Collisions and Visible/VUV Photons

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Isomer Differentiation of Trapped C₁₆H₁₀⁺ Using Low-Energy Collisions and Visible/VUV Photons