Structural investigation of doubly-dehydrogenated pyrene cations

Panchagnula, Sanjana; Bouwman, Jordy; Rap, Daniël B.; Castellanos, Patricia Cruz; Candian, Alessandra; Mackie, Cameron J.; Banhatti, Shreyak; Brünken, Sandra; Linnartz, H.; Tielens, A. G. G. M.

doi:10.1039/d0cp02272a

Cited by 25 publications

(25 citation statements)

References 56 publications

(68 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We detect an additional band at 1100 cm −1 and confirm the band at 990 cm −1 , which was detected at a very low S/N ratio in the previous work. Panchagnula et al (2020) also published an IRMPD spectrum of pyrene in their Supplementary Material, in the 600-1700 cm −1 range, which matches even better with our spectrum. They reproduced both our experimentally observed 990 and 1100 cm −1 bands with a similarly high S/N, and the shape and peak positions line up nicely.…”

Section: Pyrenesupporting

confidence: 83%

Gas-phase spectroscopy of photostable PAH ions from the mid- to far-infrared

Wiersma,

Candian,

Bakker

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

We present gas-phase InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy of cationic phenanthrene, pyrene, and perylene over the 100-1700 cm −1 (6-95 µm) spectral range. This range covers both local vibrational modes involving C-C and C-H bonds in the mid-IR, and large-amplitude skeletal modes in the far-IR. The experiments were done using the 7T Fourier-Transform Ion Cyclotron Resonance (FTICR) mass spectrometer integrated in the Free-Electron Laser for Intra-Cavity Experiments (FELICE), and findings were complemented with Density Functional Theory (DFT) calculated harmonic and anharmonic spectra, matching the experimental spectra well. The experimental configuration that enables this sensitive spectroscopy of the strongly-bound, photo-resistant Polycyclic Aromatic Hydrocarbons (PAHs) over a wide range can provide such high photon densities that even combination modes with calculated intensities as low as 0.01 km•mol −1 near 400 cm −1 (25 µm) can be detected. Experimental frequencies from this work and all currently available IRMPD spectra for PAH cations were compared to theoretical frequencies from the NASA Ames PAH IR Spectroscopic Database to verify predicted trends for far-IR vibrational modes depending on PAH shape and size, and only a relatively small redshift (6-11 cm −1 ) was found between experiment and theory. The absence of spectral congestion and the drastic reduction in bandwidth with respect to the mid-IR make the far-IR fingerprints viable candidates for theoretical benchmarking, which can aid in the search for individual large PAHs in the interstellar medium.

show abstract

Section: Pyrenesupporting

confidence: 83%

Gas-phase spectroscopy of photostable PAH ions from the mid- to far-infrared

Wiersma,

Candian,

Bakker

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

show abstract

“…For [Pyr-2H] + , we considered two isomers for which the 2H losses occur from adjacent carbons (C 1,2 or C 4,5 ). The cross sections of these two isomers have been equally weighted as suggested by the study of Panchagnula et al (2020), which compares the IR spectrum of the doubly dehydrogenated pyrene with DFT computed harmonic spectra of the fourteen possible isomers and concludes that a 1:1 mixture of 1,2-ddPyr + and 4,5-ddPyr + reproduces fairly well the measurement. For C 15 H + 9 , we considered two isomers with a five-membered ring either on the long (1) or the short (2) axis of the initial pyrene structure.…”

Section: D1 Computation Of the Photoabsorption Cross Sectionsmentioning

confidence: 99%

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Marciniak

Joblin

Mulas

et al. 2021

A&A

View full text Add to dashboard Cite

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.

show abstract

“…8 ). The cross section of these two isomers have been equally weighted as suggested by the study of Panchagnula et al (2020), which compares the IR spectrum of the doubly-dehydrogenated pyrene with DFT computed harmonic spectra of the fourteen possible isomers and concludes that a 1:1 mixture of 1,2-ddPyr + and 4,5-ddPyr + reproduces fairly well the measurement. For C 15 H + 9 , we have considered two isomers with a five-membered ring either on the long (1) or the short (2) axis of the initial pyrene structure.…”

Section: Appendix A: Additional Diagnostics To Estimate the Vuv Fluxmentioning

confidence: 99%

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Marciniak,

Joblin,

Mulas

et al. 2021

Preprint

View full text Add to dashboard Cite

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.

show abstract

Structural investigation of doubly-dehydrogenated pyrene cations

Abstract: The first infrared predissociation spectra of the pyrene and doubly-dehydrogenated pyrene cations (C16H10˙+; Py+ and C16H8˙+; ddPy+) are presented. The vibrationally resolved spectrum of ddPy+ is dominated by absorption features from two isomers.

Cited by 25 publications

References 56 publications

Gas-phase spectroscopy of photostable PAH ions from the mid- to far-infrared

Gas-phase spectroscopy of photostable PAH ions from the mid- to far-infrared

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

Contact Info

Product

Resources

About