8 research outputs found
Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands
The so-called aromatic infrared bands are attributed to emission of
polycyclic aromatic hydrocarbons. The observed variations toward different
regions in space are believed to be caused by contributions of different
classes of PAH molecules, i.e. with respect to their size, structure, and
charge state. Laboratory spectra of members of these classes are needed to
compare them to observations and to benchmark quantum-chemically computed
spectra of these species. In this paper we present the experimental infrared
spectra of three different PAH dications, naphthalene,
anthracene, and phenanthrene, in the vibrational fingerprint
region 500-1700~cm. The dications were produced by electron impact
ionization of the vapors with 70 eV electrons, and they remained stable against
dissociation and Coulomb explosion. The vibrational spectra were obtained by IR
predissociation of the PAH complexed with neon in a 22-pole cryogenic
ion trap setup coupled to a free-electron infrared laser at the Free-Electron
Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic
density-functional theory calculations for both singly and doubly charged
states of the three molecules. The experimental band positions showed excellent
agreement with the calculated band positions of the singlet electronic ground
state for all three doubly charged species, indicating its higher stability
over the triplet state. The presence of several strong combination bands and
additional weaker features in the recorded spectra, especially in the
10-15~m region of the mid-IR spectrum, required anharmonic calculations to
understand their effects on the total integrated intensity for the different
charge states. These measurements, in tandem with theoretical calculations,
will help in the identification of this specific class of doubly-charged PAHs
as carriers of AIBs.Comment: Accepted for publication in A&
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
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
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
High-resolution rovibrational spectroscopy of c-C3H2+: The v(7) C-H antisymmetric stretching band
The v(7) antisymmetric CeH stretching fundamental of c-C3H2+ has been characterized in a cryogenic 22pole ion trap by a novel type of action spectroscopy, in which the rovibrational excitation of c-C3H2+ is detected as a slowing down of the low-temperature reaction c-C3H2+ + H-2 -> C3H3+ + H. Ninety-one rovibrational transitions with partly resolved fine structure doublets were measured in high resolution. Supported by high-level quantum chemical calculations, spectroscopic parameters were determined by fitting the observed lines with an effective Hamiltonian for an asymmetric rotor in a doublet electronic ground state, (X) over tilde (2)A(1), yielding a band origin at 3113.6400(3) cm(-1). Based on these spectroscopic parameters, the rotational spectrum of this astronomically important molecule is predicted. (C) 2020 Elsevier B.V. All rights reserved
Structural investigation of doubly-dehydrogenated pyrene cations
The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation (C16H10 center dot+; Py+ and C16H8 center dot+; ddPy(+)) are presented. Infrared predissociation spectroscopy is employed to measure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. The spectrum of Py+ allows a detailed comparison with harmonic and anharmonic density functional theory (DFT) calculated normal mode frequencies. The spectrum of ddPy(+) is dominated by absorption features from two isomers (4,5-ddPy(+) and 1,2-ddPy(+)) with, at most, minor contributions from other isomers. These findings can be extended to explore the release of hydrogen from interstellar PAH species. Our results suggest that this process favours the loss of adjacent hydrogen atoms