INFRARED SPECTROSCOPY OF THE CATIONIC –H FRAGMENTS OF METHYL-PAHS

Wenzel, Gabi

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Description

Title

INFRARED SPECTROSCOPY OF THE CATIONIC –H FRAGMENTS OF METHYL-PAHS

Author(s)

Wenzel, Gabi

Contributor(s)

• Joblin, Christine
• Brünken, Sandra
• Schlemmer, Stephan
• Jusko, Pavol
• Banhatti, Shreyak
• Simon, Aude

Issue Date

2023-06-21

Keyword(s)

Mini-symposium: Infrared Spectroscopy in the JWST Era

Abstract

Cationic benzylium and tropylium are two competitive isomers formed by dissociative ionization of methylbenzene (toluene). Infrared predissociation (IRPD) spectroscopy of ions tagged with Ne has been established as a powerful tool in disentangling these cationic speciesᵃ. Methylated polycyclic aromatic hydrocarbons (PAHs) are expected to be abundant in space and their dissociative ionization could lead to the formation of both the benzylium- (XCH₂⁺) and tropylium-like (XC₇⁺) cations, which are expected to be the two lowest-energy isomers and whose isomerization process might impact their chemical evolution in photodissociation regions (PDRs). Here, we consider three methyl-PAHs as precursors, namely 1- methylpyrene, 2-methylnaphthalene, and 2-methylanthracene. Their cationic –H fragments, C₁₇H₁₁⁺, C₁₁H₉⁺, and C₁₅H₁₁⁺, were probed at the Free Electron Laser for Infrared eXperiments (FELIX) Laboratory using IRPD spectroscopy at the FE- Lion cryogenic ion trap beamline. Their strongest vibrational band is located at about 6.2 µm revealing the predominance of the XCH₂⁺ isomersᵇ. Isomer abundance measurements and spectral comparison to computed anharmonic IR spectra show that only this isomer is present for C₁₇H₁₁⁺, whereas at least two isomers are present with a large abundance for the acene-derived species. Clear spectral evidence for the XC₇⁺ isomer is found in the C₁₁H₉⁺ case consistent with a relative abundance of 30 %. These results demonstrate the important role of steric hindrance in the formation of XC₇⁺ and reveal the potential of XCH₂⁺ to account for the aromatic IR emission band (AIB) observed at 6.2 µm in astrophysical environments, NGC7027 and the PDR at the Orion Bar.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

Text

Language

eng

Handle URL

https://hdl.handle.net/2142/122580

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