Mid-infrared spectroscopic signatures of dibenzopyrene cations – The effect of symmetry on PAH IR spectroscopy

https://doi.org/10.1016/j.jms.2021.111458Get rights and content
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Highlights

  • Gas phase mid-IR spectra of three isomeric dibenzopyrene (DBP) cations are presented.

  • The spectrum of the most symmetric DBP best resembles interstellar IR emission bands.

  • The latter finding lends credit to the grandPAH hypothesis.

Abstract

In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (C2h, Cs and C1). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.

Keywords

Mid-IR spectroscopy
Interstellar medium
Polycyclic aromatic hydrocarbons
Quadrupole ion trap
Mass spectrometry

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