IRMPD spectroscopy of a PAH cation using FELICE: The infrared spectrum and photodissociation of dibenzo[a,l]pyrene

TL;DR

This study uses FELICE to record the first Far-InfraRed spectrum of dibenzo[a,l]pyrene cation, revealing how irregular PAHs' structure influences their infrared spectra and fragmentation, relevant for interstellar chemistry.

Contribution

First experimental FIR spectrum of an irregular PAH cation using FELICE, with insights into its vibrational modes and fragmentation behavior.

Findings

IR-active vibrational modes match calculations with some differences

Only one prominent out-of-plane C-H bending band at 13.5 μm

Irregular PAHs may contribute to D-class interstellar spectra and show resistance to radiation damage

Abstract

We present the experimental InfraRed Multiple Photon Dissociation (IRMPD) spectrum and fragmentation mass spectrum of the irregular, cationic PAH dibenzo[a,l]pyrene (C$_{24}$H$_{14}^+$) in the 6-40 $\mu$m / 250-1650 cm$^{-1}$ range. The use of the the Free-Electron Laser for IntraCavity Experiments (FELICE) enabled us to record its Far-InfraRed (FIR) spectrum for the first time. We aim to understand how irregularity affects the infrared spectrum and fragmentation chemistry of PAHs. Dibenzo[a,l]pyrene is an asymmetric, non-planar molecule, for which all vibrational modes are in principle IR-active. Calculated harmonic Density Function Theory (DFT) and anharmonic Density Functional based Tight Binding Molecular Dynamics (DFTB-MD) spectra show a large wealth of bands, which match the experiment well, but with a few differences. The periphery of the molecule contains several edge geometries, but out of all possible modes in the 11-14 $\mu$m out-of-plane C-H bending region, only one band at 13.5 $\mu$m is prominent. This fact and the richness of the C-C stretching range make irregular PAHs a possible contributor to D-class interstellar spectra. The fragmentation mass spectra reveal facile 2H-loss and no [2C, 2H]-loss, which is attributed to the sterically hindered, non-planar cove region, which could protect irregular PAHs from radiation damage.