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