Gas-phase spectroscopy of photostable PAH ions from the mid- to
  far-infrared
  We present gas-phase InfraRed Multiple Photon Dissociation (IRMPD)
spectroscopy of cationic phenanthrene, pyrene, and perylene over the 100$-$1700
cm$^{-1}$ (6-95 $\mu$m) spectral range. This range covers both local
vibrational modes involving C$-$C and C$-$H bonds in the mid-IR, and
large-amplitude skeletal modes in the far-IR. The experiments were done using
the 7T Fourier-Transform Ion Cyclotron Resonance (FTICR) mass spectrometer
integrated in the Free-Electron Laser for Intra-Cavity Experiments (FELICE),
and findings were complemented with Density Functional Theory (DFT) calculated
harmonic and anharmonic spectra, matching the experimental spectra well. The
experimental configuration that enables this sensitive spectroscopy of the
strongly-bound, photo-resistant Polycyclic Aromatic Hydrocarbons (PAHs) over a
wide range can provide such high photon densities that even combination modes
with calculated intensities as low as 0.01 km$\cdot$mol$^{-1}$ near 400
cm$^{-1}$ (25 $\mu$m) can be detected. Experimental frequencies from this work
and all currently available IRMPD spectra for PAH cations were compared to
theoretical frequencies from the NASA Ames PAH IR Spectroscopic Database to
verify predicted trends for far-IR vibrational modes depending on PAH shape and
size, and only a relatively small redshift (6$-$11 cm$^{-1}$) was found between
experiment and theory. The absence of spectral congestion and the drastic
reduction in bandwidth with respect to the mid-IR make the far-IR fingerprints
viable candidates for theoretical benchmarking, which can aid in the search for
individual large PAHs in the interstellar medium.
