Far-Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbons:
  Zero Kinetic Energy Photoelectron Spectroscopy of Pentacene Vaporized from
  Laser Desorption
  The distinctive set of infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6,
and 11.3{\mu}m are ubiquitously seen in a wide variety of astrophysical
environments. They are generally attributed to polycyclic aromatic hydrocarbon
(PAH) molecules. However, not a single PAH species has yet been identified in
space, as the mid-IR vibrational bands are mostly representative of functional
groups and thus do not allow one to fingerprint individual PAH molecules. In
contrast, the far-IR (FIR) bands are sensitive to the skeletal characteristics
of a molecule, hence they are important for chemical identification of unknown
species.
  With an aim to offer laboratory astrophysical data for the Herschel Space
Observatory, Stratospheric Observatory for Infrared Astronomy, and similar
future space missions, in this work we report neutral and cation FIR
spectroscopy of pentacene (C_22H_14), a five-ring PAH molecule. We report three
IR active modes of cationic pentacene at 53.3, 84.8, and 266{\mu}m that may be
detectable by space missions such as the SAFARI instrument on board SPICA.
  In the experiment, pentacene is vaporized from a laser desorption source and
cooled by a supersonic argon beam. We have obtained results from two-color
resonantly enhanced multiphoton ionization and two-color zero kinetic energy
photoelectron (ZEKE) spectroscopy. Several skeletal vibrational modes of the
first electronically excited state of the neutral species and those of the
cation are assigned, with the aid of ab initio and density functional
calculations.
