Rotational spectroscopy of interstellar PAHs
  Polycyclic aromatic hydrocarbons (PAHs) are believed to be ubiquitous in the
interstellar medium. Yet, to date no specific PAH molecule has been identified.
In this paper, a new observational avenue is suggested to detect individual
PAHs, using their rotational line emission at radio frequencies. Previous PAH
searches based on rotational spectroscopy have only targeted the bowl-shaped
corannulene molecule, with the underlying assumption that other polar PAHs are
triaxial and have a complex and diluted spectrum unusable for identification
purposes. In this paper the rotational spectrum of quasi-symmetric PAHs is
computed analytically. It is shown that the asymmetry of planar,
nitrogen-substituted symmetric PAHs is small enough that their rotational
spectrum, when observed with a resolution of about a MHz, has the appearance of
a "comb" of evenly spaced stacks of lines. The simple pattern of these "comb"
spectra allows for the use of matched-filtering techniques, which can result in
a significantly enhanced signal-to-noise ratio. Detection forecasts are
discussed for regions harbouring "anomalous microwave emission", believed to
originate from the collective PAH rotational emission. A systematic search for
PAH lines in various environments is advocated. If detected, PAH "combs" would
allow to the conclusive and unambiguous identification of specific,
free-floating interstellar PAHs.
