  Polycyclic Aromatic Hydrocarbons (PAHs) are believed to be the small-size
tail of the interstellar carbonaceous dust grain population. Their vibrational
emission is the most widely accepted source of the aromatic near-infrared
features, and their rotational radiation is a likely explanation for the
dust-correlated anomalous microwave emission (AME). Yet, no individual
interstellar PAH molecule has been identified to date. It was recently
recognized that quasi-symmetric planar PAHs ought to have a well identifiable
comb-like rotational spectrum, and suggested to search for them in
spectroscopic data with matched-filtering techniques. We report the results of
the first such search, carried out with the Green Bank Telescope, and targeting
the star-forming region IC348 in the Perseus molecular cloud, a known source of
AME. Our observations amounted to 16.75 hours and spanned a 3 GHz-wide band
extending from 23.3 to 26.3 GHz. Using frequency switching, we achieved a
sensitivity of 0.4 mJy per 0.4 MHz channel (1-sigma). The non-detection of
comb-like spectra allowed us to set upper bounds on the abundance of specific
quasi-symmetric PAH molecules (specified uniquely by their moments of inertia)
of approximately 0.1% of the total PAH abundance. This bound generically
applies to PAHs with approximately 15 to 100 carbon atoms.
