  We report the first experimental study of the attachment of multiple foreign
atoms to a cationic polycyclic aromatic hydrocarbon (PAH). The chosen PAH was
coronene, C$_{24}$H$_{12}$, which was added to liquid helium nanodroplets and
then subjected to electron bombardment. Using mass spectrometry, coronene
cations decorated with helium atoms were clearly seen and the spectrum shows
peaks with anomalously high intensities ("magic number" peaks), which represent
ion-helium complexes with added stability. The data suggest formation of a
rigid helium layer consisting of 38 helium atoms that completely covers both
faces of the coronene ion. Additional magic numbers can be seen for the further
addition of 3 and 6 helium atoms, which are thought to attach to the edge of
the coronene. The observation of magic numbers for the addition of 38 and 44
helium atoms is in good agreement with a recent path integral Monte Carlo
prediction for helium atoms on neutral coronene. An understanding of how atoms
and molecules attach to PAH ions is important for a number of reasons including
the potential role such complexes might play in the chemistry of the
interstellar medium.
