  Polycyclic Aromatic Hydrocarbons (PAHs) are considered as a major constituent
of interstellar dust. They have been proposed as the carriers of the Aromatic
Infrared Bands (AIBs) observed in emission in the mid-IR. They likely have a
significant contribution to various features of the extinction curve such as
the 220 nm bump,the far-UV rise and the diffuse interstellar bands. Emission
bands are also expected in the far-IR, which are better fingerprints of
molecular identity than the AIBs. They will be searched for with the Herschel
Space Observatory. Rotational emission is also expected in the mm range for
those molecules which carry significant dipole moments. Despite spectroscopic
studies in the laboratory, no individual PAH species could be identified. This
emphasises the need for an investigation on where interstellar PAHs come from
and how they evolve due to environmental conditions: ionisation and
dissociation upon UV irradiation, interactions with electrons, gas and dust.
There is also evidence for PAH species to contribute to the depletion of heavy
atoms from the gas phase, in particular Si and Fe. This paper illustrates how
laboratory work can be inspired from observations. In particular there is a
need for understanding the chemical properties of PAHs and PAH-related species,
including very small grains, in physical conditions that mimic those found in
interstellar space. This motivates a joint effort between astrophysicists,
physicists and chemists. Such interdisciplinary studies are currently
performed, taking advantage of the PIRENEA set-up, a cold ion trap dedicated to
astrochemistry.
