Carbon Chemistry in Photodissociation Regions

We present recent results on the carbon chemistry in photodissociation regions. We show that carbon chains and rings (CCH, c-C$_3$H$_2$ and C$_4$H) are tightly spatially correlated with each other, and with the mid-infrared emission due to PAHs (7 and 15 $\mu$m), mapped by ISOCAM. Neither the spatial distribution, nor the abundances of these species can be fit by state-of-the-art PDR models, which calls for another production mechanism. We discuss model predictions for carbon clusters and simple hydrocarbons. We show how selected abundance ratios can be used as a diagnostic of the physical conditions. We stress the need for more theoretical and laboratory work on fundamental processes relevant for the interstellar medium, which should be taken into account in the astrochemical models, but whose rates are not known accurately enough.