grains or aromatic polycyclic hydrocarbons as precursors ([@kro1987] 1987; [@pet2005] 2005), and bottom-up chemical schemes driven by cosmic rays or ultraviolet photons ([@her1989] 1989; [@fuk1998] 1998; [@cer2004] 2004).

Carbon chains are particularly conspicuous in circumstellar envelopes around carbon-rich Asymptotic Giant Branch (AGB) stars, which are also the main factories of carbonaceous dust grains in the Galaxy. Circumstellar envelopes provide an ideal laboratory in which to study the formation of carbon chains because of their relative simplicity in geometry, velocity field, and composition of the precursor material. Because chemical processes take place as matter is ejected from the star through a nearly isotropic outflow, the sequential formation of molecules can be tracked as a function of the radial position in the envelope. The central AGB star is not expected to emit at ultraviolet wavelengths and thus the main source of ultraviolet photons is the ambient radiation field from nearby stars. Chemical models point to a bottom-up scenario in which the formation of carbon chains of increasing length occurs because of ultraviolet-driven polymerization of C$_2$H$_2$ and HCN. These small precursors are injected into the expanding wind from the inner circumstellar regions and are photodissociated in the outer circumstellar layers, where the external 