A series of rapid neutral-neutral reactions involving these radicals then drives the growth of carbon chains. The schemes shown on top of Fig. \[fig:chemistry\], inspired by [@mil1994] (1994), illustrate the main processes involved in the synthesis of polyynes and cyanopolyynes. The schemes include only those processes which, according to the chemical model, are the main contributors to the formation of each species. Nevertheless, a discussion of the reliability of these schemes and whether other types of reactions may play a role follows.

The formation of carbon chains begins with the photodissociation of C$_2$H$_2$ and HCN, whose cross sections are well known from laboratory experiments. We computed photodissociation rates for the local ultraviolet radiation field of [@dra1978] (1978) and dust shielding factors (see Table \[table:rates\]) using the [Meudon PDR]{} code ([@lep2006] 2006). For C$_2$H$_2$ we adopted the photoabsorption cross section measured by [@coo1995] (1995) and subtracted the photoionization cross section from the compilation by [@hud1971] (1971) to obtain the cross section of photodissociation, which is assumed to yield the C$_2$H radical with a 100 % efficiency. For HCN, photodissociation is assumed to yield only CN radicals with the cross section measured by [@nut1982] (1982). Acetylene photodissociates faster than hydrogen cyanide, which 