are also important but to a lesser extent. Diacetylene and triacetylene are easily photodissociated owing to their large absorption cross sections in the ultraviolet. The photodissociation rate of C$_4$H$_2$ has been computed from the photoabsorption cross section measured by [@fer2009] (2009), after subtraction of the photoionization cross section measured by [@sch2012] (2012). The main channel in the photodissociation of C$_4$H$_2$ is the production of C$_4$H radicals ([@sil2008] 2008). For C$_6$H$_2$, the photoionization cross section has not been measured and therefore the photodissociation rate has been directly computed from the photoabsorption cross section measured by [@klo1974] (1974) and [@shi2003] (2003), assuming that the main channel is C$_6$H + H.

The reactions involving C$_2$ of interest here are $${\rm C_2 + C}_{2n}{\rm H_2} \rightarrow {\rm C}_{2n+2}{\rm H + H}, \label{reac:c2}$$ where the radicals C$_4$H and C$_6$H form by addition of C$_2$ subunits to C$_2$H$_2$ and C$_4$H$_2$, respectively. The reaction between C$_2$ and C$_2$H$_2$ is rapid at low temperatures, with a rate constant of $(2.4-4.8)\times10^{-10}$ cm$^3$ s$^{-1}$ in the temperature range 24-300 K ([@can2007] 2007; [@dau2008] 2008; [@par2008] 2008). The low-temperature kinetics of reactions of C$_2$ with C$_4$H$_2$ and larger polyynes has not been studied experimentally, and thus in the model we assume 