translates into a shorter photodissociation radius for C$_2$H$_2$ (see bottom panels in Fig. \[fig:chemistry\]). The photodissociation rates computed here for C$_2$H$_2$ and HCN are similar to those given in the compilation of [@hea2017] (2017). These authors, however, compute a photodissociation rate of acetylene which is about twice smaller than our value, which would shift the photodissociation radius of C$_2$H$_2$ to somewhat outer radii with respect to our predictions.

### Polyynes

The main pathway for forming polyynes of increasing chain length with an even number of carbon atoms involves successive reactions with the radical C$_2$H, $${\rm C_2H + C}_{2n}{\rm H_2} \rightarrow {\rm C}_{2n+2}{\rm H_2 + H}, \label{reac:c2h}$$ where diacetylene (C$_4$H$_2$) is synthesized when $n=1$, triacetylene (C$_6$H$_2$) is formed when $n=2$, and so on. The reaction between C$_2$H and C$_2$H$_2$ is fast at low temperatures. The measured rate constant in the temperature range 15-295 K shows a slight negative temperature dependence, with values in the range $(1.1-2.3)\times10^{-10}$ cm$^3$ s$^{-1}$ ([@cha1998] 1998). There are no measurements of the rate constant at low temperatures for $n>1$. The reaction between C$_2$H and C$_4$H$_2$ has been studied theoretically and experimentally using the crossed beams technique ([@lan2008] 2008; [@gu2009] 2009). These studies indicate that the kinetics of 