from the Meudon PDR code website [@le_petit2006], and those for CO are taken from the model of @visser2009.

Photodissociation rates and shielding functions {#sec:pd rates}
===============================================

\[sec:shielding functions\]

The photodissociation rate, $k_0$, of N$_2$ exposed to UV radiation can be calculated according to $$\label{eq:kpd}
 k_0 = \int \sigma^\mathrm{pd} (\lambda) I(\lambda)d \lambda, $$ where $\sigma^\mathrm{pd}(\lambda)$ is the N$_2$ photodissociation cross section described in Sec. \[sec:CSE model\], $I$ is the intensity of the radiation field, and $\lambda$ the wavelength. [The unattenuated interstellar radiation field (ISRF) of @draine1978, in units of photons cm$^{-2}$s$^{-1}$Å$^{-1}$sr$^{-1}$, is used in most of the following calculations and is given by $$\begin{gathered}
\label{eq:draine field differential}
 F(\lambda) = \frac{1}{4\pi} ( 3.2028 \times 10^{15} \lambda^{-3} - 5.1542 \times 10^{18} \lambda^{-4} \\ + 2.0546 \times 10^{21} \lambda^{-5} ) ,\end{gathered}$$ with $\lambda$ in Å. In the following we employ an angle-integrated intensity, $$I(\lambda) = 4\pi \chi F(\lambda),
\label{eq:draine field}$$ with an additional scaling factor relative to the Draine field, $\chi$. ]{} The integral in Eq. (\[eq:kpd\]) must be computed between 912 and 1000Å. This range is defined by the long-wavelength onset of the N$_2$ absorption spectrum and the complete suppression of ultraviolet radiation for wavelengths shorter than the atomic H ionisation limit.

The 