and will absorb extra UV radiation [@siebenmorgen]. @chaparro_molano2012a assumed a dust distribution with a minimum grain size of 0.1$\mu$m in their study of cosmic-ray induced photodissociation in a model disc. Following from this assumption they deduced a value of $x_\mathrm{dust}\sigma^\text{abs}_\mathrm{dust} = 1.47\times 10^{-22}\,\mathrm{cm^2}$, significantly below the commonly-adopted interstellar value. Here, we explore an even lower value, $x_\mathrm{dust}\sigma^\text{abs}_\mathrm{dust} = 10^{-23}\,\mathrm{cm^2}$, in order to contrast a more extreme case of grain growth in a protoplanetary disc with interplanetary dust.

Finally, the photodissociation rate of species X (per molecule X) due to cosmic ray induced photons may be calculated according to $$\label{eq:CR abs pd rate}
  k_\text{X} = \frac{1}{x_\mathrm{X}}\int R(\lambda)p_\mathrm{X}(\lambda)\,d\lambda.$$ In some previous descriptions of cosmic-ray induced photodissociation [@gredel1989; @mcelroy2013] an efficiency is defined by factoring the H$_2$ ionisation rate and dust grain albedo from Eq. (\[eq:CR abs pd rate\]). In this case, the importance of molecular line absorption in Eq. (\[eq:absorption fraction\]) prevents the factoring of dust properties.

Cosmic-ray induced photodissociation of  {#sec:CR pd N2}
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[lrrr]{}\
Dissociation rate ($\times10^{-16}\,\mathrm{s^{-1}}$): & & &\
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$\zeta=10^{-16}\,\mathrm{s^{-1}}$ & & &\
$x_\mathrm{{}^{14}N_2}=10^{-5}$ & & &\
$x_\mathrm{{}^{14}N^{15}N}=x_\mathrm{{}^{14}N_2}/225$ & & &\
$x_\mathrm{CO}=7\times 10^{-5}$ & & &\
$T_\mathrm{ex}=20\,\mathrm{K}$ & 14 & 21 & 8.1\
$b=1\,\mathrm{km\,s^{-1}}$ & & 