is completely consumed by H ionisation does not apply here. However, the N$_2$ photodissociation cross section at shorter wavelengths and approaching the photoionisation limit at 800Å contributes less than 10% to its total photodissociation rate, and is neglected here.

Shielding of a penetrating interstellar radiation field by dust grains was discussed in Sec. \[sec:shielding functions\]. The extinction of cosmic-ray induced radiation is treated differently. In this case, all photons are assumed absorbed before traversing a significant fraction of the dark interior of the interstellar cloud or disk where they are generated. Then, the detailed angular scattering of photons by dust grains is inconsequential and only the absorption cross section need be considered. The dust absorption cross section is related to its extinction cross section by $$\label{eq:dust abs ext}
  \sigma^\text{abs}_\text{dust} = \sigma^\text{ext}_\text{dust}(1-\omega),$$ where $\omega$ is the grain albedo. We adopt a commonly used value derived from observations [@savage1977; @bohlin1978] for the dust extinction cross section per hydrogen nucleus of $2\times 10^{-21}\,\mathrm{cm}^2$ and assume an albedo of $\omega=0.5$. Then, $x_\mathrm{dust}\sigma^\text{abs}_\mathrm{dust} = 10^{-21}\,\mathrm{cm^2}$ in Eq. (\[eq:absorption fraction\]).

[The assumption of locally-absorbed UV photons is only appropriate for a sufficiently large and homogeneous medium. The mean-free-path of cosmic-ray generated photons in an interstellar cloud 