here may be adapted to ionisation rates other than $\zeta=10^{-16}\,\mathrm{s}^{-1}$ by a simple scaling.

The cosmic-ray generated photons may be absorbed by dust grains and by abundant line-absorbing species: H$_2$, H, N$_2$, and CO. The idea of an absorbing column density is inappropriate because of the local nature of the photon source. Instead, the fraction of ultraviolet radiation absorbed by a particular species, X, and leading to its dissociation is determined from its relative abundance $x_\text{X}$, according to $$\label{eq:absorption fraction}
  p_\text{X}(\lambda) = \frac{x_\text{X}\sigma^\mathrm{pd}_\text{X}(\lambda)}{x_\mathrm{dust}\sigma_\mathrm{dust} + \sum_i x_i\sigma^\mathrm{abs}_i(\lambda)}.$$ Here, $x_i$ and $\sigma_i^\mathrm{abs}(\lambda)$ are the abundances and photoabsorption cross sections of various species, $x_\mathrm{dust}\sigma_\mathrm{dust}$ represents dust absorption, and $\sigma^\mathrm{pd}_\text{X}(\lambda)$ is a photodissociation cross section.

An important term in the summation in Eq. (\[eq:absorption fraction\]) arises from H$_2$. For the cases of X being or CO, self-shielding is important and must also be included. Self-shielding and shielding by are both important for the case of photodissociation. Atomic hydrogen is also a source of UV shielding at the edge of a cloud or disc but is assumed here to have too low abundance (about $10^{-4}$ relative to H$_2$) to be important in fully shielded regions. Consequently, the common assumption that all radiation shortwards of 912Å 