contrast high and low temperature limits in the present study. ]{}

[The rate of ultraviolet photons generated by cosmic ray collisions per unit spectral density per hydrogen nucleus is given by $$\label{eq:CR UV intensity}
  R(\lambda) = \zeta x_\mathrm{H_2} P(\lambda),$$ where $x_\mathrm{H_2}= n(\mathrm{H}_2) / \left[ n(\mathrm{H})+2n(\mathrm{H}_2) \right]$ is the relative abundance of molecular hydrogen with respect to H nuclei and $\zeta$ is the cosmic-ray-induced ionisation rate per H$_2$ molecule.]{} A value of $\zeta=10^{-16}\ \text{s}^{-1}$ per is adopted for the following calculations and falls within the broad range of rates estimated from observations of [ions in diffuse and translucent clouds]{} [@dalgarno2006; @padovani2009; @indriolo2012; @rimmer2012]. This collection of observations suggest a rate as high as $10^{-15}\,\mathrm{s}^{-1}$ at the edge of an interstellar cloud, which decreases with extinction to $\mathord{\sim}10^{-16}\,\mathrm{s}^{-1}$ for $A_V=10$mag as the lowest-energy cosmic rays are attenuated. Our adopted value is higher than observed in the interiors of the densest clouds with $\zeta < 10^{-17}\,\mathrm{s}^{-1}$ [@hezareh2008]. Theoretical consideration of a range of source and attenuation mechanisms influencing the cosmic ray flux [[@padovani2009; @cleeves2013]]{} suggest a smaller rate $\zeta$ than $10^{-16}\,\mathrm{s}^{-1}$ may be expected in circumstellar environments, because of the possible influence of any stellar wind and magnetic field. The photodissociation results presented 