We investigate the importance of the shielding of chemical photorates by molecular hydrogen photodissociation lines and the carbon photoionization continuum deep within models of photon dominated regions .
In particular , the photodissociation of N _ { 2 } and CN are significantly shielded by the H _ { 2 } photodissociation line spectrum .
We model this by switching off the photodissociation channels for these species behind the H i \to H _ { 2 } transition .
We also model the shielding effect of the carbon photoionization continuum as an attenuation of the incident radiation field shortwards of 1102 Å .
Using recent line and continuum cross section data , we present calculations of the direct and cosmic ray induced photorates for a range of species , as well as optically thick shielding factors for the carbon continuum .
Applying these to a time dependent PDR model we see enrichments in the abundances of N _ { 2 } , N _ { 2 } H ^ { + } , NH _ { 3 } and CN by factors of \sim 3 - 100 in the extinction band A _ { v } = 2.0 to A _ { v } = 4.0 for a range of environments .
While the precise quantitative results of this study are limited by the simplicity of our model , they highlight the importance of these mutual shielding effects , neither of which has been discussed in recent models .