fractionation of protostellar oxygen isotopes is assisted by the self-shielding of CO, which has very similar molecular structure to N$_2$ [@sheffer2002; @lyons_young2005; @smith2009]. The investigation of isotope selective N$_2$ photodissociation in interstellar clouds and protoplanetary environments is then highly warranted.

The importance of photodissociation due to the interstellar radiation field (ISRF) or an ultraviolet emitting star will decrease as the radiation is attenuated below the surface of an interstellar cloud, circumstellar envelope, or protoplanetary disc. @li2013 made a detailed assessment of the dissociation rate of and the shielding of ultraviolet radiation by relevant materials (H$_2$, H, itself, and dust grains). This work considered in detail the sharply structured absorption spectrum of N$_2$ and shielding species. This was made possible by means of an accurate and comprehensive quantum-mechanical model of N$_2$ photoabsorption and dissociation, which was informed by an extensive history of N$_2$ spectroscopy in the laboratory [e.g., @ajello_etal1989; @helm_etal1993; @sprengers_etal2003; @sprengers_etal2004b; @sprengers_etal2005b; @stark_etal2008; @lewis_etal2008a; @heays_etal2009; @heays2011b] and theoretically [e.g., @spelsberg_meyer2001; @lewis_etal2005a; @lewis_etal2005b; @haverd_etal2005; @lewis_etal2008c; @lewis_etal2008b; @ndome_etal2008]. This model, detailed in Sec. \[sec:CSE model\], explicitly accounts for the nuclear masses and so is equally applicable to the case of . In Sec. \[sec:pd rates\] of this paper, we describe new calculations 