of photodissociation rates in interstellar and blackbody radiation fields.

The interior regions of interstellar clouds and protoplanetary discs may be completely shielded from external sources of ultraviolet photons. However, collisions between high-energy cosmic rays and produce a weak local UV field that could be important for photodissociation. The cosmic ray induced photoionisation and photodissociation of molecular and atomic species is well established [@gredel1987; @gredel1989; @dalgarno2006; @padovani2009]. In Sec. \[sec:CR\] we combine our model of N$_2$ photoabsorption with a similarly thorough theoretical treatment of the H$_2$ line emission associated with cosmic rays. This is, to study in detail for the first time the photodissociation of and in cloud and disc interiors. [We also revisit calculations of the CO dissociation rate due to cosmic rays [@gredel1987] taking advantage of an updated photodissociation cross section.]{}

In Sec. \[sec:photochemical models\] of this paper we present the results of chemical models of an interstellar cloud and protoplanetary disc. These include linked chemical networks for species containing ${}^{14}$N and ${}^{15}$N and the photodissociation rates of @li2013 and Secs. \[sec:pd rates\] and \[sec:CR\]. Using these models we investigate the consequences of isotope selective photodissociation on the abundances of atomic and molecular nitrogen and other N-bearing species observed 