Schematic structure of the CSE for a C-rich AGB star, which is divided into 6 regions for modelling purposes. (I): a degenerate C/O core and He/H burning shell; (II): a convective shell; (III): a stellar atmosphere in which parent species are formed; (IV): a dust formation shell with an expanding envelope; (V): an outer CSE where daughter species are formed primarily by photodissociation; (VI): the interstellar medium (ISM). This study focusses on the outer CSE where chemistry is mainly driven by the photodissociation of molecules.