Molecular and thermal equilibrium in low density interstellar clouds. I. The chemical reaction network.

The paper describes a chemical reaction network used to compute molecular abundances in interstellar clouds with optical depths not exceeding unity in the visible range, densities of less than 10,000 per cu cm, and temperatures between 10 and 200 K. Included in the reaction network are the atoms H, C, N, and O, their singly positive ions, all their diatoms and respective singly positive ions, as well as the molecules and molecular ions H3(+), CH2, CH2(+), CH3(+), NH2(+), NH3(+), H2O, H2O(+), H3O(+), HCO(+), C2H, C2H(+), C2H2(+), HD, HD(+), and H2D(+). The relevant chemical and photodestruction reactions are listed, the dominant production processes for various hydrides are identified, and charge-exchange reactions are discussed along with neutral-neutral reactions. Photoionization cross sections and rates are examined for H2, HD, CH, CH2, OH, H2O, NH, C2, CN, CO, N2, NO, and O2.