HNC and HCN in an extended negative glow discharge: Implication to the branching ratio of the dissociative recombination of HCNH+

Submillimeter-wave transitions of HNC and HCN are observed in an extended negative glow discharge in a gas mixture of CH4 and N2. It is well established that the extended negative glow discharge is a good source for production of positive ions for microwave observations which need lower gas pressures to avoid the line broadening. The branching ratio of the dissociative recombination to lead to HCN and HNC is derived from the abundance ratio by examining the temperature dependence of the signal intensities of the lines of HCN and HNC as well as HCNH+. The branching ratio thus obtained is 3 with an estimated uncertainty of 20% for the normal species and 1.5 for the deuterated species. The larger ratio for the normal species might be caused by isomerization of HNC to HCN.