Yield of electronically excited CN molecules from the dissociative recombination of HNC$^{+}$ ion with electrons

We report flowing-afterglow measurements of the CN(B-X) and CN(A-X) emissions from the dissociative recombination (DR) of HNC$^{+}$ ions. A separate drift-tube study showed that the reaction Ar$^{+}$+ HCN, the precursor reaction used in the flow-tube experiment, produces mainly HNC$^{+}$ rather than its HCN$^{+}$ isomer. Recombining HNC$^{+}$ afterglows showed emissions of CN (B-X) and CN(B-A) but some arise from excitation transfer of metastable argon, Ar* + HCN. By adding xenon, Ar* atoms were removed and the pure recombination spectrum was recovered. Models simulating the ion-chemical processes, diffusion and gas mixing, were fitted to observed position-dependent CN band intensities. Absolute yields of CN (B) and CN (A) were inferred by comparing band intensities to those of CO bands from DR of CO$_{2}^{+}$ ions. We conclude that the 300 K recombination coefficient of HNC$^{+}$ is close to 2$\times $10$^{-7}$ cm$^{3}$/s, that CN(B) is formed with a yield of $\sim $20{\%} and CN(A) with a yield of $\sim $12{\%}. The rotational temperature of CN(B) is around 2500 K, and CN(B) and CN(A) are far more vibrationally excited than predicted by the ``impulse model'' of Bates. This finding suggests that the recombination may involve a multistep mechanism.