Knowledge Bank

The high resolution absorption spectra of the $A2\Pi \leftarrow X2\Pi 000$ electronic transitions of the isoelectronic pair of cations, $HC5N+$ and $NC4N+$ have been obtained. The cations are generated in a liquid-nitrogen cooled hollow cathode discharge and detected using a frequency-discharge double modulation technique. Rotational analyses yields accurate ground and excited state rotational constants. Neutral cyanopolyacetylenes are prevalent linear interstellar molecules detected by radioastronomy. The dicyano analogues are expected to be similarly abundant. Rotational characterization of the corresponding ionized species by means of their electronic transitions provide important information for the identification of such species as carriers of the optical diffuse interstellar bands.