High-resolution spectroscopy of [H,C,N]+: II. Ground state rotational spectrum of HCN+ (X̃2Π).

The rotational spectrum of HCN+ has been measured between 270 and 520 GHz in a cryogenic ion trap instrument employing a double resonance vibrational-rotational scheme based on leak-out spectroscopy. A total of four transitions with quantum numbers ranging from J″ = 2.5 to 5.5 have been observed for the lowest-energy spin-orbit state 2Π3/2 of this open-shell cation, which has an inverted 2Π electronic ground state. The transitions show well-resolved Λ-doubling components each displaying complex hyperfine splittings due to the presence of two nuclei with non-zero spins, 1H (I = 1/2) and 14N (I = 1). The accurate transition frequencies and ground state spectroscopic parameters determined for HCN+ in this work will enable radio astronomical searches of this important cation in the interstellar medium.