High-resolution spectroscopy of [H,C,N] ⁺ : II. Ground state rotational spectrum of HCN ⁺ ( <i>X</i> ² Π)

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 <i>J</i>″ = 2.5 to 5.5 have been observed for the lowest-energy spin-orbit state ²Π_3/2 of this open-shell cation, which has an inverted ²Π electronic ground state. The transitions show well-resolved <i>Λ</i>-doubling components each displaying complex hyperfine splittings due to the presence of two nuclei with non-zero spins, ¹H (<i>I</i> = 1/2) and ¹⁴N (<i>I</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.