The Zeeman effect in CO⁺ observed with rotational action spectroscopy

We discuss newly measured rotational transitions of CO⁺ (X2Σ+) in its vibrational ground state <i>v</i> = 0, in particular the fine-structure components of the <i>N</i> = 0−1 and <i>N</i> = 1−2 transitions. We employed a rotational action spectroscopic technique in a cryogenic ion trap for the measurements. The recorded low-temperature and high-resolution spectra of CO⁺ show resolved Zeeman splittings caused by the Earth's magnetic field. The highly accurate experimental transition frequencies and derived molecular constants agree with previous measurements and improve the spectroscopic parameters for this known interstellar molecular ion.