Laboratory Measurement of the J = 1-0 Transition of CH ⁺

The A 1 Π- X 1 Σ electronic band of the CH + ion has been used as a probe of the physical and dynamical conditions of the interstellar medium (ISM) for 65 years. In spite of being one of the first molecular species observed in the ISM, and the very large number of subsequent observations with large derived column densities, the pure rotational spectra of CH + has remained elusive in both the laboratory and, with one exception (Cernicharo et al.), the ISM as well. We report the first laboratory measurement of the pure rotation of the CH + ion J = 1-0 at 835078.950(75) MHz. Also reported are the somewhat unexpected chemical conditions that resulted in laboratory production. A mass-independent analysis of the existing optical data and the microwave transition is presented, along with predictions for rotational transition frequencies of isotopomers of CH + .