The microwave spectrum of isotopically substituted CO+ ion

The frequencies of the N = 0–1 transitions of C18O+ and 13CO+ have been measured, and those of the main isotopic form have been remeasured. The hyperfine parameters of 13CO+ have been obtained and compared to those previously determined in the laser-ion beam experiments of Carrington et al. The isotopic variations of the spin-rotation parameter γ have been studied, and evidence for breakdown of the Born–Oppenheimer approximation has been observed in the isotopic dependence of the rotational constant B0.