Single‐photon zero‐kinetic‐energy pulsed‐field‐ionization spectra have been measured for the v^+=0 and 1 levels of CO^+ (X ^2Σ^+) and the v^+=0 level of N_2^+ (X ^2Σ_g^+) by coherent XUV radiation. In spite of similarities in the electronic structure of CO and N_2, the measured ion spectra show dramatically different intensities for the Q branches. These threshold spectra are interpreted on the basis of ab initio calculations of the ion rotational distributions. Agreement between the calculated and measured spectra is very encouraging. Improved values for the ionization potentials of CO (113 025.6 and 115 211.2±1.5 cm^(−1) for v^+=0 and 1, respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed. The CO spectra show quite different behavior for the ΔN<0 transitions for v^+=0 and v^+=1 bands, which is interpreted in terms of the relative importance of rotational autoionization in the two bands
