2009A&A...494..719P

Dynamical studies of prestellar cores search for small velocity differences between different tracers. The highest radiation frequency precision is therefore required for each of these species. We want to adjust the frequency of the first three rotational transitions of N₂H⁺ and N₂D⁺ and extrapolate to the next three transitions. We compare N₂H⁺ and N₂D⁺ to NH₃, the frequency of which is more accurately known and which has the advantage of being spatially coexistent with N₂H⁺ and N₂D⁺ in dark cloud cores. With lines among the narrowest, and the N₂H⁺ and NH₃ emitting region among the largest, L183 is a good candidate for comparing these species. A correction of ∼10 kHz for the N₂H⁺ (J:1-0) transition is found (∼0.03km/s). Similar corrections from a few m/s up to ∼0.05km/s compared to previous astronomical determinations are reported for the other transitions (N₂H⁺ (J:3-2) and N₂D⁺(J:1-0), (J:2-1), and (J:3-2)). Einstein spontaneous-decay coefficients (A_ul) are included.