N2H+ and N2D+ in interstellar molecular clouds. II- Observations

We present observations of the $J$=1–0, 2–1, and 3–2 rotational transitions of N${}_2$H${}^{+}$ and N${}_2$D${}^{+}$ towards a sample of prototypical dark clouds. The data have been interpreted using non–local radiative transfer models.