First detection of [N II] 205 μm absorption in interstellar gas - Herschel-HIFI observations towards W 31C, W 49N, W 51, and G34.3+0.1C. M. Persson, M. Gerin, B. Mookerjea, J. H. Black, M. Olberg, J. R. Goicoechea, G. E. Hassel, E. Falgarone, F. Levrier, K. M. Menten and J. PetyA&A, 568 (2014) A37DOI: https://doi.org/10.1051/0004-6361/201423997
