The detection of the imidogen radical, NH, with ISO LWS
in an Extragalactic source was described first by
E. González-Alfonso, H, A. Smith, J. Fischer, and J. Cernicharo,
The Far-Infrared Spectrum of Arp 220
Astrophys. J. 613, (2004) 247261.
The two overlapping strongest fine structure components of the N = 2 1 transition were clearly detected near 153.2 μm as well as the overlapping components of 3 2 near 102 μm. There is additional evidence near the noise level for the three weaker fine structure components of 2 1.