The NH radical is one of the fundamental hydrides, and its spectroscopic properties have been studied extensively. The electronic spectrum of NH has been measured by various researchers since 1935, as compiled by Huber & Herzberg (1979 ). From the electronic spectrum, molecular constants for the ground electronic state have been obtained by Dixon (1959 ), Murai & Shimauchi (1966 ), Malicet et al. (1970 ), Ubachs et al. (1984 ), and Brazier et al. (1986 ). Notably, the investigation by Brazier et al. (1986 ) of the band near 3360 Å with a high-resolution Fourier transform (FT) spectrometer yielded relatively precise molecular constants, while the investigation of Ubachs et al. (1984 ) determined hyperfine coupling constants from molecular beam experiments. The vibration-rotation spectrum of NH has been measured and analyzed by Bernath & Amano (1982 ) and Sakai et al. (1982 ). Hyperfine coupling constants were also determined from rotational spectra recorded by Radford & Litvak (1975 ) and Wayne & Radford (1976 ) with a far infrared laser magnetic resonance (LMR) spectrometer. In addition, van den Heuvel et al. (1982 ) measured the rotational spectrum at zero magnetic field using a tunable laser-sideband spectrometer. They observed two transitions ( and ) between the fine structure levels of the rotational transition and determined molecular constants including hyperfine coupling constants. A measurement of the fine structure transition has not been reported so far.
The NH radical has also been well studied in interstellar space. It has been observed in absorption toward stars (e.g. Schmitt 1969 , Lambert & Beer 1972 , Lambert et al. 1984 , Ridgway et al. 1984 ), including the Sun (Grevesse et al. 1990 , Geller et al. 1991 ), and towards comets (e.g. Feldman et al. 1993 ) via its electronic, vibration-rotation, and high-N rotational transitions. The NH radical has also been detected in diffuse interstellar clouds toward Per and HD 27778 from its electronic absorption spectra (Meyer & Roth 1991 ). The obtained column density toward Per is 9.0 1011 cm-2. This value is 20-40 times larger than the one obtained by gas-phase chemical model calculations for Per (van Dishoeck & Black 1988 ; van Dishoeck 1992 ; Wagenblast et al. 1993 ). Accordingly, a significant contribution to the NH formation rate must occur on grain surfaces, as has already been considered by Mann & Williams (1984 ), and strongly suggested by Wagenblast et al. (1993 ).
Unlike the case of diffuse clouds, no detection of NH has been reported in dark clouds. One reason for this is that the transition falls in the 900-1000 GHz region, and is difficult to observe from ground-based observatories owing to the enhanced atmospheric opacity. A knowledge of the abundance of NH is important for the study of nitrogen chemistry in dark clouds, because it is one of the probable intermediate species in the production of NH3 from the nitrogen ion by successive hydrogenation reactions (e.g. Herbst, DeFrees, & McLean 1987 ; Galloway & Herbst 1989 ). The abundance of NH3 is high in many dark clouds, and the detection of NH can be expected in these sources. The NH2 radical, another intermediate in the formation of ammonia, has been detected in absorption towards SgrB2 (van Dishoeck et al. 1993 ).
In this Letter, we report extended frequency measurements of the pure rotational transition of NH carried out with the Cologne terahertz spectrometer. These measurements yielded precise transition frequencies from which accurate molecular constants including hyperfine coupling constants were obtained. The results are of importance to future radioastronomical observations.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998