The NH radical was produced in a dc discharge of ammonia (NH3) and helium at partial pressures of about 2 Pa and 5 Pa, respectively. The absorption cell was 2 m long with an inner diameter of 10 cm. The optimum discharge current was 200 mA, and the cell temperature was kept at about C by cooling with liquid nitrogen. The radiation source used was a backward wave oscillator (BWO OB-83-1) with a frequency range of 874-1100 GHz supplied by the ISTOK Research and Production Company (Fryazino, near Moscow). The BWO was frequency stabilized by phase-locking its output to a KVARZ synthesizer, which uses a 5 MHz reference signal from a rubidium atomic clock providing a short term relative accuracy of better than . Further details of the experimental setup can be found in Winnewisser (1995 ) and Belov et al. (1995 ). Table 1 lists the 25 observed frequencies together with their experimental uncertainties, which were estimated according to the obtained signal-to-noise ratios. For single lines with a good signal-to-noise ratio the uncertainties were estimated to be 50 kHz. For some weak or blended lines the uncertainties were increased up to 500 kHz. Figure 1 displays the recorded spectrum of the transition at 1000 GHz.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998