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Astron. Astrophys. 322, L1-L4 (1997)

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2. Experiment

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 [FORMULA] 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 [FORMULA]. 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 [FORMULA] transition at 1000 GHz.


Table 1. Observed frequencies for the [FORMULA] rotational transition of NH

[FIGURE] Fig. 1. The [FORMULA] rotational spectrum of the NH radical ([FORMULA]) recorded at 1000 GHz. Relative intensities and calculated line positions of the hfs components (Table 1) are shown below the spectrum. This 200 MHz scan consists of 887 data points with a total integration time of about 140 seconds.
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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998