In previous investigations, the pure inversion spectrum of 14 ND 3 in the v 2 ฯญ 1 state has been measured (1, 2). Observed transitions were limited by quantum numbers J ี
16 and K ี
13. In the present work, we have extended the 14 ND 3 data set by measuring high J (from 12 up to 23) lines belonging to the K ฯญ J, K ฯญ J ฯช 1, K ฯญ J ฯช 2, K ฯญ J ฯช 3 branches in the v 2 ฯญ 1 state in the region 97-161 GHz. We started this investigation with the purpose of completing the data on ammonia and testing the standard polynomial and exponential expressions for description of high J spectrum of a molecule with large amplitude motions.
A highly sensitive intracavity spectrometer based on orotron has been used for the detection of weak transitions between low populated states. This recently developed spectrometer was described in detail earlier (3,4). The main feature of this technique is that an absorption cell is placed in a high quality resonator (Q ฯท 10 4 ) of a tunable coherent oscillator, orotron. Together with a large effective absorption length, such a scheme allows one to combine the frequency of radiation with the cavity resonant peak immediately in contrast to the scheme with external resonant cell. Absorption signal in this case is detected by a variation of orotron current in a collector circuit, and thus, transmitting microwave lines between radiation source, gas cell, and detector are absent. This considerably decreases the variations in the spectrometer parameters on frequency scanning yielding a high sensitivity of measurements. A 30-kHz sine wave frequency modulation was applied to the orotron oscillator with the detection of the absorption signal at twice the modulation frequency.
The deuterated ammonia with D concentration of 99% was used for the measurements. The range of the sample pressure was between 0.05-0.10 Torr. All spectra were observed at room temperature. The accuracy of the measurements is believed to be 50 kHz, except for very weak lines.
Some 26 lines of ND 3 were measured in the v 2 ฯญ 1 state. They correspond to the pure inversion transitions (โฌJ ฯญ 0, โฌK ฯญ 0) belonging to the K ฯญ J, K ฯญ J ฯช 1, K ฯญ J ฯช 2, K ฯญ J ฯช 3 branches. To assign these lines, the first transition frequencies have been predicted on the basis of molecular parameters given in Ref. (1,2). Subsequent assignments were performed by a bootstrap method. The maximum J value was 23. The recording of one of the weakest lines J ฯญ 22, K ฯญ 20 is shown in Fig. 1. The previously measured and new transition frequencies are presented in Table 1. In the case of transitions with K ฯญ 3 splitting and hyperfine quadrupole structure, the central frequency is given. The spectrum was fit to either the standard power series (polynomial) of J and K quantum numbers or to an exponential of the power series (exponential) (5). The expressions are inv อ J, Kอ ฯญ 0 ฯฉ Eอ J, Kอ