ISSN 0030-400X, Optics and Spectroscopy, 2012, Vol. 113, No. 1, pp. 1–4. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.V. Potapov, A.A. Dolgov, L.A. Surin, 2012, published in Optika i Spektroskopiya, 2012, Vol. 113, No. 1, pp. 3–6. 1 INTRODUCTION Investigations of rotational and vibrational-rota- tional spectra of weakly bound molecular complexes are of special interest because of large-amplitude internal motions in these systems, in particular, almost free rotations of the monomers comprising the com- plexes (see, for example, [1]). The study of these motions significantly enhances our understanding of intermolecular dynamics and provides rich material for the development of ab initio calculations. In addition, van der Waals complexes that contain atoms and molecules that are abundant in planetary atmospheres and interstellar molecular clouds are also of interest for astrophysics. Detailed information on the parameters of the intermolecular interaction in such systems is required to interpret the spectral data obtained with radio-telescopes. We studied a number of complexes containing He, Ne, H 2 , N 2 , CO, and CH 4 recently [2–8]. The NH 3 –N 2 complex consid- ered in this paper also belongs to this series. Only two experimental spectroscopic studies [9, 10] were devoted to the NH 3 –N 2 complex. It was shown in both papers that, because of the small bind- ing energy, the ammonia and nitrogen molecules can rotate almost freely within the complex and their indi- vidual para- and ortho-states caused by the presence of equivalent nuclei are retained. Thus, there exist four spin modifications of the complex: ortho/paraNH 3 – ortho/paraN 2 (denoted below as o/pNH 3 –o/pN 2 ). Four rotational transitions R(0), R(1), R(2), and R(3) were measured in [9] by the molecular-beam electric-resonance method in the frequency range 6– 27 GHz for the oNH 3 –oN 2 spin modification in the state with K = 0 (K is the projection of total rotational momentum J on the intermolecular axis of the com- plex). The hyperfine structure caused by the nuclear quadrupole moments of three 14 N nitrogen atoms in the complex was partially resolved for the R(0) and R(1) transitions. In addition, one more—presumably R(1)—transition was recorded for the oNH 3 –pN 2 modification in the K = 1 state. All four possible spin modifications of the complex were studied in [10] in the frequency range of 76– 106 GHz with a pulsed molecular beam spectrometer. The recorded transitions R(11), R(12), R(13), R(14), and R(15) allowed the authors to describe a number of the lowest-lying states of the complex with K = 0 and K = 1 by sets of molecular parameters. In that case, the hyperfine structure was not observed because of high values of rotational momentum J specifying the tran- sition. In this paper, we extended the measurements of transitions in the known rotational states with K = 0 and K = 1 for different modifications of NH 3 –N 2 to the region of higher J values. EXPERIMENTAL To record the spectra of the NH 3 –N 2 complex in the frequency range 112–130 GHz, we used an orotron-based spectrometer combined with a pulsed molecular beam. The spectrometer was presented and described in detail in [11]. Briefly, the experimental setup is as follows. An electronic orotron generator of microwave radiation is placed in a vacuum chamber together with a super- sonic nozzle. An open Fabry–Perot resonator forms the oscillating system of the orotron. The molecular beam of a gas under study ejected from the nozzle passes through the spectrometer cavity perpendicu- larly to its axis. The high Q factor of the cavity ensures about 100 passes of radiation generated by the orotron SPECTROSCOPY OF ATOMS AND MOLECULES Millimeter-Wave Spectroscopy of the Weakly Bound Molecular Complex NH 3 –N 2 A. V. Potapov a, b , A. A. Dolgov a , and L. A. Surin a, b a Institute for Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow oblast, 142190 Russia b First Physical Institute, University of Cologne, Cologne, 50937 Germany e-mail: potapov@isan.troitsk.ru, surin@ph1.uni-koeln.de Received December 13, 2011 Abstract—The rotational spectrum of the van der Waals NH 3 –N 2 complex is studied in the frequency range of 112–130 GHz. The transitions are measured in a cold molecular beam with an intracavity spectrometer based on an orotron. A total of six new transitions of different spin modifications of the complex are recorded. Molecular parameters of the K = 0 ground state are determined for the orthoNH 3 –orthoN 2 modification. DOI: 10.1134/S0030400X1207017X