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A review of recent results on the millimetre-wave spectroscopy of weakly bound van der Waals complexes, mostly those which contain H 2 and He, is presented. In our work, we compared the experimental spectra to the theoretical bound state results, thus providing a critical test of the quality of the M–H 2 and M–He potential energy surfaces (PESs) which are a key issue for reliable computations of the collisional excitation and de-excitation of molecules (M = CO, NH 3 , H 2 O) in the dense interstellar medium. The intermolecular interactions with He and H 2 play also an important role for high resolution spectroscopy of helium or para -hydrogen clusters doped by a probe molecule (CO, HCN). Such experiments are directed on the detection of superfluid response of molecular rotation in the He and p -H 2 clusters.

The interaction between ammonia and helium has attracted considerable interest over many years, partly because of the observation of interstellar ammonia. The rate coefficients of NH3 –He scattering are an important ingredient for numerical modeling of astrochemical environments. Another, though quite different application in which the NH3 –He interaction can play an important role is the doping of helium clusters with NH3 molecules to perform high-resolution spectroscopy. Such experiments are directed on the detection of non-classical response of molecular rotation in helium clusters addressing fundamental questions related to the microscopic nature of superfluidity. High-resolution spectroscopy on the NH3–He complex is an important tool for increasing our understanding of intermolecular forces between NH3 and He.

The rotational spectra of the NH 3 –D 2 and ND 3 –H 2 van der Waals complexes have been observed in a supersonic molecular jet for the first time. The OROTRON intracavity spectrometer was used to detect pure rotational transitions in the frequency range from 75 to 120 GHz. The search of the spectral lines was based on the initial observation of the main isotopic species of the NH3−H2. The measured transition frequencies including hyperfine splitting were analyzed in order to determine the energy levels, molecular parameters and structure of the ortho ND 3 − ortho H 2 and ortho NH 3 − para D 2 complexes.