OROTRON intracavity millimeter-wave spectroscopy of weakly bound complexes containing molecular hydrogen

Recent results on the millimetre-wave spectroscopy of weakly bound van der Waals complexes, containing molecular hydrogen, are presented. In our experiments we used highly sensitive millimeter-wave intracavity spectrometer combined with a supersonic molecular jet expansion. The key element of the spectrometer is a tunable OROTRON oscillator, which generates the radiation (2-3 mm) through the interaction of an electron beam with the electromagnetic field of an open Fabry-Perot resonant cavity. A high quality factor (Q ≈ 10⁴) of the cavity results in 100 effective passes of the radiation through the jet. Absorption in the cavity causes changes of the electron current in the collector circuit and is detected by measuring these current changes. 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-H₂ potential energy surfaces (PESs) which are a key issue for reliable computations of the collisional excitation and de-excitation of molecules (M = CO, NH₃, H₂O) in the dense interstellar medium. The intermolecular interactions with H₂ play also an important role for high resolution spectroscopy of parahydrogen clusters doped by a probe molecule. Such experiments are directed on the detection of superfluid response of molecular rotation in the He and p-H₂ clusters.