OROTRON intracavity millimeter-wave spectroscopy of weakly bound complexes and small clusters

Much progress, both in experiment and theory, has been made in recent years for the small clusters of helium and molecular hydrogen containing a single guest molecule. Such studies allow the cluster properties to be determined with “atomic resolution” thus probing the onset of microscopic superfluidity. The rotational spectra of the He-CO and H₂-CO complexes and small He_n-CO (n ≤ 10) clusters, produced using a molecular jet expansion, were observed with intracavity millimeter-wave spectrometer (80-150 GHz). The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of an open Fabry-Perot resonant cavity. Owing to the small dimensions of the resonant absorption cell, the OROTRON spectrometer is very suitable for double resonance experiments. The overview of the spectrometer as well as recent results on molecular complexes and small clusters will be presented.