A microwave free jet absorption spectrometer and its first applications

A free jet absorption microwave spectrometer working in the frequency range 50-80 GHz has recently been built in our laboratory. Different experiments can be performed with the system: the microwave radiation can be focussed on the supersonic expansion either with a lens corrected horn system or with a semiconfocal Fabry-Perot interferometer. The microwave radiation can be either parallel or perpendicular to the jet, and Stark modulation or double resonance experiments (MW-MW or RF-MW, respectively) are possible. As first applications, studies on complicated conformational equilibria, tautomeric equilibria and molecules with puzzling nuclear hyperfine structure (for example, a molecule containing two iodine atoms) have been performed.