Millimeter-Wave Spectrometer for High-Precision Studies of Jet-Cooled Molecules and Weakly Bound Molecular Complexes

We have constructed a millimeter-wave free-space absorption spectrometer for high-resolution studies of jet-cooled molecules and weakly bound van der Waals complexes. The spectrometer employs a microwave frequency generator supplemented by active frequency multipliers and covers the spectral region from 50 to 170 GHz. A nozzle source is fixed on a rooftop mirror placed in the vacuum chamber in such a way that the millimeter-wave radiation propagates parallel to the molecular jet and makes two passes along it. Such arrangement of radiation and nozzle provides better sensitivity and higher spectral resolution compared to more common perpendicular configuration. Schottky diode detectors are used to measure absorption in the molecular gas flow. The observed linewidths are 30–40 kHz, and the accuracy of the line center determination is about 3–4 kHz. The presented spectra of the astrophysically relevant rare CO isotopologues, and the weakly bound NH₃–H₂ complexes demonstrate the potential of the newly built spectrometer. The line splittings arising from the hyperfine interactions of the nuclear spins of ¹³C for CO and ¹⁴N for NH₃–H₂ were resolved and analyzed.