Design and performance of an L-shaped Fourier transform microwave spectrometer (L-FTMW): Fabry–Perot cavity spectrometer setup

A new L-shaped molecular beam Fourier transform microwave spectrometer (L-FTMW) has been developed at Tennessee Tech University to perform both cavity and chirped-pulse rotational spectroscopy within a single platform. The instrument features an L-shaped high-vacuum chamber comprised of stainless-steel and polycarbonate sections, allowing orthogonal operation of Fabry-Perot cavity and chirped-pulse configurations without mechanical reconfiguration. This paper focuses on the design, operation, and performance of the 8-18 GHz Fabry-Perot cavity subsystem within the L-FTMW spectrometer. The cavity is formed by two 7.5-inch-diameter aluminum mirrors with 30 cm radii of curvature, arranged near-confocally and coupled to a near-coaxial pulsed molecular beam. A custom Python-based interface enables automated high-resolution mapping of cavity resonances and broadband data acquisition with minimal user intervention. The system routinely achieves 2 kHz frequency resolution, enabling precise measurement of hyperfine spectral features. Performance was validated through measurements of benchmark systems, including OCS isotopologues and their weakly bound van der Waals complexes. The 17O13CS isotopologue (natural abundance = 0.000 397 2%, corresponding to ∼40 ppb in a 1% OCS/argon mixture) was detected within 5 min of signal averaging at natural abundance with argon as the carrier gas. The simple mechanical design and open-source control software make the L-FTMW spectrometer a versatile and accessible platform for high-resolution rotational spectroscopy and future investigations of reaction dynamics and kinetics.