Passage And Resonance In Synergy – PARIS: A molecular jet chirp- and tone-excitation Fourier transform microwave spectrometer for broad acquisition and high resolution

Jet-expansion Fourier transform microwave (FTMW) spectroscopy is a powerful technique for investigating isolated molecules and molecular aggregates. The coaxial beam-resonator arrangement offers unrivalled resolution, but requires repetitive frequency retuning of a high-Q resonator for survey spectra using short, resonant single-tone excitation pulses. Chirp-excitation methods, using a fast but powerful frequency-ramp signal passing over spectral line positions, dramatically reduce survey time, but at the cost of lower resolution and sensitivity for individual lines. As a consequence, many laboratories employ both spectrometer types. However, setting up individual machines with their own vacuum systems, high-frequency electronics, and control software is costly and time-consuming. We developed and constructed the PARIS (Passage And Resonance In Synergy) FTMW spectrometer, operating in the 2-20 GHz range, integrating both tone-excitation and chirp-excitation techniques in a single, resource-efficient setup: Using a modular design, most electronic components serve dual-purpose, with the integration of reflector-refocused and resonator-enhanced interaction setups in a spherical high-vacuum chamber. The chirp and single-tone excitation setups are arranged perpendicularly, with instant mode-switching, both using microwave propagation coaxial with the molecular jet-expansion. PARIS achieves a sensitivity of a few parts-per-billion for OCS diluted in neon (1%) on the resonator axis and a few parts-per-million on the broadband axis. A resolution (full width at half maximum, FWHM, of the amplitude spectrum) of ∼2 kHz is attained with both experiments, capable of resolving complex hyperfine structures, such as those from two 14N nuclei of 4-methylpyrimidine, already in broadband operation at minimum resolvable peak separation matching the respective FWHM.