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Broadband high-resolution molecular spectroscopy with interleaved mid-infrared frequency combs

Historically, there has been a trade-off in spectroscopic measurements between high spectral resolution, broadband coverage, and acquisition time. Optical frequency combs, initially envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet region, are now commonly used for probing molecular rotational-vibrational transitions throughout broad spectral bands in the mid-infrared with superior accuracy, resolution and speed. Here we demonstrate acquisition of 2.5 million spectral points over the continuous wavelength range of 3.2 to 5.1 microns (frequency span 1200 cm^{-1}, resolution <100 kHz) via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequency-comb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying spectra with gradually shifted comb lines we fully resolved amplitude and phase spectra of molecules with narrow Doppler-broadened lines, such as ν1+ν3 band of carbon disulfide (CS2) and its isotopologues. Also, we were able to observe the chemical oxidation reaction of CS2 by detecting traces of carbonyl sulfide (OCS) via its asymmetric ν3 stretch band.