Long-wave mid-infrared cavity-enhanced frequency comb spectroscopy of cold, complex molecules.

We report the development of an instrument for cavity-enhanced absorption spectroscopy of the fundamental rovibrational transitions of molecules in the long-wave mid-infrared (LWIR) region from 6500 to 10000 nm. Our setup combines an LWIR frequency comb, a high-finesse optical enhancement cavity, a cryogenic buffer gas cooling cell, and a Fourier transform interferometer to obtain broadband, high-resolution, and high-sensitivity molecular absorption spectra. Here, we showcase the capabilities of this setup by presenting the gas-phase LWIR spectra of the ν6 band of ethane and the ν10 band of the gauche conformer of ethanol near 7200 nm under both room temperature and cryogenic conditions.