High Accuracy Line Lists of CH4 and H2CO in the 8 μm Range from Optical Frequency Comb Fourier Transform Spectroscopy

Spectral data in the 8 µm region - a water window where many molecules show strong rovibrational features - are currently derived mostly from conventional FTIR measurements, with line position uncertainties of the order of a few to few tens of MHz [1]. Recently, we developed a Fourier transform spectrometer (FTS) based on a compact 8 µm frequency comb that allows line position retrieval with sub-MHz accuracy [2]. We record and interleave spectra at different repetition rates to obtain sampling point spacing of ~1 0 MHz using the sub-nominal resolution sampling-interleaving method to analyze the FTS data [3]. Here, we use this spectrometer to measure high-resolution absorption spectra of two species important in atmospheric sensing and astrophysics: methane, Fig. 1(a), a greenhouse gas and a constituent of exoplanetary atmospheres, and formaldehyde, Fig. 1(c), a toxic pollutant and a species found in the interstellar medium. By fitting Voigt functions to the individual absorption lines in spectra measured over a wide range of partial pressures, we determine center frequencies of transitions with intensities spanning more than 3 orders of magnitude, with tvpical uncertainties of a few hundred kHz.