Optical frequency comb Fourier transform spectroscopy of 14N216O at 7.8 µm

A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy

“…We attribute this wide spread of the homogeneous widths to the fact that the SNR of most of the lines is of the order of the ratio of the Doppler to the Lorentzian widths, which limits the precision with which the latter can be determined. Moreover, we previously observed excess line broadening [41], whose origin remains unclear. Possible causes include the spectral widths of the MIR comb modes and distortions of the wavefront of the MIR beam, caused, e.g., by the multi-pass absorption cell.…”

“…The comb-based Fourier transform spectrometer has been described in detail previously [41]. In brief, it consists of three main building blocks: a mid-infrared (MIR) optical frequency comb source, a multi-pass absorption cell, and an FTS.…”

“…Behind the absorption cell, the MIR beam was re-collimated and coupled into a fast-scanning FTS [41,49]. The two out-of-phase interferometer outputs were detected by a pair of thermoelectricallycooled HgCdTe detectors in a balanced configuration.…”

“…Comb-based Fourier transform spectroscopy lends itself particularly useful to studying the congested and irregular spectrum of methane, since this technique can acquire high-resolution spectra simultaneously over large spectral intervals, and eliminates the need to probe each absorption line individually by tuning the laser source [40]. Recently, Hjältén et al [41] used optical frequency comb Fourier transform spectroscopy to record high-resolution spectra of the ν1 fundamental band and the ν1 + ν2 -ν2 hot band of nitrous oxide (N2O), achieving line positions with uncertainties of the order of 100 kHz. The results were in good agreement with the earlier study by AlSaif et al [42,43], who recorded the ν1 fundamental band using a comb-referenced continuous-wave (CW) quantum cascade laser.…”

“…Here we take advantage of the high resolution, absolute frequency calibration, large bandwidth and high spectral brightness provided by the previously developed optical frequency comb Fourier transform spectrometer (FTS) [41] to record methane absorption spectra in the range from 1250 to 1380 cm -1 . From these spectra, we retrieve line positions and intensities of over 650 absorption lines from the ν4 fundamental bands of 12 CH4 and 13 CH4, as well as from two 12 CH4 hot bands, ν2 + ν4 -ν2 and 2ν4 -ν4.…”