High-precision line positions of N2O and CH4 at 8 μm from optical frequency comb Fourier transform spectroscopy
A number of small molecules of interest to atmospheric physics and astrophysics exhibit fundamental vibrational modes in the 8 μm spectral range. Spectroscopic detection of these molecular species in the Earth’s atmosphere and in celestial objects requires accurate line lists. However, current spectroscopic databases in the 8 µm region are still largely based on conventional FTIR data with insufficient precision and accuracy. Much better precision and accuracy can be obtained using optical frequency comb spectroscopy, which provides a direct link of optical frequencies to radio frequency standards. Until now, the number of high-resolution comb-based measurements in the 8 µm range has been very limited, mostly because of the lack of reliable comb sources in this spectral region.
We recently developed an optical frequency comb Fourier transform spectrometer operating in the 8 μm range  based on a compact difference frequency generation optical frequency comb source  referenced to a GPS-disciplined Rb frequency standard. Employing the sub-nominal resolution sampling and interleaving technique [3,4], we acquired high-resolution low-pressure spectra of several vibrational bands of N2O , a greenhouse gas and ozone depleting substance, and CH4, a greenhouse gas and a constituent of (exo-)planetary atmospheres. The N2O spectra cover a range of 1251 to 1318 cm-1 with a sampling point spacing of 9 MHz, while those of CH4 span from 1240 to 1366 cm-1 at a point spacing of 14 MHz. From these spectra, we retrieved line positions with uncertainties on the level of a few hundred kHz – well below the precision of conventional FTIR-based studies. These may be used to validate and update the theoretical models at the basis of current spectroscopic databases.
We will present the comb-based spectrometer and the results obtained so far for N2O and CH4, and compare them with those of conventional FTIR spectroscopy, as well as a recent high-resolution study using a comb-referenced quantum-cascade laser .
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