Optical Frequency Comb Fourier Transform Spectroscopy of $^{14}$N$_2$$^{16}$O at 7.8 {\mu}m

TL;DR

This study employs a compact mid-infrared frequency comb Fourier transform spectrometer to precisely measure and analyze nitrous oxide spectral lines around 7.8 μm, improving the accuracy of line positions and molecular constants.

Contribution

It introduces a broadband high-resolution measurement technique using a difference frequency generation comb source for nitrous oxide spectroscopy, providing more accurate spectral line data.

Findings

Line positions of 72 lines in the fundamental band were measured with 90-600 kHz uncertainty.

Line positions of 112 hot band lines were measured with improved precision, refining previous data.

Derived upper state constants show excellent agreement with recent studies and improve upon past measurements.

Abstract

We use a Fourier transform spectrometer based on a compact mid-infrared difference frequency generation comb source to perform broadband high-resolution measurements of nitrous oxide, $^{14}$N$_2$$^{16}$O, and retrieve line center frequencies of the $\nu$$_1$ fundamental band and the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band. The spectrum spans 90 cm$^{-1}$ around 1285 cm$^{-1}$ with a sample point spacing of 3 ${\times}$ 10$^{-4}$ cm$^{-1}$ (9 MHz). We report line positions of 72 lines in the $\nu$$_1$ fundamental band between P(37) and R(38), and 112 lines in the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band (split into two components with e/f rotationless parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz. We derive upper state constants of both bands from a fit of the effective ro-vibrational Hamiltonian to the line center positions. For the fundamental band, we observe excellent agreement in the retrieved line positions and upper state constants with those reported in a recent study by AlSaif et al. using a comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf, 2018;211:172-178]. We determine the origin of the hot band with precision one order of magnitude better than previous work based on FTIR measurements by Toth [http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016 data for these bands.