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

Adrian Hjältén,Matthias Germann,Karol Krzempek,Arkadiusz Hudzikowski,Aleksander Głuszek,Dorota Tomaszewska,Grzegorz Soboń,Aleksandra Foltynowicz

doi:10.1016/j.jqsrt.2021.107734

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, 14N216O, and retrieve line center frequencies of the ν1 fundamental band and the ν1 + ν2 – ν2 hot band. The spectrum spans 90 cm−1 around 1285 cm−1 with a sample point spacing of 3 × 10−4 cm−1 (9 MHz). We report line positions of 72 lines in the ν1 fundamental band between P(37) and R(38), and 112 lines in the ν1 + ν2 – ν2 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.

Highlights

Nitrous oxide (N2O) is of significant importance for atmospheric physics and chemistry
We fixed the Doppler widths to the theoretical values calculated at 23 oC, which are around 71 MHz (FWHM)
We demonstrated a Fourier transform spectrometer based on a compact fiber-based difference frequency generation (DFG) optical frequency comb source and a multi-pass cell capable of measuring low-pressure spectra with center frequency precision of the order of 100 kHz in the atmospheric window around 8 μm

Summary

Introduction

Nitrous oxide (N2O) is of significant importance for atmospheric physics and chemistry. We use a Fourier transform spectrometer and a recently developed compact all-fiber-based fceo-free DFG comb emitting at 8 μm [25] to perform high-resolution absorption measurements of the ν11 fundamental band and the ν1 + ν2 – ν2 hot band of 14N216O. We estimate that the contribution of the frep lock to the linewidth of the MIR comb modes is at most 25 kHz. Behind the OP-GaP crystal, an optical long-pass filter blocks the near-IR radiation emitted by the dualwavelength source while transmitting the MIR idler produced in the DFG process. We used the method described in Refs [36, 37] in order to match the frequency domain sampling points to the comb mode positions at each frep step and minimize the influence of the instrumental line shape. We interleaved the averaged and baselinecorrected absorption spectra recorded at different frep steps to obtain the final spectrum with a sample point spacing of 9 MHz in the optical domain

Line by line fitting

Line positions of the ν1 band

Conclusions