Precision frequency comb spectroscopy of the $^{14}$N$_{2}$O, $^{14}$N$^{15}$NO, $^{15}$N$^{14}$NO, and $^{15}$N$_{2}$O isotopocules in the 3300-3550 cm$^{-1}$ range

Abstract

Nitrous oxide is a long-lived greenhouse gas. Its isotopic composition provides valuable insights into sources and sinks, and about the mechanism of formation. A major challenge in the spectroscopic analysis of the isotopocule compositions is the availability of accurate spectroscopic parameters, particularly for the minor $^{15}$N isotopocules. In this work, we introduce high-resolution spectroscopic measurements of four isotopocules of nitrous oxide: $^{14}$N$_{2}$O, $^{14}$N$^{15}$NO, $^{15}$N$^{14}$NO, and $^{15}$N$_{2}$O in the mid-infrared range of 3300 - 3550 cm$^{-1}$, using a frequency comb-based Fourier transform spectrometer. The nitrous oxide samples were obtained from a chemical synthesis involving acid-catalyzed amine-borane reduction of equimolar amounts of $^{15}$N isotopically enriched sodium nitrite and $^{14}$N sodium nitrite. The high-resolution spectra, measured in a temperature-controlled single-pass absorption cell, were used to retrieve line center frequencies and relative intensities for a total of 426 rovibrational transitions of the ${\nu}$$_{1}$ + ${\nu}$$_{3}$ band of the four isotopocules, and of the one order of magnitude weaker 2${\nu}$$_{2}$ + ${\nu}$$_{3}$ and ${\nu}$$_{1}$ + ${\nu}$$_{2}$ + ${\nu}$$_{3}$ - ${\nu}$$_{2}$ bands in the same spectral region. We compare the determined line center frequencies and relative intensities with spectroscopic parameters available in high-resolution molecular databases. For $^{14}$N$_{2}$O, $^{14}$N$^{15}$NO and $^{15}$N$^{14}$NO we find good agreement with the HITRAN database. The $^{15}$N$_{2}$O isotopocule is missing in HITRAN, and we find that its line center frequencies in the GEISA database, the Institute of Atmospheric Optics database, as well as the Ames-1 line list deviate severely from the comb measurements.