Precision frequency comb spectroscopy of the 14N2O, 14N15NO, 15N14NO, and 15N2O 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 mechanisms of formation. A major challenge in the spectroscopic analysis of the isotopocule compositions is the availability of accurate spectroscopic parameters, particularly for the minor ¹⁵N isotopocules. In this work, we introduce high-resolution spectroscopic measurements of four isotopocules of nitrous oxide: ¹⁴N₂O, ¹⁴N¹⁵NO, ¹⁵N¹⁴NO, and ¹⁵N₂O 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 ¹⁵N isotopically enriched sodium nitrite and ¹⁴N 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 ν₁ + ν₃ band of the four isotopocules, and of the one order of magnitude weaker 2ν₂ + ν₃ and ν₁ + ν₂ + ν₃ – ν₂ 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 ¹⁴N₂O, ¹⁴N¹⁵NO and ¹⁵N¹⁴NO we find good agreement with the HITRAN database. The ¹⁵N₂O 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.