High sensitivity CW-cavity ring down spectroscopy of N2O near 1.5 μm (I)

The absorption spectrum of nitrous oxide, N2O, in natural isotopic abundance has been recorded by CW-Cavity Ring Down Spectroscopy between 6000 and 6833 cm−1. The spectra were obtained at Doppler limited resolution by using a CW-CRDS spectrometer based on a series of fibered DFB lasers. The typical sensitivity of 2 × 10−10 cm−1, allowed for the detection of lines with intensity as weak as 2 × 10−29 cm/molecule while the minimum intensity value provided by HITRAN in the considered spectral region is 2 × 10−25 cm/molecule. More than 6000 line positions of five isotopologues contributing to the spectra (14N216O, 15N14N16O, 14N15N16O, 14N218O and 14N217O), were measured with a typical accuracy of 1.5 × 10−3 cm−1 and rovibrationally assigned on the basis of their respective global effective Hamiltonian models. Highly excited rovibrational levels corresponding to J values larger than 80 could be detected for the stronger vibrational bands. The band by band analysis led to the determination of the rovibrational parameters of a total of 68 bands, 49 of them being newly reported. The rms value of the deviations of the predictions of the effective Hamiltonian models from the observed line positions is 0.010 cm−1. As expected, the quality of the predictions degrades for the minor isotopologues for which important deviations up to a few wavenumbers were evidenced. Most of the bands were found unperturbed but in a few cases, local rovibrational perturbations were evidenced. The interaction mechanisms and the perturbers were univocally assigned on the basis of the effective Hamiltonian model. In particular, interpolyad couplings were evidenced indicating that the polyad version of the effective Hamiltonian has to be extended to include Coriolis and interpolyad anharmonic interactions.