High sensitivity Cavity Ring Down spectroscopy of 18O enriched carbon dioxide between 5850 and 7000 cm−1: I. Analysis and theoretical modeling of the 16O12C18O spectrum

The room temperature absorption spectrum of highly 18O enriched carbon dioxide has been recorded by very high sensitivity CW-Cavity Ring Down spectroscopy between 5851 and 6990 cm−1 (1.71–1.43 μm). The achieved sensitivity (noise equivalent absorption αmin∼5×10−10−5×10–11 cm−1) has allowed the detection of more than 19,000 transitions belonging to eleven isotopologues of carbon dioxide. This first report is devoted to the analysis of the 16O12C18O data. A total of 6452 transitions belonging to 71 bands were rovibrationnally assigned on the basis of the predictions of the effective Hamiltonian model. For comparison, only twelve 16O12C18O bands were previously measured by Fourier Transform spectroscopy in the region. Line intensities of the weakest transitions are on the order of 2×10–29 cm/molecule. The studied spectral region is formed by ΔP=8, 9 and 10 series of transitions, where P=2V1+V2+3V3 is the polyad number (Vi are vibrational quantum numbers). The band-by-band analysis has allowed deriving accurate spectroscopic parameters of 69 bands from a fit of the measured line positions. A few resonance perturbations were identified. In particular, the 31112 (P=10) and 51105 (P=11) states belonging to different polyads are anharmonically coupled. The resulting intensity transfer leads to the observation of several extra lines of the 51105-01101 hot band which could be assigned. A major result of the present work is the measurement of the ΔP=8 bands of 16O12C18O near 5900 cm−1 which are the main responsible of the absorption of carbon dioxide in natural isotopic abundance in the region but are absent in the current version of the CO2 spectroscopic databases (HITRAN, GEISA, and CDSD). These ΔP=8 bands may significantly impact the transmission of CO2 in the 1.70 µm transparency window of importance for Venus spectra. Using the large set of newly measured line positions and those collected from the literature, the global modeling of the 16O12C18O line positions within the effective Hamiltonian approach has been performed and a new set of Hamiltonian parameters has been obtained. Using a similar approach, the global fits of the obtained intensity values of the ΔP=8, 9 and 10 series of transitions were used to derive the corresponding set of effective dipole moment parameters. The obtained results will help to improve importantly the quality of the 16O12C18O line positions and line intensities in the most currently used spectroscopic databases of carbon dioxide. -- Highlights: • Cavity Ring Down spectroscopy of 18O enriched CO2 between 5851 and 6990 cm−1. • 6452 transitions belonging to 71 bands of 16O12C18O are rovibrationnally assigned. • A new set of Hamiltonian parameters is been obtained for 16O12C18O. • Effective dipole moment parameters for ΔP=8–10 series of bands are determined. • 16O12C18O is the major contributor to the absorption of CO2 in the 1.70 µm window