High sensitivity CRDS of CO2 in the 1.18 µm transparency window. Validation tests of current spectroscopic databases

Abstract

The very weak absorption spectrum of natural CO2 is studied at high sensitivity near 1.18 µm. The investigated region corresponds to a transparency window of particular interest for Venus. Low pressure (10 Torr) spectra are recorded with unprecedented sensitivity in the 8311–8679 cm−1 interval. A noise equivalent absorption, αmin, on the order of 5 × 10−12 cm−1 is achieved using a cavity ring down spectrometer (CRDS) coupled with a self-referenced frequency comb. 669 lines are accurately measured and rovibrationnally assigned to 15 bands of the three first carbon dioxide isotopologues, 12C16O2, 13C16O2 and 16O12C18O. For all but one band, line intensities are smaller than 10−28 cm/molecule at 296 K and the weakest lines have intensity as low as 5 × 10−31 cm/molecule. In the large 8310–8645 cm−1 interval where no CO2 absorption lines were previously detected, about 500 lines are measured. In spite of the weakness of the considered transitions, line centers could be accurately determined and spectroscopic parameters are determined from standard band-by-band fit of the line positions (typical rms deviations are 5 × 10−4 cm−1).These newly observed bands provide critical validation tests for the most recent spectroscopic databases of carbon dioxide. The comparison to the recent updated version of the Carbon Dioxide Spectroscopic Databank (CDSD2019), to the current HITRAN2016 database and ab initio line lists is presented. The CDSD and HITRAN positions deviate from the measurements by less than 0.01 cm−1. Measured line intensities show an overall good agreement in particular with ab initio values but reveal that a few bands have inaccurate intensities in HITRAN2016 and CDSD2019. Three very weak bands are missing in CDSD2019. The reported experimental information will help to improve the quality of the CO2 spectroscopic databases in the region.