High sensitivity Cavity Ring Down spectroscopy of carbon dioxide in the 1.19-1.26 µm region

- A new CRDS spectrometer using a fibered External Cavity Diode Laser is developed.
- The CO2 absorption spectrum is recorded with unprecedented sensitivity near 1.2 μm.
- 3425 Transitions are assigned to 61 bands of six isotopologues of carbon dioxide.
- The spectroscopic parameters of 57 bands are derived from a fit of the line positions.
- ΔP=11 effective dipole moment parameters are determined for the six isotopologues.

A Cavity Ring Down spectrometer using a fiber-connected External Cavity Diode Laser (ECDL) as light source has been developed to access the 1.26-1.19 μm region. We present here the first application of this newly developed CW-CRDS spectrometer to the study of the highly sensitive spectrum of natural carbon dioxide between 7909 and 8370 cm−1. The spectrum is dominated by the ν1+3ν3 dyad at 8192.55 and 8293.95 cm−1 which forms the low energy border of the 1.2 µm transparency window of importance for planetary applications. The achieved sensitivity (noise equivalent absorption, αmin, in the 10−10-10−11 cm−1 range) allowed detection of numerous new transitions with intensity values down to 10−30 cm/molecule, in particular hot bands reaching upper states with energy up to about 10,600 cm−1.More than 3400 transitions belonging to the six major isotopologues of 'natural' carbon dioxide were assigned using the predictions of effective Hamiltonian (EH) models. A total of 2027, 442, 548, 303, 92 and 13 transitions belonging to 37, 9, 8, 4, 2 and 1 bands were rovibrationnally assigned for 12C16O2, 13C16O2, 16O12C18O, 16O12C17O, 16O13C18O and 16O13C17O, respectively. For comparison, only 14 12C16O2 absorption bands were previously known in the region (mostly from Venus spectra). Intensity values range between 2.1×10−30 and 2.4×10−24 cm/molecule. All the identified bands correspond to the ΔP=11 series of transitions, where P=2V1+V2+3V3 is the polyad number (Vi are vibrational quantum numbers). The overall agreement of the EH predicted and measured line positions is very good (for instance, average and rms deviations of 0.64×10−3 and 3.8×10−3 cm−1 for 12C16O2, respectively).The band-by-band analysis provided accurate spectroscopic parameters of 57 bands from a fit of the measured line positions. The global fits of the obtained intensity values of the ΔP=11 series of transitions were used to refine the corresponding set of effective dipole moment (EDM) parameters of the six studied isotopologues.The obtained results will help to improve the spectral line parameters of carbon dioxide in the most currently used spectroscopic databases. In particular, the calculated band intensities of a few weak hot bands are now in good agreement with the observations.