The CO₂ absorption continuum near 2.3 µm is determined for a series of sub atmospheric pressures (250–750 Torr) by high sensitivity Cavity Ring Down Spectroscopy. An experimental procedure consisting in injecting successively a gas flow of CO₂ and synthetic air, keeping constant the gas pressure in the CRDS cell, has been developed. This procedure insures a high stability of the spectra baseline by avoiding changes of the optical alignment due to pressure changes.

The CO₂ continuum was obtained as the difference between the CO₂ absorption coefficient and a local lines simulation using a Voigt profile truncated at ±25 cm⁻¹. Following the results of the preceding analysis of the CO₂ rovibrational lines (Vasilchenko S et al. J Quant Spectrosc Radiat Transfer 10.1016/j.jqsrt.2016.07.002, a CO₂ line list with intensities obtained by variational calculations and empirical line positions was preferred to the HITRAN line list. A quadratic pressure dependence of the absorption continuum is observed, with an average binary absorption coefficient increasing from 2 to 4×10⁻⁸ cm⁻¹ amagat⁻² between 4320 and 4380 cm⁻¹.

The obtained continuum is found in good agreement with a previous measurement using much higher densities (20 amagat) and a low resolution grating spectrograph and is consistent with values currently used in the analysis of Venus spectra.