PhD Defense: Temperature Dependence of Water Vapor Continua by high sensitivity Laser Absorption Spectroscopy - Morgane Toureille (LAME, LIPhy)

This thesis addresses the characterization of the water vapor absorption continuum, a slowly-varying spectral contribution that adds to monomer absorption lines and plays a major role in the atmospheric radiative budget, particularly within transparency windows. This work focuses on the 1.6 µm window, where the continuum reaches some of its lowest values across the infrared, and relies on cavity ring-down spectroscopy (CRDS) for its high sensitivity and baseline stability.
The self-continuum of pure water vapor was measured at five temperatures between 284.9 K and 332.2 K over 30 spectral points, including the first determination below room temperature in this window. A marked decrease with temperature is observed (a factor of 2.2 over 47 K at 5875 cm⁻¹), in overall good agreement with the MT_CKD4.3 model, except at the low-energy edge where the model overestimates the values. Comparison with a model combining the far wings of absorption lines (Ma and Tipping) and quasi-bound dimers (Simonova et al.) reproduces the measured behavior over most of the window, with a residual discrepancy at the center suggesting an additional contribution, possibly from bound dimers.
The foreign-continuum was studied for the first time as a function of temperature (296–352 K) in this window, for H₂O-air, H₂O-O₂, and H₂O-N₂ mixtures at two spectral points. In contrast to the 1/T dependence implemented in MT_CKD4.3 and to far-wing model predictions, a clear and systematic increase of the continuum with temperature is evidenced — a novel result whose physical origin remains to be explained. Finally, the H₂O-CO₂ foreign-continuum, relevant to modeling CO₂-rich atmospheres (Mars, telluric exoplanets), was characterized at three spectral points; no significant temperature dependence was observed, in contrast with the other collision partners studied.
These results provide a new and robust experimental basis for improving the semi-empirical continuum models used in atmospheric radiative transfer.