Stefani, Stefania (2011) Optical Properties of gases under planetary conditions: measurements and models. [Tesi di dottorato] (Unpublished)


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Item Type: Tesi di dottorato
Uncontrolled Keywords: absorption coefficients, carbon dioxide, high pressure high temperature, models, line mixing, far wings
Date Deposited: 09 Dec 2011 09:23
Last Modified: 30 Apr 2014 19:49


The conventional spectroscopic database, likely HITRAN, HITEMP, CDSD, does not contain information about optical properties of gases at high pressure and high temperature. Beyond while extensive data sets exist for radiative transfer calculations concerning the Earth’s atmosphere, important information and a thorough understanding of the behaviour of dense planetary atmospheres is still missing. For these reasons, we building up an experimental setup to characterized the optical properties of CO2, the principal gas of the Venus atmosphere, at high pressure and high temperature. The CO2 absorption spectra have been recorded by a Bruker-Vertex 80 Fourier Transform InfraRed (FT-IR) interferometer, able to work in a big spectral range, from 350 to 25000cm-1 (0.4 to 29 m ), with a relatively high spectral resolution, from 10 to 0.07 cm-1 . A special customer gas cell, certified to support pressure up to 350 bar and temperature about 300 °C, has been integrated with the interferometer. To reproduce same conditions found in the deep atmosphere of Venus, we have varied the pressure from 1 bar to 32 bar and the temperature from 294 K to 630 K. The results will be presented in this paper. Measurements have been compared with synthetic spectra obtained using three different models: one implementing a line by line calculation. The second takes into account the line mixing effect in the strong collision approximation. The last one is a new approach to the line mixing effect and far wings. These comparisons leads us to conclude that for gases under extreme conditions, the shape of the spectral lines no longer follows the conventional Voigt form. Another interesting phenomenon, occurring at high pressure is the collision induced absorption (CIA). Even when the pressure is of a few bar, we have observed the CO2 CIA forbidden bands for the symmetric 16O12C16O molecule. The interaction of the molecules with their neighbours induces indeed a dipole moment which yields to pressure-induced-absorption bands. The observed CIA bands integrated intensities shows a quadratic dependence versus density and basically depends on the temperature. An aspect which is probably underestimated in typical remote sensing methods of radiative transfer inverse calculation is the CIA effect, which has to be more deeply studied and modelled. The impact of CIA is currently under study.

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