  Most exoplanets detected so far have atmospheric T significantly higher than
300K. Often close to their star, they receive an intense UV photons flux that
triggers important photodissociation processes. The T dependency of VUV
absorption cross sections are poorly known, leading to an undefined uncertainty
in atmospheric models. Similarly, data measured at low T similar to that of the
high atmosphere of Mars, Venus, and Titan are often lacking. Our aim is to
quantify the T dependency of the abs. cross section of important molecules in
planetary atmospheres. We want to provide both high-resolution data at T
prevailing in these media and a simple parameterization of the absorption in
order to simplify its use in photochemical models. This study focuses on carbon
dioxide. We performed experimental measurements of CO$_2$ absorption cross
section with synchrotron radiation for the wavelength range (115--200nm). For
longer wavelengths (195--230nm), we used a deuterium lamp and a 1.5m Jobin-Yvon
spectrometer. We used these data in our 1D thermo-photochemical model in order
to study their impact on the predicted atmospheric compositions. The cross
section of CO$_2$ increases with T. It can be separated in two parts: a
continuum and a fine structure superimposed on the continuum. The variation of
the continuum of absorption can be represented by the sum of three gaussian
functions. Using data at high T in thermo-photochemical models modifies
significantly the abundance and the photodissociation rates of many species, in
addition to CO$_2$, such as methane and ammonia. These deviations have an
impact on synthetic transmission spectra, leading to variations of up to 5 ppm.
We present a full set of HR ($\Delta \lambda$=0.03nm) absorption cross sections
of CO$_2$ from 115 to 230nm for T ranging from 150 to 800K.
