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\title{PRESSURE DEPENDENT LINE INTENSITY AND CONTINUUM ABSORPTION FOR PURE CO$_2$: PREDICTIONS BY REQUANTIZED MOLECULAR DYNAMICS SIMULATIONS}

\author{\underline{\FirstName H. \LastName TRAN},
  \underline{\FirstName J.-M. \LastName HARTMANN}}
\address{LMD, IPSL, Sorbonne Universite, Ecole Polytechnique, CNRS, Paris, France}

\author{\underline{\FirstName M. \LastName BIRK},
  \underline{\FirstName C. \LastName ROSKE} and 
  \underline{\FirstName G. \LastName WAGNER}}
\address{Remote Sensing Technology Institute, German Aerospace Center (DLR), D-82234 Wessling, Germany}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
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We investigate the pressure dependence of the line intensities retrieved from fits,
using accurate isolated line shapes, of pure CO$_2$ spectra predicted using classical 
molecular dynamics simulations (rCMDS). For that, rCMDS have been carried out, at 
296 K and 215 K, for pressure from 0.5 to 1 atm. An accurate ab initio intermolecular 
potential has been used to represent the CO$_2$-CO$_2$ interactions. The usual 
quadratic speed dependent hard collision model, taking into account the 
first-order line-mixing, is used to fit the simulated spectra. The results 
show that the retrieved line intensities linearly decrease with increasing 
pressure and that this decrease is larger at low temperature, consistently 
with the experimental results \footnote{M. Birk et al., Pressure dependent line 
intensity and continuum absorption for pure CO2: experimental results, this conference}. 
Quantitatively, at 1 atm and room temperature (resp. 215 K), our predictions show that the retrieved intensity can be up to 
2\% (resp. 4\%) smaller than the intensity obtained at zero pressure. The effect 
decreases with increasing rotational quantum number. From the spectra fit 
baselines, a continuum absorption proportional with the square of pressure, 
is deduced. 
Our analysis shows that the observed pressure dependent intensity and the 
continuum are mostly due to incomplete or ongoing collisions, which govern 
the dipole auto-correlation function at very short times. These collisions 
transfer a fraction of the intensity from the core region of the line to 
a broad and weak continuum, and thus reduce its area.

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