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\title{A NEW INSTRUMENTATION FOR SIMULTANEOUS THZ AND MID-IR QUANTITATIVE SPECTROSCOPY IN CORROSIVE GASEOUS MIXTURES}

\author{\underline{\FirstName W. \LastName TCHANA BETNGA},
  \FirstName F. \LastName KWABIA TCHANA, \FirstName J.-M. \LastName FLAUD}
\address{Universit\'e Paris Cit\'e and Univ Paris Est Creteil, CNRS, LISA, F-75013 Paris, France}

\author{\FirstName F. \LastName HINDLE, \FirstName A. \LastName CUISSET, \FirstName G. \LastName MOURET, \FirstName R. \LastName BOCQUET}
\address{Laboratoire de Physico-Chimie de l'Atmosph\`ere, Universit\'e du Littoral C\^ote d'Opale, Dunkerque, France}

\author{\FirstName L. \LastName MANCERON, \FirstName P. \LastName ROY}
\address{Ligne AILES, Synchrotron SOLEIL, L'Orme des Merisiers, St-Aubin BP48, 91192 Gif-sur-Yvette Cedex, France}

\author{\FirstName J. \LastName VANDER AUWERA}
\address{Spectroscopy, Quantum Chemistry and Atmospheric Remote Sensing (SQUARES), C.P. 160/09, Universit\'e Libre de Bruxelles, B-1050 Brussels, Belgium}

\author{\FirstName A. \LastName PERRIN}
\address{Laboratoire de M\'et\'eorologie Dynamique/IPSL, UMR CNRS 8539, Ecole Polytechnique, Universit\'e Paris-Saclay, RD36, 91128 Palaiseau Cedex, France}

\author{\FirstName O. \LastName JOHANNES}
\address{Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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\maketitle  

For precise laboratory measurements of infrared absorption intensities and line profiles of molecules of relevance 
for atmospheric chemistry in planetary and upper earth atmospheric layers, precise measurements between $10^{-3}$ and a few mbars in the 200 to 300 K temperature range are necessary.
For some gases, such as nitrous or hypobromous acids which always appear in complex chemical equilibria, one cannot, however, 
rely on absolute pressure measurements to measure optical absolute integrated absorption cross sections or infrared line intensities.\\
An original experimental development is described here, involving the simultaneous use of two instruments: a dual-beam Mid-IR/THz experiment. 
We have built a new coolable cell (200-300 K), made of inert materials, combining mid-infrared and THz absorption channels for measurements on the same gaseous sample at the same temperature.
The THz channel records pure rotational lines of these unstable molecules, for which the dipole moment is previously known at high precision from Stark effect measurements.
This enables a precise (better than $10\%$) computation of the partial pressure within the gaseous mixture and using this information for retrieving absolute line intensities for the Mid-IR range, useful for remote sensing of the atmosphere.\\
This new instrument opens up the way for many experiments in quantitative spectroscopy by improving the precision of spectroscopic parameters for unstable molecules of atmospheric interest, which only exist in the laboratory in a mixture characterized by chemical equilibrium.
The design and performance of the equipment are briefly presented and illustrated on spectroscopic examples.

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