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\title{HIGH-RESOLUTION PRESSURE-BROADENING PARAMETERS FOR ROVIBRONIC TRANSITIONS IN EXOMOLECULES}

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\author{\underline{\FirstName J. \LastName BULDYREVA}$^{*,**}$,
  		\FirstName R.P. \LastName BRADY$^{**}$,
		\FirstName S.N. \LastName YURCHENKO$^{**}$,
		\FirstName J. \LastName TENNYSON$^{**}$}
\address{Institut UTINAM, UMR CNRS 6213, Universit\'{e} de Franche-Comt\'{e}, 16 route de Gray, 25030 Besan\c{c}on cedex, France ($^*$); Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, United Kingdom ($^{**}$)}


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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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Spectroscopic studies of extrasolar planets simultaneously in visible and 
infrared regions constitute the main goal of next-generation space missions, 
such as the James Webb Space Telescope (JWST) launched late 2021 and Atmospheric 
Remote-sensing Infrared Exoplanet Large-survey (ARIEL) adopted by the European Space 
Agency for launch in 2029 [1]. Analyses of the recorded spectra will strongly 
rely on a new and important source of information: high-resolution line-shape 
parameters for rovibrational and rovibronic transitions, in particular 
line-broadening parameters [2] which contribute to absorption in atmospheric 
transparency windows and spectral wings and influence the continuum modelling. 
Given elevated temperatures, optically active species to be considered include 
not solely already detected and expected "exotic" molecules but also molecular 
ions. Moreover, because of a wide variety of exoplanet atmospheres to be probed, 
various perturbers should be taken into account: hydrogen and helium, as for gas 
giants, but also carbon dioxide, nitrogen, oxygen, water vapor, carbon monoxide, 
nitric monoxide, methane and ammonia. Thus, a large amount of line-shape data 
for "exotic" molecular pairs is required.

Extensive line lists of positions and intensities for over 90 "exomolecules" are 
provided by the ExoMol database [3] and molecular species relevant to the 
Earth's and planetary atmospheres are included in specialized databases, such as 
HITRAN [4], GEISA [5], TheoReTS [6], MoLList [7], etc. Although air-broadening 
and air-induced shift coefficients for vibration bands are generally available 
in HITRAN for "standard" atmospheric molecules and within temperature ranges of 
atmospheric interest, pressure broadening and shifting parameters as well as 
their temperature dependence for required exomolecules are extremely scarce or 
(more often) completely missing. In addition, the specific conditions of hot 
atmospheres and chemical reactions occurring in them are hardly reproducible in 
laboratory, so that experimental studies of line-shape parameters are also 
deficient. The only alternative to produce the large amount of essential but 
missing data on line-shape parameters is offered, therefore, by theoretical 
approaches.

Whereas the pressure broadening and pressure-induced shift of molecular spectral 
lines corresponding to pure rotational and vibrotational transitions have been 
extensively addressed by theoreticians, these parameters for transitions 
involving a change in electronic state have received much less attention.
We present a theoretical study of collisional broadening of rovibronic lines in 
the framework of phase-shift theory developed for atoms but applicable also to 
molecules because of high transition energies and common line-shape features 
conformed by electronic absorption spectra. We start by a general analysis for 
arbitrary molecular partners which is conducted in terms of a dimensionless 
parameter depending on 12-6 Lennard-Jones parameters describing the 
intermolecular interactions in the ground and excited electronic absorber's 
states. The mean-thermal-velocity approximation as well as the Maxwell-Boltzmann 
average over velocities are considered. The latter allows, in particular, 
addressing the validity of the traditional power law for widths and shifts. To 
check the validity of the theory, after having computed the required 
potential-energy surfaces, we present practical computations of linewidths and 
shifts at various temperatures for some representative molecular pairs and 
compare them to available measurements. The influence of the trajectory model 
for the relative molecular motion and the role of the Lennard-Jones form for the 
isotropic intermolecular potential representation are discussed. Concluding 
remarks outline some prospectives of improvements.
\vspace{0.5cm}\\
\begin{small}
\mbox{[1]} https://arielmission.space/   \\
\mbox{[2]} J.J. Fortney et al., arXiv:1905.07064 (2019).\\
\mbox{[3]} I.E. Gordon et al., JQSRT 277, 107949 (2022).\\ 
\mbox{[4]} T. Delahaye et al., J Mol Spectrosc 380, 111510 (2021).\\
\mbox{[5]} J. Tennyson et al., JQSRT 255, 107228 (2020).\\
\mbox{[6]} A.V. Nikitin et al., J Mol Spectrosc 327, 138 (2016).\\ 
\mbox{[7]} P.F. Bernath, JQSRT 240, 106687 (2020).
\end{small}



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