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\title{SPECTROSCOPIC MODEL AND PREDISSOCIATION LIFETIMES OF THE HYDROXYL RADICAL}

\author{\underline{\FirstName G.~B. \LastName MITEV},
  \FirstName J. \LastName TENNYSON,
  \FirstName S.~N. \LastName YURCHENKO}
\address{University College London, Gower St, London WC1E 6BT}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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OH spectroscopy has been heavily studied due to its importance in combustion, atmospheric and interstellar chemistry, and as a key constituent of the Earth's atmosphere. It is  found in  planetary atmospheres, comets, stellar atmospheres and interstellar clouds. Furthermore, recently OH has been detected in the atmospheres of Ultra-hot Jupiters WASP-76b and WASP-33b.\footnote{\href{https://doi.org/10.1051/0004-6361/202141696}{doi: 10.1051/0004-6361/202141696}, R. Landman et al.
%, A. S\'{a}nchez-L\'{o}pez, P. Molli\`{e}re, A. Y. Kesseli, A. J. Louca, A. J. Louca, 
Detection of OH in the ultra-hot Jupiter WASP-76b, A\&A 656, A119 (2021)}$^,$\footnote{\href{https://arxiv.org/abs/2305.11071}{doi: 10.48550/arXiv.2305.11071} O. Wright et al. A spectroscopic thermometer: individual
vibrational band spectroscopy with the example of OH in the atmosphere of WASP-33b, A\&AS, in press (2023).} 

Novel multi-reference configuration interaction calculations for the X $^2 \Pi$, A $^2 \Sigma^+$, 1 $^2 \Sigma^-$, 1 $^4 \Sigma^-$, and 1 $^4 \Pi$ potential energy curves (PECs), associated coupling curves and (transition) dipole moments are presented. Curves are refined against empirical energy levels and transitions from the latest MARVEL OH data\footnote{\href{https://doi.org/10.1039/D2CP02240K}{doi: 10.1039/D2CP02240K}, T. Furtenbacher, S. T. Hegedus, J. Tennyson, A. G. Cs\'{a}sz\'{a}r, Analysis of measured high-resolution doublet rovibronic spectra and related line lists of $^{12}$CH and $^{16}$OH, Phys. Chem. Chem. Phys. 24, 19287 (2022)} using the bound state nuclear motion code Duo \footnote{\href{https://doi.org/10.1016/j.cpc.2015.12.021}{doi: 10.1016/j.cpc.2015.12.021}, S. N. Yurchenko, L. Lodi, J. Tennyson, A. V. Stolyarov, Duo: A general program for calculating spectra of diatomic molecules, Comp. Phys. Comm., 202, 262 (2016), \href{https://github.com/exomol/duo}{https://github.com/exomol/duo}}.  

The effect of predissociation of the A $^2 \Sigma^+$ energy levels caused by a spin-orbit interaction with with repulsive (unbound) electronic states $^2 \Sigma^-$ and $^4 \Sigma^-$, $^4 \Pi$ is studied. Predissociation is one of the the main  sources of line broadening of the A--X rovibronic transitions. 

In Duo, continuum states are treated as bound states  by adding  infinite potential barriers to the unbound  PECs at the long-range boundary defined by the internuclear distance ($R_{c}$).  Varying $R_{c}$ allows us to scan continuum states, giving a series of crossings between (quasi) bound states associated with the A $^2 \Sigma^+$ and continuum states, from which we are able to extract spin-orbit induced predissociation lifetimes with good agreement to experimental data. 

Python package, \textbf{binSLT}, a parallelised code to calculate predissociation lifetimes, associated line broadening parameters, and uncertainties from Duo *.states files are presented and will be available on the \href{https://github.com/exomol}{ExoMol GitHub}. 
The potential energy and other curves from this study will be used to produce a rovibronic  ExoMol linelist and temperature dependent photodissociation cross sections for the  for hydroxyl radical.
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