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Predissociation Broadening of Sulfur Monodeuteride

Students from Kingsmead School Enfield with Orbyts fellows Oleksiy Smola & Noor-Ines Boudjema

The spin-orbit coupling (the interaction between electronic spin and electronic orbital motion) between excited and intersecting unbound electronic states in diatomic molecules may lead to the premature dissociation of the molecule, through transitions from the excited state to the unbound states. This effect manifests in the molecular spectra as a broadening of spectral lines in states belonging to the bound electronic state. As a result, observations of such spectra during molecular retrievals in astrophysical environments can often prove challenging if the characterisation of the associated line broadening in the spectrum is not well described. It is therefore useful for molecular line lists intended for such retrievals, to contain the necessary parameters to describe such broadening.

The ExoMol group makes use of the variational nuclear motion code Duo to diagonalise the molecular Hamiltonian and compute the energies of bound ro-vibronic states. Duo cannot however determine unbound ro-vibronic states in the continuum, which are instead treated as artificially bound using a cut-off at some bond length L. The energy and density of these continuum states varies with bond length, and so by adjusting L one can sample the entire continuum space of unbound states. A bound ro-vibronic state will interact and experience perturbations from neighbouring continuum states, so by plotting the computed energy of the state as a function of bond length L, the broading of the spectral lines associated with the state can be characterised by binning the distribution in terms of energy and then fitting a Lorentzian profile to the resulting histogram (see figure below).


Using Duo computed energies, students from Kingsmead School Enfield investigated the predissociative states of sulfur monodeuteride, making use of provided Python code to fit Lorentzian distributions to the histogrammed energy against bond length plots. By experimenting with the various parameters of the Lorentzian fits, they obtained predissociation broadening parameters for predissociative states of sulfur monodeuteride. A similar investigation and study was carried out by students of Parmiters School Watford and Ryan Brady for the sulfur monohydride isotopologue. We hope to write a paper on results in the near future.

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