Clark, Victoria H. J.; (2021) Development of Computational Spectroscopic Methods for the Analysis of Molecular Reactions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The overarching aim of this thesis is to develop new methodologies for the spectroscopic analysis of dissociative molecular reactions. The reversible reaction SiH2 +SiH4→Si2H6 was chosen as a test system. The first ro-vibration linelist for the electronic ground state (X̃ 1A1) of SiH2 has been produced using the ExoMol procedure. The linelist is suitable for temperatures up to 2000 K, wavenumber range 0–10 000 cm−1 and rotational excitation up to J = 51. A new refined potential energy surface and a new ab initio dipole moment surface for SiH2 have been produced for the first time. Next, using the SiH2 linelist as a reference spectrum, the computational methodologies for modelling spectra of polyatomic molecules produced in reactive or dissociative environments were developed. Two approaches for modelling the disequilibrium vibrational populations are introduced: a simplistic 1D approach based on the harmonic approximation and a full 3D model incorporating accurate vibrational wavefunctions computed with the TROVE (Theoretical ROVibrational Energy) program. The developed methodologies use the accurate and extensive ro-vibrational ExoMol linelists and allow a full and systematic analysis of the vibrational spectra for dissociated species. The developed 1D methodology was used to model the emission spectra of di- and triatomic molecules formed via the dissociation of formamide, and the results compared with the experimental spectra. The fragments modelled include CO, N2 , CN, CO2 , and HCN. A further bi-temperature approach was introduced in order to fully and correctly model all fragments. The computational methodologies developed here produced an excellent agreement with experiment and our theoretical analysis is directly responsible for the determination of a experimental self-absorption within the CO2 spectrum that had remained unnoticed until this work. Finally, the SiH2 linelist was used to investigate the rotational energy level clustering of silylene for the first time, and the critical total angular momentum quantum number determined to be Jcr. ≈ 17.

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