Gorman, MN; (2016) Calculation of linelists for Chromium Hydride (CrH) & Manganese Hydride (MnH). Doctoral thesis , UCL (University College London).

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Abstract

New linelists (list of wavelengths with associated frequencies) for the open-shell transition metal diatomics Chromium Hydride (CrH) and Manganese Hydride (MnH) have been calculated. These linelists have been calculated from first principles making use of the Born-Oppenheimer approximation which decomposes the Schr\"{o}dinger equation for a molecule into an electronic equation and rovibronic equation. To solve the electronic Schr\"{o}dinger equation and thus produce Potential Energy Curves (PECs), Dipole Moment Curves (DMCs) and all relevant couplings (transition dipole, electronic angular momentum, spin-orbit) the MRCI (Multi-Reference Configuration Interaction) method as implemented in the \textit{ab initio} \texttt{MOLPRO} package was used. The electronic states considered were those up to $20\;000$ cm$^{-1}$ as this region is of most importance to astronomers to whom we are creating the linelists for within the ExoMol project. The construction of linelists for transition metal diatomics is very much at a developmental stage due to the myriad of low-lying electronic states of high multiplicity which couple together. Hence during the calculation of \textit{ab initio} curves a systematic study of the CASSCF orbitals used for the proceeding MRCI calculations was undertaken for both CrH and MnH. Also the effect of changing the configuration space of electrons was found to have profound effects on the behaviour of the PECs obtained. Additionally, the variation in properties obtained by changing the number of states calculated within a single MRCI calculation was investigated. A choice selection of these \textit{ab initio} curves were then implemented into the in-house programme Duo to produce linelists for CrH and MnH. These linelists were refined using available experimental data. At present, the existing linelist available for CrH in literature has 2 electronic states. Our new linelist is composed from 8 low-lying electronic states of CrH. CrH is used to classify L type dwarfs under the widely accepted classification scheme of Kirkpatrick. Additionally it has been shown theoretically that CrH could be used as a sensitive probe of magnetic fields of stars and also in the so-called ``deuterium test'' to probe the evolutionary history of sub-stellar objects. Due to the limited coverage of the existing linelist, a new linelist is sought after by astronomers. At present MnH has not been discovered in outer space but due to the favourable abundance of manganese it has been speculated that it could be present in the ISM. Hence the creation of a linelist opens up the possibility of the first astronomical detection of this molecule. Since January 2016 I have been working as a teaching fellow at Aberystwyth University. Hence preliminary work has been done on creating projects for undergraduate students for the molecules of BeH, MgH and CaH. A summary of literature has also been created for FeH with a view to in future creating a new linelist for this molecule which is of considerable interest both from an astronomical and theoretical perspective.

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