Absorption spectra of tetragonal single crystals of Cs<sub>3</sub>MnCl<sub>5</sub> have been measured at 4.2K over the wavelength range 5200-2600Å. Spin forbidden bands of low oscillator strengths were observed for the <sup>4</sup>Γ ← <sup>6</sup>A<sub>1</sub> transitions of the MnCl<sub>4</sub><sup>2-</sup> chromophore. All the intensity arose from the electric dipole mechanism, but for small magnetic dipole contributions to the intensities of <sup>4</sup>E(G) and <sup>4</sup>T<sub>2</sub>(F). Magnetic circular dichroism spectra measured at 4.2K in a field of 4.75 Teslas were dominated by C-term signals. The band intensities were assumed to arise via the spin orbit interaction, allowing "intensity borrowing" from higher energy charge transfer transitions. A model based on tetrahedral symmetry for the chromophore ion allowed the calculation of MCD:absorption intensity ratios for the quartet states. Good agreement was found with the experimental values for most quartets. Crystal Field calculations were performed in support of the MCD-based band assignments. Covalency was introduced into Stout's electrostatic matrices via the Koide and Pryce parameter andepsilon;. The following parameter values were found: B = 600cm<sup>-1</sup> , C = 2940cm<sup>-1</sup>, Dq = 350cm<sup>-1</sup>, and ε = 0.03, giving the correct ordering of the states and an rms. deviation from the observed energies of 520cm<sup>-1</sup>. <sup>4</sup>A<sub>2</sub>(F) was placed above <sup>4</sup>T<sub>1</sub>(F) in energy, a departure from the "conventional" <sup>4</sup>F term assignment. A simplified model for the perturbation of the cubic states by the tetragonal component of the crystal field was developed to account for the observed linear polarisations. The spectra were examined in detail to identify the electronic origins and assign them using a tetrahedral or tetragonal scheme. The latter appeared to be the more realistic model for most states. Assisted by data from the Raman spectrum, the vibrational sidebands were also assigned, When very sharp origin lines were present, some of the sidebands showed small splittings attributable to the presence of the two chlorine isotopes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:474448 |
| Date | January 1977 |
| Creators | Tacon, R. John |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:1a66d823-5602-44b3-afa5-a04b613de321 |
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