The optical properties of caesium pentachloromanganate (II)

Thesis

Absorption spectra of tetragonal single crystals of Cs₃MnCl₅ have been measured at 4.2K over the wavelength range 5200 2600Å. Spin forbidden bands of low oscillator strengths were observed for the ⁴Γ ← ⁶A₁ transitions of the MnCl₄²⁻ chromophore. All the intensity arose from the electric dipole mechanism, but for small magnetic dipole contributions to the intensities of ⁴E(G) and ⁴T₂(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 ε. The following parameter values were found: B = 600cm⁻¹ , C = 2940cm⁻¹, Dq = 350cm⁻¹, and ε = 0.03, giving the correct ordering of the states and an rms. deviation from the observed energies of 520cm⁻¹. ⁴A₂(F) was placed above ⁴T₁(F) in energy, a departure from the "conventional" ⁴F 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.

Authors: Tacon, J; Tacon, John

• Publication date: 1977
• DOI: 10.5287/ora-6rogzj6ky
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English