Interpretation of the absorption spectrum of single crystal UCl4

Temperature‐dependent polarized absorption spectra of single crystal UCl₄ have been investigated in the range 4000–25 000 Å. The spectra have been assigned and analyzed using the U⁴⁺ ion in a field of D_2d point group symmetry as a model. An rms deviation of 91 cm⁻¹ in the fit to 34 assigned levels was achieved using the following set of parameters: F₂=172.6±1.8; F₄=38.79±1.13; F₆=2.565±0.101; ζ=1666±11; ${\mathrm{A\hspace{0.17em}}}_2^0{\mathrm{〈\; r}}^2\text{〉=−291.3± 82.6}$ ; ${\mathrm{A\hspace{0.17em}}}_4^0{\mathrm{〈\; r}}^4\text{〉=−378.4± 30.8}$ ; ${\mathrm{A\hspace{0.17em}}}_4^4{\mathrm{〈\; r}}^4\text{〉=−5940± 195}$ ; ${\mathrm{A\hspace{0.17em}}}_6^0{\mathrm{〈\; r}}^6\text{〉=−141.4± 30.3}$ ; ${\mathrm{A\hspace{0.17em}}}_6^4{\mathrm{〈\; r}}^6\text{〉=557.6± 184.6}$ (all units in cm⁻¹). These optical data are interpreted to show that the ground state belongs to the Γ₄ irreducible representation, with the first excited state being a Γ₅ level at approximately 140 cm⁻¹. The parameters are discussed in terms of the bonding characteristic of the coordinated complex ion.