Electronic and vibronic states of uranium hexafluoride in the gas and in the solid phase at very low temperatures

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Abstract
Complex vibronic structure has been observed in the electric‐dipole‐forbidden charge transfer absorption bands of solid and matrix‐isolated UF6 in the temperature range 8–14 K. These bands have their maximum intensity near 260 and 375 nm. Associated with the 260 nm band are four electronic transitions with origins at 30 331, 31 032, 32 120, and 32 821 cm−1, the first two being observed directly. Two more no‐phonon transitions are associated with the weak band at 375 nm, one at 24 564 cm−1 and another at 25 265 cm−1. These levels are assigned via a weak j‐j coupling scheme as excitations from the ligand t1uσ orbital to empty uranium 5f orbitals. Uranium spin–orbit coupling in UF6 charge transfer states strongly resembles that of UF6 −. Several progressions in the symmetric stretch frequency ν1=580–595 cm−1 are present which have as their origins various combinations of the above electronic levels with the bending modes ν4, ν5, and ν6 or their overtones 2ν4, 2ν5, and 2ν6. Temperature effects, which account for the principal differences between the gas and solid spectra, are also discussed. A much more intense band at 214 nm is lacking in vibronic structure and is attributed to an allowed charge transfer transition.

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