Electronic Absorption Spectra of Matrix-Isolated Uranium Tetrachloride and Uranium Tetrabromide Molecules

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Abstract
The electronic absorption spectra of UCl4 and UBr4 molecules isolated in nitrogen matrices cooled to liquid-helium temperatures have been observed and measured in the range 400–50 000 cm−1. The more than 30 narrow absorption bands, found in the 4000–24 000-cm−1 region for each of the two molecules, have been assigned to pure electronic transitions from the H43(Γ5) ground state to excited states of the f2 configuration. Energy-level calculations have been performed by simultaneously diagonalizing the electrostatic, the spin–orbit coupling, and the (tetrahedral) crystal-field interactions. The crystal-field parameters of fourth and sixth degree, designated as A 〈r4〉 and B 〈r6〉, were found to be − 593 and − 22 cm−1 for UCl4 and − 490 and − 15 cm−1 for UBr4. These values of the parameters were obtained by a least-squares fit of observed-to-calculated energy levels allowing A 〈r4〉 and B 〈r6〉 to vary while maintaining the values of F2, F4, F6, and ζ5f at 191, 34, 4, and 1796 cm−1, respectively. The magnitudes of A 〈r4〉 and B 〈r6〉 calculated on the basis of a point-charge model including induced dipole contributions were found to be − 345 and 26 cm−1 for UCl4 and − 309 and 22 cm−1 for UBr4.

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