Study of the electronic structure of the actinide tetrabromides ThBr4 and UBr4 using ultraviolet photoelectron spectroscopy and density functional calculations

Ultraviolet photoelectron spectra of UBr4 and ThBr4 have been recorded in t he gas-phase and interpreted using relativistic density functional calculat ions. For ThBr4, eight bands were observed which are interpreted as ionizat ion from the five Br 4p symmetry orbitals of a Br-4 unit in T-d symmetry, w ith three of the bands [the (4t(2))(-1), (1t(1))(-1), and (3t(2))(-1) ioniz ations] each being split into two by spin-orbit interaction. The observed s plittings are rationalized in terms of the Th 6p and Br 4p contributions to the orbitals and by comparison with the known ultraviolet photoelectron sp ectra of ThF4 and ThCl4. The first vertical ionization energy (VIE) of ThBr 4 was measured as (10.92 +/-0.03) eV. UBr4 shows a very similar photoelectr on spectrum with an extra band at (9.65 +/-0.02) eV VIE. This is associated with a (5t(2))(-1) (U 5f,6d) ionization. Supporting matrix isolation infra red experiments were also carried out under very similar vaporization condi tions to those used in the photoelectron spectroscopy experiments to check the composition of the vapor beams used. In these experiments, the T-2 stre tching modes of ThBr4 and UBr4 have been measured as 230 +/-2 and 239 +/-2 cm(-1), respectively. Both the photoelectron and infrared matrix isolation spectra are consistent with an effective tetrahedral geometry for UBr4 and ThBr4. (C) 2001 American Institute of Physics.