Absolute oscillator strengths of 5f-5f transitions of U4+ in ThBr4 and in hydrobromic acid solutions

Absolute oscillator strengths of 5f² transitions of U⁴⁺ in ThBr₄ and in hydrobromic acid solutions are found to be about 10^-4 which is two orders of magnitude larger than those for 4f-4f transitions (lanthanide series) and one order of magnitude larger than 5f³ ones (U³⁺). Applicability of Judd's theory to 5f² transitions is discussed with particular attention to the roles played by reduced matrix elements of odd rank tensor U^λ and by the proximity of the 6d configuration.