PARAMETRIZATION OF 5F-5F TRANSITION-PROBABILITIES BETWEEN STARK LEVELS OF U3+ IN LIYF4

In order to simulate intensities of the 5f-5f transitions of U3+ in Li YF4, we applied the Judd-Ofelt theory. Because of the large crystal-fi eld splitting of the J-multiplets in the actinides, a set of phenomeno logical intensity parameters B-lambda kq is introduced to describe the transition probabilities between the crystal-field sublevels of U3+. The intensities of the absorption transitions between the crystal fiel d levels calculated and a set of six phenomenological intensity parame ters give a rather good simulation of the experimental intensities, an d the applicability of the Judd-Ofelt theory is discussed for the 5f - -> 5f transitions of the actinides. From these values, the oscillator strengths between the excited states involved in the laser transition I-4(11/2) --> I-4(9/2) are calculated and the corresponding radiative lifetime is compared to the experimental one. Moreover, a comparison b etween the actinide ion U3+ (5f(3)) and the lanthanide ions Nd3+ (4f(3 )) and Er3+ (4f(11)) in LiYF4 is made.