Relaxation Process in U 3+ :Lacl 3 Detected by Resonant Self‐Absorption of Fluorescence

Using a pulse saturation method with optical detection of the recovery of fluorescent light, the temperature dependence and angular anisotropy of the spin‐lattice relaxation rate T   1 −1of U 3+ in anhydrous lanthanum trichloride has been measured. In the range 1.5 K ≦ T ≦ 4.2 K the direct relaxation process was found to be dominant, leading to a relaxation rate which can be fitted to the expression T   ld −1= 4.16(5) T s −1 with the anisotropy coefficient of the direct process A (θ = 90°)/ A (θ = 0°) ≈ 10. Both results are comparable with the data measured for the corresponding 4f 3 ion, Nd 3+ in LaCl 3 .