ENDOR and relaxations in systems with I=S=1/2:169Tm2+and171Yb3+in CaF2

It is shown that equal intensities of the two ENDOR transitions for 169 Tm 2+ and 171 Yb 3+ in CaF 2 indicate that the relaxation rates T x -1 ( mod 1 / 2 ,- 1 / 2 ( to or from mod - 1 / 2 ,- 1 / 2 )) and T x' -1 ( mod 1 / 2 , 1 / 2 ( to or from mod - 1 / 2 ,- 1 / 2 )) are approximately equal. The relative simplicity of the wavefunctions for the 4f 13 configuration in a cubic crystal field, and of the Hamiltonian for the orbit-lattice interaction, enables one to make an explicit calculation of the various relaxation rates in terms of the orbit lattice parameters V( Gamma i ,l). Matrix elements of the hyperfine interaction between ground and excited states of the J= 7 / 2 manifold are found to be important in leading to calculated values for T x -1 which are comparable to those for T x' -1 . Similar magnitudes for T x -1 and T x' -1 are shown to be expected for systems with large components of unquenched orbital angular momentum, in contrast to systems with little orbital angular momentum where T x' -1 approximately=0.