Configuration interaction in rare earth ions. III. Trees parameters for Pr2+and Pr3+

For pt. II see ibid., vol.4, 1388 (1971). Numerical estimates of the Trees parameters alpha , beta , gamma for Pr 2+ (4f 3 ) are reported. Configuration interactions through second order excitations to bound states are considered in detail. It is shown that excitations of the type 4d to 4f and 4f to 5d are dominant. The results are compared with recent work on Pr 3+ . Bound state excitations alone grossly overestimate the parameters for both ions. Introduction of a screening factor of 1 / 5 for the excitations of two 4f electrons from d states gives reasonable agreement with experiment. Excitations to the continuum are crudely estimated but they do not appear to give sufficiently large corrections to the values of alpha , beta and gamma to lead to good agreement with experimental data.