Evolution of the population of outer 6s and 5d shells in rare-earth metals

The shifts of the Kα1 and Kβ1 lines of all rare-earth (RE) metals (from La to Lu) have been measured experimentally by the x-ray shift method. The population of the RE-metal 6s and 5d shells has been determined by comparing the experimental and theoretical shifts obtained within the Dirac-Fock (Koopmans) model. Trivalent metals exhibit a monotonic cross-over from the 6s≈25d≈1 to 6s≈15d≈2 configuration with increasing atomic number.