The Arc Spectrum of Samarium and Gadolinium. Normal Electron Configurations of the Rare Earths

A partial analysis which includes most of the low temperature lines is presented for Sa I and Gd I: 453 lines, 175 levels for Sa I; 71 lines, 35 levels for Gd I. The normal configuration and lowest state of Sa I is $4f^{6}6s^2{}_{}{}^7{}_{}{}^{}F$; for Gd I is $4f^{7}5d6s^2{}_{}{}^9{}_{}{}^{}D$. It is also shown that the normal configuration for Tm I is probably $4f^{13}6s^2$. There exists in the rare earth group the same type of variation of relative binding between the $4f$ and $5d$ electrons as exists between the $s$ and $d$ electrons in the long periods. In the rare earth group the $5d$ electron is most tightly bound at the beginning and middle of the group and has a minimum just before the middle and at the end of the group.