On the tensor polarizabilities in the 4f N 6s 2 ground levels of the neutral rare-earth atoms

A systematic study of the Stark splitting of the 4f ^N 6s ² ground levels in the lanthanides was performed by the nonlinear level-crossing technique and the optical pumping method with rf detection in parallel electric and magnetic fields. The tensor polarizabilities of the hyperfine structure levels in¹⁴¹Pr I,¹⁵⁹Tb I,¹⁶⁵Ho I,¹⁶⁹Tm I and of the fine structure levels in¹⁴²Nd I,¹⁶⁴Dy I, and¹⁶⁶Er I were deduced from the level-crossing and rf signals. Values of the tensor polarizability of the 4f electron were evaluated from the experimental results. On average these values decrease with the increasing number of 4f electrons from the value of¹⁴¹Pr (4f ³ 6s ²) α₂(4f)=1.16(12) kHz/(kV/cm)² to the value of¹⁶⁹Tm (4f ¹³6s ²) α₂(4f)=0.68(4) kHz/(kV/cm)². This decrease is caused by the lanthanides contraction.