Abstract. The optical pumping method with rf detection and the nonlinear level crossing technique were used in parallel electric and magnetic fields to investigate the Stark splitting of all the fine structure levels of the ground multiplet 4f7(8S)5d6s 2 9D in Gd I. The tensor potarizabilities were deduced from the level crossing and the rf signals. The variation of the tensor polarizabilities is well reproduced by the LS coupling approximation, except for the small value of the 9D 5 level. A value of the tensor polarizability of the 5 d eletron is evaluated from the experimental results, ~2(5 d)= 2.00(6) kHz/(kV/cm) 2. It is shown that the ratio between the electric quadrupole constant and the tensor polarizability is constant, except for the 9D 5 ratio, which is caused by a breakdown of the central field model. PACS. 32.60. + i; 32.80.Bx
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