Abstract High-resolution optical spectroscopy of Rb atoms in an intermediate-Zeeman magnetic field regime is performed. Magnetic fields between 0. 05 T and 0. 13 T are measured. We introduce a measurement approach based on three-level Λ systems where the open-loop frequency energy separation is determined by the applied magnetic field. Using this method an accuracy better than 100 ppm is reached in a measurement series. In parallel, on the basis of complementary measurements based on three-level V systems we obtain the first experimental value of the Landé g-factor of the first excited state for ^85 85 Rb, ^85gJ (5P₁/₂) 85 g J (5 P 1 / 2) =0. 66709 (7), and for ^87 87 Rb, ^87gJ (5P₁/₂) 87 g J (5 P 1 / 2) =0. 6663 (2). Our approach is unprecedented in the literature for Rb. Graphic abstract Experimental results for the Land´e g-factor of the first excited state of Rb
Scotto et al. (Thu,) studied this question.