Fine-structure measurements in high-L n=17 and 20 Rydberg states of barium

Fine-structure intervals in n=17 and 20 barium Rydberg levels with 6≤L≤11 have been measured precisely with the resonant excitation Stark ionization spectroscopy microwave technique. The measured intervals were analyzed using the long-range polarization model, including allowance for the large nonadiabatic effects of the Ba+ 5d level and the contributions of higher-order terms. The analysis determined improved values in atomic units of both dipole and quadrupole polarizabilities of Ba+, α1=123.88(5) and α2=4420(250)