Fine-structure intervals in n = 17 and 20 barium Rydberg levels with 6 Յ L Յ 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 + , ␣ 1 = 123.88͑5͒ and ␣ 2 = 4420͑250͒.
The fine structure of high-angular-momentum n = 9 and 10 Rydberg states of barium has been measured precisely, using the resonant excitation Stark ionization spectroscopy method. Optical transitions corresponding to ͑n , nЈ͒ = ͑10, 30͒, ͑9,17͒, and ͑9,20͒ were induced with a Doppler-tuned CO 2 laser, determining the finestructure energies corresponding to all n = 9 and 10 levels with L ജ 6. The pattern of these fine-structure energies conforms closely with an effective potential model, by comparison with which the dipole and quadrupole polarizabilities of Ba + can be determined. Combining our data with earlier measurements made it possible to deduce, in addition, the portion of ␣ 2 due to the lowest excited D state of Ba + . Our best estimates of these three properties are ␣ 1 = 124.30͑16͒a 0 3 , ␣ 2 = 2462͑361͒a 0 5 , and ␣ 2 0 = 1828͑88͒a 0 5 .
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