Excited states of the neutron-rich nuclei97;99 Rb were populated for the first time using the multistep Coulomb excitation of radioactive beams. Comparisons of the results with particle-rotor model calculations provide clear identification for the ground-state rotational band of 97 Rb as being built on the πg 9=2 ½431 3=2 þ Nilsson-model configuration. The ground-state excitation spectra of the Rb isotopes show a marked distinction between single-particle-like structures below N ¼ 60 and rotational bands above. The present study defines the limits of the deformed region around A ∼ 100 and indicates that the deformation of 97 Rb is essentially the same as that observed well inside the deformed region. It further highlights the power of the Coulomb-excitation technique for obtaining spectroscopic information far from stability. The 99 Rb case demonstrates the challenges of studies with very short-lived postaccelerated radioactive beams. DOI: 10.1103/PhysRevLett.115.172501 PACS numbers: 27.60.+j, 23.20.Lv, 25.70.De, 29.38.-c The spherical symmetry of atomic nuclei is well established for the cases where both the proton (Z) and neutron (N) numbers are near magic numbers. Most atomic nuclei, however, have nonspherical shapes. The best-known and well-studied region of prolate-deformed nuclei is the "rare-earth region," centered between 50 < Z < 82 and 82 < N < 126. A less known region of deformed nuclei, which is predicted to show even larger deformations [1], is centered around mass 100 (A ∼ 100) between the 28 < Z < 50 and 50 < N < 82 major shells. These nuclei are neutron rich, and well away from the valley of stability, so they are challenging to study experimentally.The A ∼ 100 prolate-deformed Sr-Zr region (Z ¼ 38; 40) has attracted considerable attention since its prediction [2] and experimental observation [3]. Spectroscopic studies of these neutron-rich nuclei were undertaken at on-line mass separators and by γ-ray spectroscopy in spontaneous fission [4][5][6]. A key feature is the sudden onset of deformation when progressing from neutron number N ¼ 58 to N ¼ 60. However, the abrupt change of the deformation quickly washes out when moving away from Z ¼ 38 [4].Nuclei at the border of this deformed region, in which the addition or the removal of a single nucleon results in a large shape change, hold the key to its understanding. Tracking