Direct Evidence for the νd_{5/2} Orbital in ^{69}Ni: Implications for the N=40 Island of Inversion

Shape coexistence, a collective manifestation of nuclear structure, emerges from the underlying single-particle dynamics and is prominently observed in the region below ^{68}Ni. Theoretical studies have emphasized the key role of the νd_{5/2} orbital, the quadrupole partner of νg_{9/2}, in driving deformation. However, experimental constraints on the location and properties of neutron orbitals in neutron-rich isotopes in this region remain scarce. In this Letter, the single-particle structure of ^{69}Ni was investigated via the ^{68}Ni(d,p) reaction in inverse kinematics, performed at the ISOLDE Solenoidal Spectrometer at CERN. Several new excited states were observed, and comparisons with adiabatic distorted wave approximation (ADWA) calculations enabled ℓ-value assignments for the most strongly populated states. In particular, a state at 2.56 MeV is interpreted as the dominant fragment of the νd_{5/2} strength. The experimental findings are well reproduced by Large-Scale Shell Model calculations using a modified LNPS interaction with an increased νg_{9/2}-νd_{5/2} energy gap. These results provide new insight into the structure of ^{69}Ni and underscore the crucial role of the νd_{5/2} orbital in the onset of collectivity at the N=40 island of inversion.