Background: The systematic trend in mean-square charge radii as a function of proton or neutron number exhibits a discontinuity at the nucleon-shell closures. While the established N = 28 shell closure is evident in the 10 charge radii of the isotopic chains of K through Mn, a similar signature of the N = 20 shell-closure is absent in the Ca region.Purpose: The isotope shift between neutron-deficient 36 K and 37 K was determined to investigate the change of the mean-square charge radii across N = 20 in the K isotopic chain.Methods: The D1 atomic hyperfine spectra of 36 K and 37 K were measured using an optical pumping and 15 subsequent β-decay asymmetry detection technique. Atomic rate equations were solved to fit the resonant line shape. The result was compared to Skyrme energy-density functional and shell-model calculations.Results: The isotope shift was obtained as δν 37,36 = −139(4)(3) MHz. Using a re-evaluated isotope shift, δν 39,37 = −264(2)(3) MHz, the isotope shift relative to 39 K was determined to be δν 39,36 = −403(5)(4) MHz. The differential mean-square charge radius was then deduced as δ r 2 39,36 = −0.16 (5)(8) Conclusions:The absence of the shell-closure signature at N = 20 in the K isotopic chain is understood as a balance between the monopole and the quadrupole proton-core polarizations below and above N = 20, respectively.

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The population of low-energy metastable states in Ni I subsequent to chargeexchange reactions and cascade decay was studied for 29.85 keV Ni ions impinging on a sodium vapor. The charge-exchange cross sections were calculated using a semi-classical time-dependent perturbation theory and redistribution of the population via rapid spontaneous decays was simulated. Collinear laser spectroscopy experiments were performed on four emission lines in Ni I to determine populations in low-energy metastable states: 3d 9 4s 3 D 3 (204.787 cm −1), 3d 9 4s 3 D 2 (879.816 cm −1), 3d 9 4s 3 D 1 (1713.087 cm −1) and 3d 9 4s 1 D 2 (3409.937 cm −1). Fair agreement was obtained between the simulation and experimental results. The low-energy metastable states were preferentially populated after the cascade decay in contrast to weak populations in near-resonant high-energy states in Ni I.