X-ray transitions of Ni-like and neighbouring charge states of Bi

We present high-resolution X-ray spectra of Zn-like, Cu-like, and Ni-like bismuth ions recorded at the National Institute of Standards and Technology (NIST) electron beam ion trap (EBIT) facility. Measurements were performed using a microcalorimeter array of transition-edge sensors (TES) at a nominal electron beam energy of 5.5 keV. By comparing observed spectra with simulations from a collisional-radiative model (NOMAD), we identified 43 previously unreported transitions, including forbidden electric quadrupole (E2) and magnetic octupole (M3) lines. Additionally, we analyzed high-n Rydberg transitions converging toward the ionization limit to determine the ionization potential of Ni-like Bi. The resulting experimental value of 5593.26(60) eV provides a significant improvement in precision over previous theoretical recommendations and serves as a benchmark for relativistic atomic structure calculations and high-precision mass spectrometry.