New experimental and theoretical energy levels, lifetimes, and oscillator strengths in singly ionised zirconium

Context. Neutron-capture elements are believed to make up almost all elements heavier than iron in the periodic table. By studying the abundance of these elements throughout the Galaxy, it is possible to put constraints on how and where neutron-capture processes occur. Determining elemental abundances is made possible through the correct interpretation and modelling of astrophysical spectra. Key ingredients for this, in turn, are accurate and complete sets of atomic data. Aims. We investigate the spectrum of singly ionised zirconium with the aim of reporting level energies and radiative lifetimes for previously experimentally unknown high-lying even 4d26s and 4d25d levels, as well as improved energies for odd 4d25p and 4d5s5p levels. We also aim to provide wavelengths, branching fractions, and oscillator strengths (log gf values) for lines from these upper even 4d26s and 4d25d levels. Methods. The energies, wavelengths, and branching fractions were derived from hollow cathode spectra recorded with a Fourier transform spectrometer. The radiative lifetimes were measured using a two-step laser-induced fluorescence technique. Theoretical calculations using the pseudo-relativistic Hartree-Fock method, modified to account for core polarisation effects, were also performed and show good general agreement with the experimental results. Results. We report for the first time level energies and radiative lifetimes for 19 high-lying even 4d26s and 4d25d levels and improved energies for odd 15 4d25p and 4d5s5p levels in Zr II. We also report wavelengths, branching fractions, and oscillator strengths for 79 lines from upper levels of 4d26s and 4d25d.