Transitions between the $4f$-core-excited states in Ir$^{16+}$, Ir$^{17+}$, and Ir$^{18+}$ ions for clock applications

Iridium ions near $4f$-$5s$ level crossings are the leading candidates for a new type of atomic clocks with a high projected accuracy and a very high sensitivity to the temporal variation of the fine structure constant $\alpha$. To identify spectra of these ions in experiment accurate calculations of the spectra and electromagnetic transition probabilities should be performed. Properties of the $4f$-core-excited states in Ir$^{16+}$, Ir$^{17+}$, and Ir$^{18+}$ ions are evaluated using relativistic many-body perturbation theory and Hartree-Fock-Relativistic method (COWAN code). We evaluate excitation energies, wavelengths, oscillator strengths, and transition rates. Our large-scale calculations included the following set of configurations: $4f^{14}5s$, $4f^{14}5p$, $4f^{13}5s^2$, $4f^{13}5p^2$, $4f^{13}5s5p$, $4f^{12}5s^25p$, and $4f^{12}5s5p^2$ in Pm-like Ir$^{16+}$ ; $4f^{14}$, $4f^{13}5s$, $4f^{13}5p$, $4f^{12}5s^2$, $4f^{12}5s5p$, and $4f^{12}5p^2$ in Nd-like Ir$^{17+}$; and $4f^{13}$, $4f^{12}5s$, $4f^{12}5p$, $4f^{11}5s^2$, and $4f^{11}5s5p$ in Pr-like Ir$^{18+}$. The $5s-5p$ transitions are illustrated by the synthetic spectra in the 180 - 200~\AA ~range. Large contributions of magnetic-dipole transitions to lifetimes of low-lying states in the region 2.5~Ry