Ni- and Co-like Xe Ion EUV Spectra Produced by Excitation Around the Ionisation Threshold of Xe XXVII

A high-resolution flat-field grating spectrometer has been employed at the Livermore EBIT-I electron beam ion trap for observations of extreme-uv spectra of Ni-like ions Xe${}^{26+}$ and Co-like ions Xe${}^{27+}$. Multistep ionisation involving the long-lived 3d${}^9$ 4s ${}^3$D${}_3$ level in the Ni-like ion as a stepping stone has a significant influence on the charge state distribution at a given electron beam energy, as has been reported elsewhere. Complementing those observations of 3d-4s $E2$ and $M3$ transitions from long-lived levels, the present report shows spectra of 3d-4p and 3d-4f $E1$ transitions that arise from the decays of short-lived levels in both ions and their neighbouring ions of higher charge states and provide bright reference signals for the changes in the charge state distribution. Their observation is serendipitously furthered by the visual absence of 3d-4d transitions from the observed spectra, although $M1$ and $E2$ transitions between these configurations are permitted.