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 Xe26+ and Co-like ions Xe27+. Multistep ionisation involving the long-lived 3d9 4s 3D3 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.