Time-resolved soft-x-ray spectroscopy of a magnetic octupole transition in nickel-like xenon, cesium, and barium ions

A microcalorimeter with event mode capability for time-resolved soft-x-ray spectroscopy, and a high-resolution flat-field extreme ultraviolet spectrometer have been employed at the Livermore EBIT-I electron beam ion trap for observations and wavelength measurements of $M1$, $E2$, and $M3$ decays of long-lived levels in the Ni-like ions Xe²⁶⁺, Cs²⁷⁺, and Ba²⁸⁺. Of particular interest is the lowest excited level, 3d⁹4s0.2em0ex³D₃, which can only decay via a magnetic octupole $(M3)$ transition. For this level in Xe, an excitation energy of (590.40±0.030.3em0exeV) and a level lifetime of (11.5±0.50.3em0exms) have been determined.