Abstract
We report on a striking improvement in spectral resolution in the soft x-ray range to 1.0 meV at 64.1 eV, measured via the $\ensuremath{\mu}\mathrm{eV}$-wide 2, ${\ensuremath{-}1}_{3}$ double-excitation resonance of helium. This ultrahigh resolution combined with the high photon flux at undulator beam line 9.0.1 of the Advanced Light Source have allowed observation of new Rydberg series and resonances below the $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ threshold of doubly excited He. The obtained resonance parameters (energies, lifetime widths, and Fano-q parameters) are in excellent agreement with the results of state-of-the-art calculations.
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