Core-hole-induced degeneracy of the valence subshells in the $5p$ photoemission of atomic europium

The $5p$ photoemission spectrum of atomic europium has been analyzed using high-resolution photoelectron spectroscopy. In contrast to the expected simple multiplet structure of a ${5p}^{-1}{4f}^7{(}^{8}S)$ configuration with six main lines, the observed Eu $5p$ photoemission shows a very complex spectrum. Configuration-interaction calculations reveal that the $5p$ core hole gives rise to a near degeneracy of the valence subshells in the final ionic states, so that the main multiplet structure of the spectrum can only be described by a strong mixture of the configurations ${6s}^2,$ $6s5d,$ ${5d}^2,$ and ${6p}^2.$ This mixing also results in a strong broadening of the lines at higher binding energy.