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^-14f⁷(⁸S) 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 6s², 6s5d, 5d², and 6p². This mixing also results in a strong broadening of the lines at higher binding energy.