Observation of Electric Octupole Emission Lines Strongly Enhanced by an Anomalous Behavior of Cascading Contribution

Abstract

We present extreme ultraviolet spectra of Ag-like W${}^{27+}$ observed with an electron beam ion trap. In the spectra, the $4f_{7/2,5/2}$ -- $5s$ transitions are identified as the first observation of spontaneous electric octupole emission. Our theoretical investigation shows that the emission line intensity is strongly and specifically enhanced at the atomic number 74 by an anomalous behavior of cascading contribution to $5s$ via $5p\leftarrow 5d$.